Measuring Soil Water Content with Ground Penetrating Radar A Review

Measuring Soil Water Content with Ground Penetrating Radar:A Review

J.A.Huisman,*S.S.Hubbard,J.D.Redman,and A.P.Annan

ABSTRACT termines the diurnal evolution of the atmospheric bound-

ary layer(Callies et al.,1998).Currently,there is a need We present a comprehensive review of methods to measure soil

to establish and quantify the contribution of soil water water content with ground penetrating radar(GPR).We distinguish

content–regulated land–atmosphere coupling to regional four methodologies:soil water content determined from reflected

climate anomalies,such as continental droughts and wave velocity,soil water content determined from ground wave veloc-

large-scale precipitation events(Entekhabi et al.,1999). ity,soil water content determined from transmitted wave velocity

At the catchment scale,soil water content partly con-between boreholes,and soil water content determined from the sur-

trols the separation of precipitation into infiltration, face reflection coefficient.For each of these four methodologies,we

evaporation,and runoff and therefore has a large influ-discuss the basic principles,illustrate the quality of the data with field

ence on soil erosion and river discharge.Recent studies examples,discuss the possibilities and limitations,and identify areas

have shown that including soil water content heteroge-where future research is required.We hope that this review will further

neity in spatially distributed hydrological models can stimulate the community to consider ground penetrating radar as one

improve discharge predictions(Merz and Plate,1997; of the possible tools to measure soil water content.

Merz and Bardossy,1998;Pauwels et al.,2001).How-

ever,there is no consensus on how to incorporate spatial

soil water content heterogeneity in these models.Some W ater at the land surface is a vital resource for have suggested that it is sufficient to include the statistics both human needs and natural ecosystems.Soci-(spatial mean and variance)of measured soil water con-ety’s fresh water needs for agriculture,sanitation,mu-tent structure(e.g.,Pauwels et al.,2001),whereas others nicipal,and industrial supply are ever increasing.At have suggested that discharge predictions also improve the same time,natural hazards involving water,such as when a full description of soil water content variation floods,droughts and landslides are major natural threats is included(Merz and Bardossy,1998).Furthermore, to society in many countries(Entekhabi et al.,1999).Merz and Plate(1997)argued that the improvement of The vadose zone,which may be defined as the transition discharge predictions by including soil water content

heterogeneity will strongly depend on the event charac-zone between the atmosphere and groundwater reser-

teristics,including antecedent soil water conditions and voirs,is important for water resource management,be-

rainfall intensities.

cause it regulates the water availability for vegetation,

At the field scale,information on the spatial distribu-including crops,while at the same time provides a pro-

tion of soil water is important for precision agriculture tective buffer zone between land surface and groundwa-

programs.With too much water,crop quality could de-ter against solutes and pollutants(Rubin,2003).

crease due to the adverse effects of waterlogged plant Hydrologists,soil scientists,ecologists,meteorolo-

roots(e.g.,reduced root respiration due to depletion of

gists,and agronomists all study the space and time vari-O

2and increased availability of toxic ions under reduc-

ability of water in the vadose zone,hereafter referred ing soil conditions).With too little water,crops can be to as soil water content,over a range of scales and for irreversibly damaged due to drought stress.In addition a variety of reasons.At the regional to continental scale,to the impact of water content on the quality and quan-the exchange of moisture and energy between soil,vege-tity of crops,the outlay of resources and energy concom-tation,and the atmosphere has an impact on near-sur-itant with crop irrigation is critical in water-scarce re-face atmospheric moisture and temperature,which in gions,especially as competition for water resources

between rural and agricultural land users increases. turn define the regional climate.For example,soil water

Clearly there is a need for soil water content measure-content determines to a large extent the relative magni-

ments across a range of spatial scales.High-frequency tudes of sensible and latent heat fluxes and therefore de-

electromagnetic techniques are the most promising cate-

gory of soil water content sensors to fulfill this need J.A.Huisman,Center for Geo-Ecological Research(ICG),Institute because this category contains a range of techniques for Biodiversity and Ecosystem Dynamics(IBED)–Physical Geogra-that measure the same soil water content proxy,namely phy,Universiteit van Amsterdam,The Netherlands(currently Dep.dielectric permittivity,at different spatial scales.Re-of Landscape Ecology and Resources Management,Justus-Liebig-mote sensing with either passive microwave radiometry University Giessen,Heinrich-Buff-Ring26-32,35372Giessen,Ger-

or active radar instruments is the most promising tech-many);S.S.Hubbard,Lawrence Berkeley National Laboratory and

University of California,Berkeley,CA;J.D.Redman and A.P.Annan,nique for measuring soil water content variations over Sensors and Software Inc.,Mississauga,ON,Canada.Received13Mar.

2003.Special Section—Advances in Measurement and Monitoring Meth-

Abbreviations:CMP,Common-MidPoint;CPT,cone penetrometer; ods.*Corresponding author(sander.huisman@agrar.uni-giessen.de).

GPR,ground penetrating radar;MOP,multi-offset profile;SWC,soil

water content;TDR,time domain reflectometry;VRP,vertical radar Published in Vadose Zone Journal2:476–491(2003).

Soil Science Society of America profiling;WARR,Wide Angle Reflection and Refraction;ZOP,zero

offset profile.

677S.Segoe Rd.,Madison,WI53711USA

476

https://www.doczj.com/doc/249395081.html,477 large regions(Jackson et al.,1996;Ulaby et al.,1996,allow more accurate travel time measurements,which

are needed for soil water content determination with Famiglietti et al.,1999;van Oevelen,2000).The passive

instruments have low spatial resolution and can either GPR.In this review,we present an overview of the

available methods for estimating soil water content with be airborne with pixel sizes of thousands of square me-

ters or satellite-borne with footprints in the order of GPR.Additionally,we discuss the possibilities and limi-

tations of each of the methods.Hopefully,this review tens of square kilometers.In the near future,passive

satellite remote sensing will provide global coverage of will further stimulate readers to consider GPR as one of

the viable options for soil water content determination. critical hydrological data,including soil water content

(Entekhabi et al.,1999).Active radar instruments have

smaller pixel sizes ranging from1to several1000m2.

PRINCIPLES OF

Although remote sensing will surely play an important

ELECTROMAGNETIC METHODS

role in many future hydrological studies,currently there

is still a need to establish transfer functions between Electromagnetic Wave Propagation

remote sensing and the more familiar in situ soil water

The propagation velocity of electromagnetic waves,v(m s?1), content measurements.Additionally,because remote is determined by the complex dielectric permittivity,ε*(f)?

sensing approaches for estimating water content esti-ε?(f)?jε″(f)

mate water content in the uppermost0.05m of the soil

and require that the vegetation cover is minimal(Jack-v(f)?c

?ε?(f)?r1?√1?tan2?

2[1]

son et al.,1996),remote sensing methods are not applic-

able in all types of vadose zone studies.

A well-established in situ electromagnetic technique

for soil water content investigations is time domain re-with the loss tangent?defined as flectometry(TDR),which was introduced in vadose zone

hydrology in the early1980s(Topp et al.,1980).Time do-

main reflectometry has developed into a reliable method tan??ε″(f)??dc

2?fε0

ε?(f)

[2]

for soil water content determination that can easily be

automated(Heimovaara and Bouten,1990).Further-

and where c is free space electromagnetic propagation velocity more,TDR can simultaneously measure dielectric per-

(3?108m s?1),f is the frequency of the electromagnetic field mittivity and bulk soil conductivity(Dalton et al.,1984;

(Hz),ε?(f)is the real part of the relative dielectric permittivity, Topp et al.,1988),which allows the study of water andε″(f)is the imaginary part of the relative dielectric permittivity,

solute transport within the same soil volume.Although?

r is the relative magnetic permeability,?dc is the DC conduc-

TDR is highly suited for monitoring the development tivity(S m?1),andε

0is the free space permittivity(8.854?

of soil water content at one location with a high temporal10?12F m?1).For nonmagnetic soils,?

r equals1in the GPR

frequency range(van Dam et al.,2002).Throughout this re-resolution,the small measurement volume(?dm3)makes

view,permittivity is understood to represent the relative di-it sensitive to small-scale soil water content variation

electric permittivity,εr,that is,the permittivity relative to free (e.g.,macropores,air gaps due to TDR insertion)within

space as calculated by the absolute permittivity,ε(F m?1), this volume(Ferre′et al.,1996).Furthermore,assess-

divided by the free space permittivityε0(F m?1):

ment of spatial soil water content variation with TDR

is labor intensive because TDR sensors need to be in-

εr?ε

ε0[3]

stalled at each measurement location.

Clearly,there is a scale gap between remote sensing

The imaginary part of the permittivity,ε″(f),is associated and TDR measurements of soil water content.At inter-

with the energy dissipation,and the real part of the permittivity, mediate spatial scales,such as agricultural land and small

ε?(f),is associated with the capability to store energy when catchments,reliance on sparse TDR measurements or

an alternating electrical field is applied.The complex per-coarse remote sensing measurements might not provide

mittivity of most materials varies considerably with the fre-the accurate soil water content information required at

quency of the applied electric field.An important process these scales(e.g.,crop management,precision farming).contributing to the frequency dependence of permittivity is

Therefore,there is a need for soil water content mea-the polarization arising from the orientation with the imposed surement techniques that can provide dense and accu-electric field of molecules that have permanent dipole mo-rate measurements at an intermediate scale.Since the ments.The mathematical formulation of Debye describes this

process for pure polar materials(Debye,1929):

early days of electromagnetic measurement techniques,

ground penetrating radar(GPR)has been conceived as

the natural intermediate-scale counterpart of TDR forε*(f)?ε∞?εs?ε∞

1??i f f rel??j?dc

2?fε0

[4]

soil water content measurements.Although the number

of TDR applications has increased immensely in the

past20yr,the number of GPR applications for measur-

whereε∞represents the permittivity at frequencies so high ing soil water content has only recently increased.Prob-that molecular orientation does not have time to contribute

ably,the most important reason behind this delay is the to the polarization,ε

s represents the static permittivity(i.e.,

more complicated behavior of the unguided waves used the value at zero frequency),and f

rel (Hz)is the relaxation

in GPR as compared with waves guided by a TDR sensor.frequency,defined as the frequency at which the permittivity

equals(εs?ε∞)/2(Nelson,1994).The separation of Eq.[4] Furthermore,recent improvements in GPR technology

478

VADOSE ZONE J.,VOL.2,NOVEMBER 2003

and εis based on dielectric mixing models,which use the

volume fractions and the dielectric permittivity of each soil constituent to derive a relationship (e.g.,Dobson et al.,1985;Roth et al.,1990;Friedman,1998;Jones and Friedman,2000).In dielectric mixing models,the bulk permittivity of a soil–water–air system,εb ,may be expressed with the Complex Re-fractive Index Model (CRIM):

εb ?

?

?ε?

w ?(1?n )ε?

s ?(n ??)ε?a

?

1

?[7]

where n (m 3m ?3)is the soil porosity;εw ,εs ,and εa are the permittivities of water,soil particles and air,respectively;and ?is a factor accounting for the orientation of the electrical field with respect to the geometry of the medium (??1for an electrical field parallel to soil layers,???1for an electrical field perpendicular to soil layers,and ??0.5for an isotropic Fig.1.Example of the Debye model for the real part (solid line)and

medium).After rearranging Eq.[7],the following expression imaginary part (dashed line)of the permittivity.

can be obtained for soil water content:

into its real and imaginary part is shown in Fig.1for an ??

ε?b

?(1?n )ε?s ?n ε?a ε?w ?ε?a

[8]

idealized medium with εs ?20,ε∞?15,f rel ?108.47Hz (300MHz)and ?dc ?0S m ?1.Figure 1shows that at frequencies very low and very high with respect to relaxation processes,After substitution of εa ?1and assuming ??0.5,Eq.[8]

the permittivity has constant values and zero losses.At inter-reduces to

mediate frequencies,the permittivity undergoes a dispersion,and dielectric losses occur with the peak loss at f ?f rel .Water ??

1

√εw ?1

√εb ?

(1?n )√εs ?n

√εw ?1

[9]

in its liquid state is a prime example of a polar dielectric.The Debye parameters of water are εs ?80.1,ε∞?4.2,and f rel ?which gives a physical interpretation of a simple soil water 1010.2Hz (17.1GHz)at 25?C (Hasted,1973).In sandy soils,content–εrelationship suggested by Ledieu et al.(1986)and most water is effectively in its free liquid state.In high clay Herkelrath et al.(1991):

content soils,pore water is not necessarily in its free liquid state.Sometimes it is physically absorbed in capillaries,limited ??a √b ?b [10]

in motion by electrostatic interaction with clay particles.Di-electric relaxation of absorbed water takes place at lower where a and b are calibration parameters and √εb is also re-frequencies than the relaxation of free water (Hasted,1973;ferred to as refractive index (n a ).This relationship has an Or and Wraith,1999).

accuracy of 0.0188m 3m ?3,as determined by an independent In the case of GPR measurements,which commonly have validation on mineral soils (Jacobsen and Schj?nning,1994).a frequency bandwidth from 10MHz to 1GHz,ε″(f )is often For more information,the reader is referred to Robinson et small compared with ε?(f ).Furthermore,many soils do not al.(2003)and the references therein.

show relaxation of permittivity in the frequency range of 10It is important to realize that most available calibration MHz to 1GHz.Under these conditions,Eq.[1]reduces to

equations between permittivity and water content were derived using TDR,which mainly operates in the frequency range from 500to 1000MHz (see Robinson et al.2003).However,it has v ?

c √ε?

[5]

long been recognized that high clay content soils exhibit signif-icant permittivity dispersion at low frequencies (e.g.,Olhoeft,for nonsaline soils (Wyseure et al.,1997).The real part of 1987).Recently,West et al.(2003)presented frequency-the permittivity of water within the megahertz to gigahertz dependent permittivity measurements of fine-grained sand-bandwidth is approximately 80,whereas the permittivity of stone samples containing up to 5%clay and soil samples con-air is 1and of most other common soil constituents is about taining Ottawa sand and varying amounts of montmorillonite 3to 10.This large contrast in permittivity explains the success clay.Their result showed that both the sandstone and soil of soil water content measurements with electromagnetic tech-samples showed a significant frequency dispersion below 350niques working within this frequency bandwidth.

MHz.This implies that site-specific calibration may be re-quired for those applications that require accurate water con-Water Content–Permittivity Relationships

tent measurements with lower antenna frequencies,such as the commonly used 100-MHz antenna.Clearly,this is an im-The most commonly used relationship between apparent portant research topic for the near feature.However,even permittivity,ε,and volumetric soil water content,?(m 3m ?3),when using published petrophysical relationships derived with was proposed by Topp et al.(1980):

TDR (such as Eq.[6])with permittivity values obtained from ???5.3?10?2?2.92?10?2ε?5.5?10?4ε2

GPR data,reasonable information about water content varia-tion and spatial patterns can be obtained,as will be shown ?4.3?10?6ε3[6]

below.

and was determined empirically for mineral soils having vari-ous textures.It has an accuracy of 0.022m 3m ?3determined

PRINCIPLES OF GROUND in an independent validation on mineral soils (Jacobsen and PENETRATING RADAR

Schj?nning,1994).The term apparent is used because the per-mittivity used in this equation is determined from the mea-Ground penetrating radar is a geophysical measurement tech-nique that has been extensively used to noninvasively map sured electromagnetic propagation velocity in the soil.

A more theoretical approach to relating soil water content

subsurface features at scales from kilometers for geologic fea-

https://www.doczj.com/doc/249395081.html,479

Fig.3.Propagation paths of electromagnetic waves in a soil with

two layers of contrasting dielectric permittivity(ε1andε2)(after

Sperl,1999).

Fig.2.Wave fronts around a dipole source on the soil surface.A and MEASURING SOIL WATER CONTENT

B are spherical waves in the air and soil,respectively.Wave

C is

WITH GPR the lateral or head wave in the soil,and D is the ground wave in

the air(after Annan,1973).Soil Water Content Measurements with

Reflected Waves

tures to centimeters for rebar in concrete structures.Extensive

Two classes of methods to estimate soil water content from literature describing GPR applications is available.An excel-

reflected wave travel time data can be distinguished.The first lent introduction to GPR in hydrogeological applications is

class contains the methods that use a single antenna separation available in Davis and Annan(1989).

for soil water content estimation(e.g.,soil water content esti-The GPR technique is similar in principle to seismic and

mation from scattering objects and traditional GPR sections). sonar methods.In the case of the most commonly used bistatic

The second class contains the methods that require multiple systems,one antenna,the transmitter,radiates short pulses of

measurements with different antenna separations.

high-frequency(MHz to GHz)electromagnetic waves,and the

other antenna,the receiver,measures the signal from the trans-

Single(or Common)Offset Reflection Methods

mitter as a function of time.When the source antenna is placed

on the surface,spherical waves are radiated both upward into The energy that GPR transmits into the soil will be(partly) the air and downward into the soil as indicated by wave fronts reflected when contrasts in soil permittivity are encountered.

A and

B in Fig.2.Because of the continuity requirements for Figure4(right)shows an idealized GPR section measured the electromagnetic field at the soil surface,the propagating with surface radar and a fixed antenna separation(single off-spherical air wave(A in Fig.2)gives rise to a lateral wave set)over an anomaly(e.g.,a water-filled pipe)having a differ-front(

C in Fig.2)in the soil.Similarly,the spherical wave ent permittivity than the host material as shown in Fig.4(left). propagating in the soil gives rise to the ground wave(

D in Because GPR emits waves in all directions,reflected energy Fig.2).The ground wave amplitude is known to decrease is measured before the GPR is directly over it(Fig.4,left). strongly with distance above the soil surface,and therefore The reflected events in the radar section trace out a hyperbola the ground wave is not presented as a wave front in Fig.2.(B in Fig.4,right)because the reflected energy of the GPR Two important aspects of GPR are resolution and depth measurement directly above the anomaly has the shortest penetration.GPR resolution is determined by the period of travel distance(time)and all other waves will have a larger the emitted pulse,which is controlled by the frequency band-distance to travel.The average wave velocity in the soil deter-

mines the convexity of the reflection hyperbola B;i.e.,it deter-width of the GPR system.Because impulse radar systems are

mines how much longer the waves need to travel the extra designed to achieve bandwidths that are about equal to the

distance.The average velocity between the ground surface center frequency,the resolution of GPR increases with in-

and the anomaly,v soil,can be determined from a GPR transect creasing center frequency(Davis and Annan,1989).Depth

by fitting the following hyperbola to measured arrival times penetration of GPR measurements is strongly controlled by

at several positions x

the soil electrical conductivity combined with the center fre-

quency of the GPR system.In low-conductivity media,such

as dry sand and gravel,low-frequency GPR systems(e.g.,50-v

soil?2√x2?d2

t rw,x

[11]

or100-MHz antennas)can achieve penetration up to several

tens of meters,and high-frequency systems(e.g.,450-or900-

where x is the position relative to the position of the scattering MHz antennas)achieve penetration of one to several meters.

object(apex of the hyperbola),d is the depth of the scattering

For silty sands and clays,depth penetration will be significantly object,and t

rw,x is the arrival time of the reflected wave at po-

less.It is important to realize that this high sensitivity to soil sition x that has been zero time corrected.If the GPR section texture and electrical conductivity reduces the range of soils is measured with a significant antenna separation,a,this should where GPR can successfully be applied.also be included in the velocity determination as follows:

Figure3presents possible propagation paths for surface

GPR energy.Principally,all these waves can be used to mea-

v soil?√(x?0.5a)2?d2?√(x?0.5a)2?d2

t rw,x

[12]

sure soil water content.In the following section,we focus on

soil water content estimation using reflected and ground wave

travel time data.In addition,we also discuss the estimation Most common GPR analysis software provides routines of water content using borehole GPR travel time data and where the velocity can be determined interactively by manu-

ally fitting hyperbola to the limbs of the reflections of the using ground surface reflection amplitude data.

480

VADOSE ZONE J.,VOL.2,NOVEMBER 2003

Fig.4.Idealized ground penetrating radar (GPR)transect measured with a fixed antenna separation over an anomalous wetter zone and a horizontal groundwater table (GWT).A marks the air wave,B marks the point reflector,and C marks the reflection from the groundwater table (after Davis and Annan,1989).

scattering object.The velocity can then be used to calculate v soil ?

2d t rw

[13]

soil permittivity (Eq.[5])and soil water content (e.g.,Eq.[6]).The zero time correction of arrival times is required to correct for the additional travel time at the beginning of each where t rw is the two-way travel time of the reflected wave that measurement,which is mainly due to the travel time in the has been zero time corrected and d is the water table depth.cables of the radar system.A commonly used correction proce-When antenna separation is significant,this equation becomes

dure consists of (i)aligning the arrival times of the air wave to correct for drift in the zero time (e.g.,caused by temperature v soil

?

2√d 2?(0.5a )2t rw

[14]

changes affecting the radar system and the cables),(ii)estimat-ing the average arrival time of the air wave,and (iii)calculating To be useful for estimating water content,the single offset the zero time correction from the average arrival time and GPR reflection method requires sufficient signal penetration,the known antenna separation.It should be noted that the the presence of a subsurface dielectric contrast that yields a methods presented in this review have different requirements clear (and preferably continuous)GPR reflector,and good for the accuracy of the zero time correction.The accuracy of control on the depth of the reflector.Shallow studies using borehole GPR and single offset ground wave data depends this technique have been successfully performed using buried on accurate zero time corrections,whereas the accuracy of reflectors and engineered materials,where the depth to the multi-offset measurements does not depend strongly on accu-reflector is well constrained.Grote et al.(2002)used the reflec-rate zero time corrections.

tion travel time associated with shallow (?1m)reflectors Although it is simple and straightforward to determine ve-buried within a constructed sandy test pit to estimate water locity from scattering objects,it is a method that has not been content values.Their estimates were within 0.01m 3m ?3of used often for soil water content determination.The main the measured values obtained using gravimetric techniques.drawback of this method is that it can only be used in soils Grote et al.(2002)also used 1.2-GHz time-lapse single offset where scattering objects can be observed in the GPR section.GPR reflection data to monitor water infiltration conducted Even when scattering objects are present,this method only within a pavement section consisting of permeable aggregate provides the average soil water content to the depth of the base layers overlain by concrete,all having a prescribed thick-reflector;that is,the user has no control over the depth resolu-ness.Under these engineered conditions,they obtained esti-tion of the soil water content measurements.An early appli-mates of volumetric water content within the permeable layers cation was presented by Vellidis et al.(1990),who used a as a function of depth using the GPR reflection method,which reflection from a buried pipe to determine the average velocity agreed with measurements obtained using gravimetric tech-above the wetting front,which allowed the monitoring of wet-niques to within 0.01m 3m ?3.Stoffregen et al.(2002)estimated ting front movement by assuming a homogeneous soil water volumetric water content seasonally using 1-GHz GPR reflec-content distribution above the wetting front.

tions from the base of a 1.5-m-deep lysimeter,which was filled The reflection from the top of the saturated zone,just above with sandy soils.They found that the standard deviation be-the groundwater table,is an example of a (semi-)horizontal tween the GPR estimates and the lysimeter measurements of contrast in soil permittivity (marked with C in Fig.4,right).water content was on average 0.01m 3m ?3and that the esti-It can be seen that the anomalous wetter zone results in a mated changes in water content corresponded with seasonal pull-down of the arrival time of the wave reflected from the moisture variations.

horizontal groundwater table because the average wave veloc-The accuracy of the single offset GPR reflection method ity to the groundwater table is lower for GPR measurements for estimating water content under natural conditions is not recorded above the anomaly.Unfortunately,the groundwater yet well established.Some researchers have tested the concept table reflection in a single offset measurement cannot be used of using GPR reflections under natural conditions to estimate to calculate the v soil without knowledge of the water table depth.water content using depth measurements to the reflectors ob-Of course,the water table depth (or depth of any other reflec-tained at discrete locations from,for example,noting lithologic tor)can be determined independently,and in such cases the transitions during drilling (Weiler et al.,1998)or water table arrival time of the water table reflection can easily be con-observations (van Overmeeren et al.,1997).The use of single offset GPR reflection data for estimating spatially variable

verted to average soil water content of the vadose zone with

https://www.doczj.com/doc/249395081.html,

481

https://www.doczj.com/doc/249395081.html,mon-midpoint (CMP,top)and wide angle reflection and https://www.doczj.com/doc/249395081.html,mon-midpoint (CMP)measurement made with a 100-refraction (WARR,bottom)acquisition,where S denotes the trans-MHz antenna at the Cambridge Research Station,University of mitter location and R denotes the receiver locations.

Guelph,ON,Canada.

water content under naturally heterogeneous conditions at and Dubois,1995).Most common GPR analysis software pro-the field scale is a topic of active research.vides routines where the velocity can be determined by manu-ally fitting hyperbola to the reflected waves in the multi-offset Multi-Offset Reflection Methods

measurements.Multi-offset measurements also permit veloc-ity determination from arrivals other than the reflected waves.Single offset measurements cannot be used to determine water For example,Bohidar and Hermance (2002)showed how to content from reflecting soil layers if no information about the use critically refracted air waves and subsurface refracted depth of the reflector is available.In that case,one can use a waves for soil water content determination.

multi-offset GPR acquisition geometry to determine soil water To avoid subjective soil water content estimates and to content from radar reflections.Two commonly used multi-offset speed up the analysis,(semi-)automated approaches for veloc-GPR acquisition geometries are called Common-MidPoint ity determination from reflected GPR waves have been devel-(CMP)and Wide Angle Reflection and Refraction (WARR)oped that are analogous to the velocity analysis approaches measurements (Fig.5).In CMP acquisition,the distance be-developed for use with seismic data (e.g.,Yilmaz,1987).A tween the antennas is increased stepwise while keeping a com-well-known method is semblance analysis.The aim of sem-mon midpoint.In WARR acquisition,the distance between blance analysis is to find the velocity and travel time for which the antennas is increased stepwise with the transmitter at a the reflection energy of a reflected wave in a multi-offset fixed position.A schematic outcome of a multi-offset GPR measurement collapses to a point.This is done with help of measurement is given in Fig.6.If consistent reflected waves the semblance plot,which is constructed by recalculating the are present in the multi-offset GPR measurement,they can arrival times of the CMP for a range of velocities (x axis of be used to calculate soil water content directly by fitting

semblance plot)and summing the normalized energy for each arrival time (y axis)for each velocity.High values in the v soil ?

2√d 2?(0.5a )2

t rw,a

[15]

semblance plot indicate that the reflected waves at that partic-ular arrival time are well described by that particular velocity.to the zero time corrected arrival times of the reflected wave,The manually or semiautomatically determined velocities t rw,a ,for different antenna separations,a ,and solving for depth,from the multi-offset measurement are average velocities to d ,and the average velocity to the reflecting layer,v soil (Tillard

the depth of the reflector.To convert these average velocities to interval velocities of each layer,v int,n ,the Dix formula (Dix,1955;Yilmaz,1987)can be used:

v int,n ?

?

t rw,n v 2soil,n ?t rw,n ?1v 2

soil,n ?1

t rw,n ?t rw,n ?1

[16]

where v soil,n is the average velocity from the surface down to the bottom of layer n ,v soil,n ?1is the average velocity down to the bottom of layer n ?1,t rw,n is the two-way travel time to the bottom of layer n ,t rw,n ?1is the two-way travel time to the bottom of layer n ?1,and n ?1is the upper layer of the soil.The requirements for useful water content estimates with the multi-offset GPR reflection method are similar to those of single offset measurements:decent signal penetration and the presence of subsurface dielectric contrasts that yield clear (and preferably continuous)GPR reflectors.There are numer-ous applications of multi-offset measurements for soil water content determination (e.g.,Tillard and Dubois,1995;Greaves et al.,1996;van Overmeeren et al.,1997;Dannowski and Yara-Fig.6.Schematic wide angle reflection and refraction (WARR )mea-manci,1999;Endres et al.,2000;Nakashima et al.,2001;Bohi-surement.The ground wave can be identified as a wave with a dar and Hermance,2002;Garambois et al.,2002).Figure 7linear move out starting from the origin of the x –t plot.In the shows a CMP measurement made with 100-MHz antennas at slope equations,c is the electromagnetic velocity in air and x is the antenna separation (after Sperl,1999).

the Cambridge Research Station (University of Guelph,ON,

482

VADOSE ZONE J.,VOL.2,NOVEMBER 2003

Fig.9.Wide angle reflection and refraction (WARR)measurement recorded on loamy sand with the 225-MHz antennas.The velocity of the ground wave is v and of the air wave is c ,both in meters per nanosecond.

Fig.8.Semblance plot of the common-midpoint (CMP)measurement shown in Fig.7to illustrate automatic extraction of velocity vs.The ground wave can easily be recognized on data collected time.Blue colors indicate a high semblance.

using a multi-offset GPR acquisition geometry,by the ob-served linear relationship between antenna separation and Canada).The air wave,ground wave,and several reflected ground wave travel times,which starts at the origin of the waves can clearly be recognized.Figure 8presents the corre-multi-offset measurement set (see Fig.6,7,and 9).The slope sponding semblance analysis plot.The semblance analysis of the ground wave in a multi-offset measurement is directly shows that the strong first reflection starting at about 35ns in related to the ground wave velocity and can,therefore,be Fig.7corresponds with a velocity of about 0.13m ns ?1for used for soil water content determination.The accuracy of this the upper soil layer.The second reflected wave starts at ap-soil water content measurement technique was determined by proximately 65ns,and the semblance plot indicates that the Huisman et al.(2001)by analyzing a set of 24multi-offset average velocity is somewhat higher at 0.14m ns ?1.Inserting measurements collected using 225-MHz antennas and inde-these average velocities into Eq.[16]results in an interval pendent gravimetric samples.The resulting calibration equa-velocity of 0.15m ns ?1for the second layer.Figure 8also tion (??0.1087n a ?0.1076)is shown in Fig.10and has an shows that semblance analysis does not result in very accurate accuracy of 0.024m 3m ?3.Grote et al.(2003)compared 29velocity estimates.The width of the velocity spectrum in Fig.estimates of water content obtained using multi-offset GPR 8indicates an accuracy of about 0.02m ns ?1for the velocity with independent gravimetric measurements.They reported estimate,which translates to an accuracy of 0.03m 3m ?3for root mean squared errors of 0.022and 0.015m 3m ?3using soil water content measurements in dry soils.

450-and 900-MHz antennas,respectively,for calibration equa-Although multi-offset measurements are widely used in tions in the form of Eq.[10].Both Huisman et al.(2001)and GPR data processing for determining velocity profiles with Grote et al.(2003)reported that the permittivity determined depth,there are some distinct disadvantages to soil water from the ground wave velocity also agreed well with the TDR content determination with this method:

measurements at these frequencies,as is illustrated in Fig.11(from Huisman et al.,2001).

1.As with soil water content determination from single Estimation of soil water content using multi-offset GPR offset measurements,there is no control over the mea-measurements is cumbersome and time-consuming,as was surement depth resolution.

mentioned before.The ground wave velocity can also be deter-2.Multi-offset measurements are cumbersome to make and mined from a single offset GPR measurement,provided that do not allow reconnaissance studies of soil water con-the approximate arrival time of the ground wave is known tent variation.

from a multi-offset GPR measurement.Therefore,Du (1996)

3.For heterogeneous media,soil water content determined from multi-offset measurements is biased toward the common midpoint in the case of a CMP acquisition ge-

ometry and toward the position of the fixed antenna in the case of a WARR acquisition geometry (Huisman and Bouten,2003).

Soil Water Content Measurements with

the Ground Wave

The measurement principle of soil water content estimation with the ground wave is illustrated in Fig.3.The ground wave is the part of the radiated energy that travels between the transmitter and receiver through the top of the soil.The ground wave is detected by the GPR receiver,even in the absence of clearly reflecting soil layers (Du,1996;Berktold et al.,1998;Sperl,1999).The evanescent character of the Fig.10.Calibration equation between gravimetrically determined soil ground wave (see Fig.2)measured by the GPR receiver an-water content (SWC)and refractive index (n WARR )determined from tenna at the soil surface requires that both the transmitter the ground wave velocity obtained with 225-MHz ground penetrat-ing radar (GPR)antennas.

and the receiver be placed close to the soil surface.

https://www.doczj.com/doc/249395081.html,

483

resulting soil water increase map for TDR is presented in Fig.

12(right).Clearly,there is a good general agreement between both methods since they show similar patterns in soil water increase caused by the heterogeneous application of water.Furthermore,the higher number of GPR measurements ac-quired within a shorter time span has resulted in a much higher resolution,as evidenced by the clear circular boundaries of the increase in soil water content due to the impact sprinklers located at (17.5,42.5)and (35,25).Using 450-and 900-MHz single offset GPR ground wave data,Grote et al.(2003)esti-mated the spatial and temporal variations in near surface water content over a large-scale agricultural site during the course of a year.They found that soil texture exerted an influence on the spatial distribution of water content over the field https://www.doczj.com/doc/249395081.html,parison of n TDR and n WARR (225MHz)for the same mea-Using a 100-MHz antenna,Lesmes et al.(1999)generally surements shown in Fig.10.

found similar trends in soil water content estimated with GPR ground wave data and reference small-scale measurements.and Sperl (1999)proposed the following procedure for soil However,soil water content estimated using 100-MHz GPR water content mapping with the ground wave of GPR:ground wave data typically underestimated water content com-pared with the reference measurements collected between 01.Identify an approximate ground wave arrival time for and 0.30m below ground surface.This discrepancy may be different antenna separations in a multi-offset GPR mea-associated with the deeper zone of influence associated with surement,

the lower-frequency GPR antennas as compared with the 2.Choose an antenna separation where the ground wave higher-frequency antennas used in the studies described above.is clearly separated from the air and reflected waves and Although the results with ground wave data generally are https://www.doczj.com/doc/249395081.html,e this antenna separation for single offset GPR mea-promising,there are still some uncertainties associated with surements and relate the changes in ground wave arrival this method.An important,but unresolved issue is the effec-time to changes in soil permittivity.tive measurement volume over which the ground wave aver-The most straightforward relationship between ground wave ages.Du (1996)suggested that the influence depth is approxi-arrival time t GW (s),antenna separation x (m),and soil per-mately one-half of the wavelength [??c /(f ε)1/2],which would,mittivity is given by Sperl (1999):

for example,mean that for a central frequency of 225MHz,the depth of influence could vary from 0.50m (ε?4.0)to ε??c v ?2?

?c (t GW ?t AW )?x x ?

2

[17]

0.22m (ε?20.0).Sperl (1999)reported that the depth of influence was indeed a function of wavelength,but from a where t AW (s)is the air wave arrival time.The t AW is included

modeling exercise he concluded that the influence depth is as part of the zero time correction,as was already discussed approximately 0.145?1/2,which would suggest that the influ-in the previous section.Accurate zero time corrections and ence depth ranges from 0.15m (ε?4.0)to 0.10m (ε?20.0)accurate travel time determination are important for soil water for the 225-MHz antennas.The results of Sperl (1999)do not content measurements derived from ground wave travel time contradict the results of Huisman et al.(2001),who concluded data.Typically,the onset of the wave or the first maximum of that soil water content measurements based on ground wave the wave are picked,but Huisman and Bouten (2003)showed data are similar to measurements with 0.10-m-long TDR that these phases can be quite difficult to pick when mapping probes for both the 225-and 450-MHz GPR antennas.Grote water content variation,especially for the air wave.Instead,et al.(2003)compared estimates of water content obtained Huisman and Bouten (2003)proposed to use the zero-crossing using 450-and 900-MHz common offset GPR ground wave between the maximum and minimum wave amplitude because data with measurements collected using gravimetric tech-the steep derivative of the wave at this phase allows the most niques in soils having various textures and at depths from 0accurate travel time determination.It should be noted that to 0.10,0.10to 0.20,and 0to 0.20m below ground surface.the onset of the wave should be preferred over the latter They found that the estimates obtained using these frequen-approach when there are clear indications for waveform dis-cies showed the highest correlation with the soil water content persion (Annan,1996).

values averaged across the 0-to 0.20-m range and the least Recent experiments by Lesmes et al.(1999),Hubbard et correlation with the water content measurements taken from al.(2002),Huisman et al.(2002,2003),and Grote et al.(2003)the 0.10-to 0.20-m interval.Clearly,further research is needed have confirmed that soil water content mapping with ground to better understand the ground wave zone of influence.wave travel time data works well.In an irrigation experiment,Other drawbacks of using the GPR ground wave to estimate Huisman et al.(2002)created a heterogeneous pattern in soil water content include the following:

soil water content by using different types of sprinklers with varying ranges and intensities.Figure 12(left)shows the in- 1.It might be difficult to separate the ground wave arrival crease in water content over a 60by 60m area measured with in the clutter of critically refracted and reflected waves.GPR,which was calculated by subtracting the water content 2.It might be difficult to choose an antenna separation for map before irrigation from the water content map after irriga-which the arrival times of the air and ground wave can tion.Ground penetrating radar measurements were made with consistently be picked despite moving the antennas a 225-MHz antenna on lines 5m apart.The antenna separation across a field with varying soil water content.

was 1.54m,and the distance between GPR measurements 3.The ground wave is attenuated more quickly than other was 0.5m.The measurements were performed within 1h in waves,which limits the range of antenna separation at the field.Time domain reflectometry measurements to 0.1m deep were made within 1.5h at 5-m station spacing.The

which the ground wave can be observed.

484VADOSE ZONE J.,VOL.2,NOVEMBER2003

Fig.12.Maps illustrating the increase in soil water content(m3m?3)due to irrigation obtained using ground penetrating radar(GPR)ground wave and time domain reflectometry(TDR)measurements.The maps were obtained by subtracting interpolated maps of soil water content obtained using GPR and TDR data collected before and after irrigation.

mographic data was presented in detail by Peterson(2001), Soil Water Content Measurements with

who also discussed methods for recognizing and correcting Borehole GPR

for errors caused by incorrect station geometry,incorrect zero For borehole GPR applications,the transmitting and receiv-time and zero time drift,geometric spreading,transmitter ing antenna are lowered into a pair of vertical access tubes.radiation pattern,transmitter amplitude,and high angle ray-In the zero offset profile(ZOP)mode,the antennas are low-paths.The two-dimensional tomogram is obtained by discret-ered such that their midpoints are always at the same depth izing the area between the boreholes in rectangular cells of (Fig.13,left).The arrival time of the direct wave between constant velocity and determining the velocity of each cell by the boreholes and the known borehole separation is used to minimizing the difference between measured arrival times and calculate the velocity and soil permittivity.The ZOP mode is arrival times calculated for raypaths passing through these an attractive approach for measuring the soil water content cells.When necessary,three-dimensional tomograms can also profile of the vadose zone with a high spatial resolution and a be reconstructed(e.g.,Eppstein and Dougherty,1998).The large sampling volume(Gilson et al.,1996;Knoll and Clement,drawback of acquiring the two-dimensional tomogram is the 1999;Parkin et al.,2000;Binley et al.,2001,2002,Rucker and much longer time(typically several hours)required to obtain Ferre′,2003).Furthermore,each borehole GPR measurement all the required measurements.Therefore,the MOP mode is requires only a couple of seconds and,therefore,the ZOP

best suited for steady-state water content conditions.For cases method is potentially capable of measuring transient processes

where the subsurface variations occur on similar time scales within the unsaturated zone.

as the radar data acquisition,Day-Lewis et al.(2002)recently Soil water content can also be determined from a multi-

described a sequential approach for inverting time-lapse tomo-offset profile(MOP,Fig.13,right).The first arrival times of

graphic radar data to assist in more accurate imaging.

all multi-offset measurements can be used to reconstruct a

The sampling volume of borehole GPR measurements can (tomographic)two-dimensional image of the soil water con-

be approximated by the volume of the first Fresnel zone(Cer-tent distribution between the boreholes(Hubbard et al.,1997;

veny and Soares,1992).The Fresnel volume is an elongated Parkin et al.,2000;Binley et al.,2001;Alumbaugh et al.,2002).

rotational ellipsoid with its foci at the location of the transmit-To extract high-resolution,quantitative information from ra-

ter and the receiver.For a homogeneous medium,the Fresnel dar tomographic data,it is important to process the data as

volume,V,depends on the path length of the ray trace,L accurately as possible.The procedure for inverting radar to-

(m),and the wavelength of the radar signal,?(m):

V?4

3

?abc[18]

where a,b,and c are the semi-axes of the ellipsoid defined as

a?1

2?

?

2

?L?[19]

b,c?1

2?

?2

4

?L??12[20]

The wavelength,?,can be calculated from the center fre-

quency or the bandwidth of the transmitted GPR pulse.The Fig.13.Schematic diagrams of borehole ground penetrating radar

Fresnel zone,a circular region,is the cross section of the Fres-(GPR)wave paths between transmitter and receiver for ZOP(left)

and MOP(right).nel volume in a plane perpendicular to the raypath.The maxi-

https://www.doczj.com/doc/249395081.html,485 mum diameter of the Fresnel zone along the raypath(2b)is et al.,1998).In Fig.14b,the solid lines illustrate the soil water often given as the spatial resolution in tomography.

content estimates obtained from the ZOP measurements,ac-Several studies have compared water content estimates ob-quired prior and subsequent to the MOP measurements,while tained from GPR travel time data collected between boreholes

the dashed line indicates the estimates obtained from the ZOP with one-dimensional measurements obtained from data col-measurements extracted from the MOP gathers.These ZOP lected within the corresponding boreholes(Hubbard et al.,

measurements have been converted into water content using 1997;Binley et al.,2001;Alumbaugh et al.,2002).Errors in the Topp relation and without accounting for the zero time water content estimates obtained using borehole GPR data

drift as was mentioned above and described in detail by Pe-can potentially arise from acquisition procedures(ZOP and terson(2001).A gradual shift in zero time is illustrated by MOP),inversion procedures(MOP),as well as from the petro-

this figure,which is indicative of system changes associated physical relationship used to convert the GPR velocity esti-with components in the radar system warming up.This figure mates to water content(ZOP and MOP).Using a site-specific

highlights how important it is to correct for zero time drifts petrophysical relationship,Alumbaugh et al.(2002)found that using radar tomographic data.For example,in this case,if the estimates of volumetric water content obtained from tomo-

zero time drift is not accounted for,estimates of soil water graphic radar velocity estimates had a root mean square error content obtained from radar velocity measurements could eas-of0.02to0.03m3m?3compared with neutron log derived

ily be off by1%.Finally,Fig.14c illustrates the estimates of values obtained from corresponding boreholes,and that the soil water content obtained by converting the tomographic errors were greatest in wetter zones.Repeatability measure-

(MOP)travel time data,collected between X2and X3and ments at the same study site suggested that the precision corrected for zero time drift,into estimates of water content associated with the radar acquisition system accounted for

using the Topp https://www.doczj.com/doc/249395081.html,parison of the estimates of water about0.5%of the error in water content estimation.content from the one-dimensional CPT data,the horizontally Time-lapse radar tomograms are often presented as“differ-

averaged ZOP data,and the two-dimensional tomographic ence”tomograms to illustrate the change in geophysical attri-data illustrate the similarity in water content spatial distribu-bute(or corresponding estimated moisture content)with time.

tions revealed by the borehole GPR estimates and the log These tomograms are constructed by subtracting the geophysi-data under conditions of only minor lateral heterogeneity. cal responses collected at one point in time from the responses

Similar results have been found on comparison of water con-collected at the same location but at another point in time,tent estimates obtained from borehole GPR and from neutron or by inverting the differences in the electromagnetic travel

probe data.

time picks.By displaying differences only,subtle changes asso-Crucial to the analysis of borehole GPR measurements is ciated with a dynamic event,such as moisture migration due

the correct identification of the raypath of the wave arriving to forced or natural infiltration,can be highlighted.For exam-first(Hammon et al.,2003;Rucker and Ferre′,2003;Ferre′

et al.,2003).Figure15(left)shows possible raypaths for ZOP ple,if the geophysical signal is influenced by both lithology

and moisture content in the unsaturated zone,by collecting measurements in a soil with a high-velocity(dry)layer sur-data during an infiltration experiment at one point in time

rounded by soil with a lower velocity.Figure15(right)presents and subtracting it from another data set collected at another the corresponding schematic ZOP measurement(right).For point in time,the changes in the geophysical signature associ-

the high-velocity zone,the direct wave labeled1arrives first, ated only with the infiltrating fluid are illuminated,and the and the known borehole separation can be used to calculate effects of the geology are suppressed.This approach has been

the velocity.The reflections from the top and bottom of the used with time-lapse radar tomographic data to delineate per-high velocity zone(marked2and3)arrive later.When the meable pathways and moisture migration(Hubbard et al.,

antennas are in the low-velocity zone,refracted waves(4and 1997;Eppstein and Dougherty,1998).Binley et al.(2002)5),rather than direct waves(6),might arrive first,and the converted time lapse ZOP radar data sets to volumetric water

velocity estimate will be in error when a direct wave path is content using a dielectric mixing model(West et al.,2003)to assumed.This is of particular importance for borehole GPR obtain an estimated change in volumetric water content.Their

measurements near the surface,where the refracted(air)wave change in water content estimates ranged between0.5and(4)will most likely arrive first.For refracted waves partly 2%and agreed favorably with monthly rainfall measurements.

traveling in air,the velocity–depth profile,V(z)can be esti-As described by Peterson(2001),extreme care must be mated from(Hammon et al.,2003)

taken to determine the zero time when using crosshole radar

data for time-lapse monitoring,as zero time drift associated V(z)?1

?1c2?14V?(z)2[21] with the radar system can easily overwhelm velocity changes

caused by the dynamic event that is being monitored.These

changes can be caused by system electronics,such as changing

power transmitters or cycling the power to any of the system where V?(z)is the apparent velocity at depth z that can be components.Loose connections or temperature changes as determined from the slope,d z/d t,in Fig.14(right).Rucker the system components warm up can cause more subtle shifts.and Ferre′(2003)successfully used Eq.[21]to determine soil Peterson(2001)suggested collecting ZOP data prior and sub-water content from refracted air waves.They also predicted sequent to acquiring MOP radar data to assess zero time drifts refraction termination depths below which direct waves are that occur during acquisition and subsequently accounting for expected to arrive before refracted air waves,which can be them in the MOP processing procedure.helpful in analyzing borehole GPR measurements.

A comparison between estimates of water content obtained Borehole GPR has not only been used in the vertical ZOP from simple transformations of data obtained using200-MHz and MOP mode.For example,Parkin et al.(2000)and Gala-borehole GPR and logging tools is given in Fig.14(modified gedara et al.(2002)used horizontal boreholes to monitor the from Majer et al.,2002).These data were collected within a soil water content in a two-dimensional horizontal plane below sandy and unsaturated section of the DOE Hanford Site in a wastewater trench.Other interesting applications of hori-Washington.Figure14a illustrates estimates of soil water con-zontal boreholes are conceivable,such as measurements of tent(SWC)obtained at two well bores(X2and X3)using cone fingered flow and solute breakthrough.Knoll and Clement penetrometer(CPT)capacitance probe measurements(Shinn

(1999)and Buursink et al.(2002)used an acquisition scheme

486VADOSE ZONE J.,VOL.2,NOVEMBER2003

Fig.14.A comparison between estimates of soil water content(SWC)obtained from simple transformations of200-MHz zero offset profiling (ZOP)and multi-offset profiling(MOP)borehole ground penetrating radar(GPR)as well as cone penetrometer(CPT)data at the DOE Hanford Site in Washington(modified from Majer et al.,2002).

called vertical radar profiling(VRP)to estimate water content.schemes is that only one borehole is required.However,the In VRP,one antenna is moved down the borehole while the

use of only one borehole restricts the depth of investigation other antenna remains stationary at the soil surface.As with and reduces the accuracy as compared with the ZOP and

MOP acquisition schemes.

conventional borehole acquisition schemes,the travel time of

the direct arrivals is determined to estimate water content.The An interesting extension of borehole GPR inversion proce-advantage of VRP over the other borehole GPR acquisition

dures is the consideration of radar velocity anisotropy(Vasco

Fig.15.Schematic raypaths(left)and common-offset data(right)for borehole ground penetrating radar(GPR)measurements made in a soil with a high velocity layer.

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487

Fig.16.An elevated 500-MHz ground penetrating radar (GPR)sys-tem being used to measure surface reflection

amplitude.

et al.,1997)and the simultaneous analysis of velocity and Fig.17.Water content profiles measured with air-launched 500-MHz attenuation (e.g.,Ollson et al.,1992;Zhou et al.,2001).Consid-ground penetrating radar (GPR)surface reflectivity method com-ering the impact of simultaneous measurement of bulk per-pared with measurements obtained using 20-cm-long time domain mittivity and bulk soil conductivity with TDR in soil physics reflectometry (TDR)probes.

and hydrology (e.g.,Robinson et al.,2003),it seems worth-while to direct more attention to this type of borehole GPR radar (Davis and Annan,2002;Redman et al.,2002):

application.

Recent publications show that borehole GPR is quickly becoming an important site-specific investigation tool in hy-drogeological studies.An obvious but important difference εsoil

?

?1?

A r A m

1?A r

A m

?

2

[23]

from the other GPR methods presented in this review is that borehole GPR is not suitable as a reconnaissance tool.Despite the strong increase of borehole GPR applications,there are several points that require attention when using this technique:The εsoil obtained from surface reflectivity is a nonlinear average of the permittivity variation with depth.The footprint 1.To obtain quantitative information from borehole radar of the radar can be approximated by the diameter of the first data,it is important to recognize and correct for errors Fresnel zone (FZD):

caused by acquisition procedures and transmission char-acteristics as described by Peterson (2001),and to recog-nize the accuracy limitations as described by Alumbaugh FZD ??

?2

4

?2h ??

0.5

[24]

et al.(2002).

2.It is important to consider the impact of refracted waves,where ?is the wavelength (c /f for air)at the center frequency especially those traveling in the soil (labeled 6in Fig.of the GPR antenna and h is the height of the antenna above 14),on the accuracy of soil water content measurements the surface.Although the definition of the FZD applies to a with borehole GPR.

monochromatic source,it has been shown that this is also a 3.The length of the borehole GPR antenna,the borehole good approximation for surface reflectivity measurements per-separation distance,and the antenna frequencies have formed with GPR,which uses a broadband source (Redman an impact on the maximum spatial resolution that can et al.,2002).The FZD criterion suggests a calibration foot-be achieved;

print of 0.79by 0.79m for 1-GHz antennas operated 1.0m 4.Soil heterogeneity affects the sampling volume of bore-above the soil surface and a footprint of 1.76by 1.76m for hole GPR.

225-MHz antennas operated at the same height.Clearly,sur-face reflection measurements are more practical with high-Soil Water Content Measurements with

frequency GPR antennas.

Surface Reflections

Figure 17shows a sample of data acquired with the 500-MHz GPR surface reflection measurement set up shown in The measurement principle of soil water content measure-Fig.16.Generally,the soil water content measured with GPR ments with air-launched surface reflections is illustrated in is similar to the soil water content measured with 0.20-Fig.16.The GPR antennas are operated at some distance m-long TDR sensors.However,there seems to be quite a above the ground by mounting them on a vehicle or a low-large amount of very short distance variation in soil water flying air platform.The soil property being measured is the content,which was also reported by Redman et al.(2002).reflection coefficient of the air–soil interface,R ,which is re-Three likely explanations for the observed large variation of lated to the soil permittivity,εsoil ,by

?0.10m 3m ?3in soil water content include (i)the impact of the soil water content profile with depth on the reflection R ?

1?√εsoil 1?√εsoil

[22]

coefficient,(ii)the impact of surface roughness on the reflec-tion coefficient,and (iii)reliability and accuracy of ampli-tude measurements.

The reflection coefficient is determined from the measured amplitude,A r ,relative to the amplitude of a “perfect”reflec-The determination of soil water content from the reflection coefficient requires accurate amplitude measurements.Figure

tor,A m ,such as a metal plate larger than the footprint of the

488VADOSE ZONE J.,VOL.2,NOVEMBER2003

https://www.doczj.com/doc/249395081.html,),Mala?Geosciences(Mala?,Sweden;

www.malags.se),and Sensors and Software(Mississauga,ON,

Canada;www.sensoft.ca).These manufacturers provide multi-

purpose GPR systems that can typically be operated at differ-

ent frequency ranges from10to1400MHz.The GPR market

as a whole is moving toward easy-to-use GPR systems dedi-

cated to specific tasks.Besides the aforementioned manufac-

turers of GPR equipment,there are other manufacturers,who

mostly provide purpose-built systems operating at around

1000MHz(Davis and Annan,2002).

Important aspects to note when choosing a GPR system

for soil water content measurements include the availability

of the required range of antenna frequencies,the possibility

to separate antennas for multi-offset measurements,and the

availability of borehole antennas.It is also useful to have

shielded antennas for higher quality measurements in the pres-

ence of cultural noise.Some GPR systems allow multichannel Fig.18.Reflection coefficient as a function of soil water content.Soil

acquisition,which can considerably speed up the acquisition water content was calculated from soil permittivity with Topp’s

of multi-offset data.Although this is helpful for soil water equation(Eq.[6]).

content measurements based on reflected wave and ground

wave data,it remains to been seen whether the extra costs of 18shows the relation between the reflection coefficient associ-

multiple antennas is worth the reduced acquisition effort for ated with the air–soil interface and soil water content,assum-

hydrological applications.

ing Topp’s equation for the relation between soil permittivity

In many studies,accurate positioning of radar measure-and soil water content.The relationship reveals that the reflec-

ments is important.Traditional surveying with marked posi-tion coefficients are more sensitive to variations in SWC at lower

tions in the field is very time-consuming.Most GPR systems water content ranges than at higher water content ranges.

allow interfacing with a GPS,which can provide the freedom Simple calculations show that an error of only1%in measured

of movement not available in traditional surveying.However, amplitudes can result in a significant error of?0.02m3m?3

for some applications the accuracy of regular GPS systems might for a soil water content of0.30m3m?3.Redman et al.(2002)

not be sufficient.Higher positional accuracy can be obtained reported that measured repeat profiles have typical differences

by more advanced differential GPS systems or by a self-of?0.02m3m?3,which suggests that the required accurate am-

tracking theodolite(e.g.,Lehmann and Green,1999). plitude measurements can be achieved with the current gen-

eration of GPR instrumentation and that the amplitude accu-

racy was not a major contributor to the observed variability CONCLUSION

in Fig.17.

Surface roughness and varying soil water content profiles In this review,we have presented an overview of the with depth will both cause significant scattering,which leads available methods for measuring soil water content with to a decrease in reflection coefficient and an underestimation GPR.Although the GPR technology has been devel-of soil water content if it is not accounted for.Chanzy et al.oped in the same period as the successful TDR method-(1996)used the Rayleigh criterion(?1/8of the wavelength)ology,there is still a large difference in acceptance to determine the upper limit of surface roughness,below which

between these methods for soil water content deter-the surface can be considered smooth.For example,for the

mination.This can partly be attributed to the limited 1-GHz antennas the surface roughness must be below0.038

suitability of the first generation GPR instrumentation m for an adequate approximation of a smooth surface,and

for accurate soil water content measurements.However, for the225-MHz a surface roughness below0.167m is suffi-

the current generation of GPR instrumentation pro-cient.The Rayleigh criterion implies that lower antenna fre-

quencies will be less sensitive to surface roughness and will,vides much more accurate measurements,which has therefore,show less soil water content variation,which was resulted in a revival of the use of GPR for water content confirmed by Redman et al.(2002).The effective measure-determination as evidenced by the increasing number ment depth and the impact of soil water content variations of publications in recent years.

with depth are topics of active research,but little is known at The extensive research in the past years on all of the the moment.

available methods has clearly demonstrated that GPR Clearly,the impact of surface roughness and soil water con-

can be used by experienced researchers to provide reli-tent profiles on the surface reflection coefficient are two key

able estimates of subsurface water content.The excep-issues that need to be addressed when applying this technique.

tion to this is the GPR surface reflectivity method that, It is interesting to note that these problems are similar to those

although promising,does not yet provide the accuracy in active remote sensing(e.g.,Jackson et al.,1996),and it is

possible that this soil water content measurement technique and robustness of the other methods.In our opinion,the can profit from developments in the highly sponsored field of complexity of data acquisition and processing currently remote sensing.limits the applicability of GPR for water content estima-

tion to the research community.However,continued GROUND PENETRATING development of specific and easy-to-use GPR instru-RADAR INSTRUMENTATION mentation,coupled with increased experience and appli-

cation of GPR for water content estimation,should help There are a number of commercial GPR systems available

to bridge the gap between the advances made in the on the market.The three largest manufacturers are Geophysi-

cal Survey Systems Incorporated(GSSI,North Salem,NH;research community and the practical need for straight-

https://www.doczj.com/doc/249395081.html,489

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important research tool for studying infiltration pro-tional soil moisture analysis from atmospheric observations.J.Hy-

cesses and contaminant transport in the saturated and

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Geophysics57:902–915.

tions outside the research community for monitoring

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clay content soils should help resolve this issue.The

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the ground wave of ground penetrating radar.Ph.D.thesis.Ludwig-second issue is the sampling volume and spatial resolu-

Maximilians-Universita¨t,Munich,Germany.

tion of all of the GPR methods,particularly under het-

Endres,A.L.,W.P.Clement,and D.L.Rudolph.2000.Ground pene-erogeneous(field)conditions.Here,both numerical trating radar imaging of an aquifer during a pumping test.Ground

modeling and field studies will be required to determine Water38:566–576.

these values.

Entekhabi,D.,G.R.Asrar,A.K.Betts,K.J.Beven,R.L.Bras,C.J.

Duffy,T.Dunne,R.D.Koster,D.P.Lettenmaier,D.B.McLaughlin,

W.J.Shuttleworth,M.Th.van Genuchten,M.Y.Wei,and E.F.

ACKNOWLEDGMENTS

Wood.1999.An agenda for land surface hydrology research and This study was financially supported by NWO-ALW grant

a call for the second international hydrological decade.Bull.Am.

Meteorol.Soc.80:2043–2058.

number750-19-804(J.A.Huisman)and by USDA grant num-

Eppstein,M.J.,and D.E.Dougherty.1998.Efficient three-dimensional ber2001-35102-09866(Y.Rubin).T.P.A.Ferre′is thanked for

data inversion:Soil characterization and moisture monitoring from initiating this review.Three anonymous reviewers are thanked

cross-well ground-penetrating radar at a Vermont test site.Wat. for their constructive comments.

Resour.Res.34:1889–1900.

Famiglietti,J.S.,J.A.Deveraux, https://www.doczj.com/doc/249395081.html,ymon,T.Tsegaye,P.R.

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安全生产演讲稿(完整版)

安全生产演讲稿 安全生产演讲稿 第一篇： 安全生产演讲稿 “百日安全无事故”——安全与梦想齐飞 我想先和大家提一个我们从小到大都会谈论的话题。－“梦想”。我想问大家梦想对我们而言意味着什么？是一辆跑车？一座豪宅？还是花不完的钱，还是至高的荣誉？有人说年轻就是无限，有梦想就有动力。年轻的我们都不曾忘记那份豪情壮志。我们努力奋斗，为的就是实现心中的梦想。 可是，你们有没有想过，在我们怀抱着梦想努力工作的时候，也许在某一天，我们会因为自己的一次麻痹失误，一次不起眼的违章作业而让自己的梦想变成幻想呢？ 在我们的工作生活中，我们最基本的需求就是安全，“安全第一，预防为主”这是我们应牢记在心的，企业是时时讲、周周学、月月喊。作为萍钢这样的大型钢铁企业，为了确保安全，工厂里所有的设备都专门编制了操作规程，针对特殊工种还特配了相关岗位说明书。除此之外，公司还在明显的位置张贴、悬挂各种类型的安全标语和安全标致，为的就是要提高我们对安全的意识。生产部还定期组织对公司进行安全隐患排查，及时发现并消除隐患，这些都是为了保证我们能够在一个确保安全的情况下生产。 在公司，老员工的安全意识向来都比较强烈，因为他们都见证了安全意识淡薄、不遵守规章制度所造成的后果。发生事故的往往都是

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安全源于长期警惕，事故出于瞬间麻痹，思想上的松懈麻痹必然会导 致安全隐患的存在和违章现象的发生，哪怕一次偶然现象的发生，哪怕一 次偶然违章，就有可能一失足成千古恨。 没有了安全就丧失了和和美美的家庭，更没有企业的可持续发展，依 法遵章，所有的事故都可以避免，松懈麻痹，所有的事故都可能发生，遵 章二字，正是水之源，树之根，安全之本。 一花一世界， 一叶一菩提， 每一个人的生命都是天地之魂， 万物之灵， 生命因此而珍贵，我们要热爱生命，热爱生活，珍惜眼前的每一天；让我 们永远牢记安全责任， 警钟长鸣， 才会让安全之花扎根于我们的生活之中。 篇二《六月，让我们共同走进安全月》 刚刚送走激情、温馨的五月，六月的祝福已亭亭玉立在我们面前。 13 亿双期盼的眼光一起向"安全月"汇聚，在这个阳光铺满的季节里， 铿锵走来了"安全月"的脚步。 安全是什么？安全就是生命！安全意味着什么？对于一个人，安全意 味着健康；对于一个家庭，安全意味着和睦；对于一个企业，安全意味着 发展；对于一个国家，安全意味着强大。 没有安全，就没有一切。 安全是什么？安全就是一首歌，需要我们天天来弹唱；安全就是一首 诗，需要我们日日来吟诵；安全就是一盘棋，需要我们走一步看两步；安 全就是标准化，要求建设者严格规范的操作；安全就是有序的节奏，推进 科学化管理上水平；安全就是沸腾的生活，预示我们生机勃勃的明天；安 全就是一根七彩的丝带，连接一个又一个美好的愿望。

放飞梦想演讲稿10篇

放飞梦想演讲稿10篇 【篇一】 我喜欢飞翔的感觉，像空气一样轻盈，像雄鹰一样自由。而几乎，每一个向往飞翔的人，都拥有一个生着一双美丽翅膀的梦想，因为，只有让梦想插上了翅膀，让它更高更高的飞翔，人生才会发出璀璨的光芒，才能成就不休和辉煌。 春秋季世，诸侯纷争，战火连天，太阳也失了光芒，大地上到处是悲哭和苍凉，血污的现实让好多人，或是不问世事终老山林，或是得过且过苟延残喘，而，孔子，正是在这样一个人性压抑的时代，依然将梦想放飞，他的梦想自由而骄傲——那就是推行礼制，宣扬仁政，还世界一个完美而合理的秩序。尽管遭遇了无数的磨难，权贵的排挤，暴力的围攻，路途的坎坷，但这些都没有使孔子屈服和放下，他的梦想，飞翔前方，高扬复高扬，让他心中充满期望，让他“知其不可而为之”，最终成就了一个“万世之师”的人生。清朝末年，又是一个痛苦和绝望的年代，帝国列强在中华大地上肆虐横行，天地变色，草木含悲，满目疮痍，屈辱和彷徨充斥着人心，而，孙中山，正是在中华民族这样生死存亡的关头，依然将梦想放飞，他梦想着驱除鞑虏恢复中华，他梦想着与所有拥有相同梦想的仁人志士一齐，救亡图存，振兴中华。正是在这样伟大梦想的鼓舞下，他身陷囹圄而不悔，黄花岗血洒而不退，直到听到辛亥革命那划破长夜的枪声，直到看到民族完全独立，东方睡狮发出震撼世界的长长怒吼。梦想是期望，是拂向寒冷大地的一缕春风，是穿过漫漫黑夜的一线光明，是荒蛮大原上的一点火种，是浩瀚沙漠中的一块绿州。有梦就有期望，梦想有多远，路就有多远。 相反，没有梦想的民族，不会长久;没有梦想的国家，不会强大;同样，没有梦想的人生，不可想象。 如果没有一向飞扬的梦想，摩西的子孙不会在流浪世界近千年，历尽世世苦难之后，终回到耶路撒冷的热土建立起富强的以色列;如果没有一向飞扬的梦想，一穷二白的炎黄后代，不会在伤痕累累疮痍处处的中华大地，独立自主艰苦奋斗，励精图治勇于改革，让这个古老的民族重焕生机，以大国的形象屹立在世界的东方。如果没有飞扬的梦想，毛泽东不会在他人生最艰难的关头依然坚守信仰永不言弃，成为中国人民的伟大领袖。 将梦想放飞吧，让梦想拥有一双美丽而矫健的翅膀，让它在人生的天空飞得更高，飞得更高。放飞的梦想之于人生，就像那无边无际的深夜里一盏刺破黑暗的明灯，它能引导出一个光明的世界。放飞的梦想之于人生，就像那广袤的大地上永不干涸的河流，它能慢慢向东汇聚，就会构成蔚蓝的海洋。 放飞的梦想能照亮我们前进的人生。冰雪覆盖的时候，它能帮我们创造出一团熊熊燃烧的烈火，温暖我们的疲惫身体;暗夜无边的时候，它能帮我们更好的领悟星光的好处，点化我们人生的迷茫;前途困顿的时候，它能帮我们开启尘封的心智之光，寻到到真正属于自己的人生航标。 放飞的梦想能温暖我们的生命。梦想是我们生命的温床和灵魂的栖息地。梦想不在，生命和灵魂便永远不能安寝。梦想让每一个如风一样轻的生命，增加了存在尊严和重量。梦想让我们的生命，在俗世中独焕斑斓，甚至让我们美丽的人性，得如幽兰一般升华。我们的生命所负载的心胸，因为梦想而变得宽如大海，阔如天空;我们的视野，因为梦想而“一览众山小”着眼处尽是天蓝云白水碧花红芳草青青。 将梦想放飞吧，让梦想挥动长长的翅膀，在人生美丽的天空自由而骄傲地飞翔，且得更高，更高…… 【篇二】 尊敬的老师、亲爱的同学： 大家好！我演讲的题目是《放飞梦想》。

广州新版八年级英语下册unit1

Unit 1 Helping those in need 一、要点概括 1.短语 in need, voluntary work, ask peimission, suffer from, raise one’s spirits, in order to, continue to do, pay for, work as 2.句型 (1)There are many children without parents. (2)They have difficulty walking or moving. (3)I am thingking about doing some voluntary work. (4)I am going to do some voluntary work. (5)Since then, this project has helped millions of girls return to school. (6)I wish to help other girls the way the Spring Bud Project helped me. 二、疑难宝典 1. courage 勇气，勇敢 amazing courage 惊人的胆量cool courage 镇定勇气 He hurt his leg in an accident, but he has lots of courage. They do not have the courage to apologise for their actions. 2. illness （某种）病 bear illness 忍受疼痛cure illness 治愈疾病combat illness 与疾病斗争 He died at the age of 83 after a long illness. 3. in order to 目的在于，为了 In order to raise money for children in need, we are going to sell old books. She worked all summer in order to save enough money for a holiday. 4. lonely adj. 孤独的，寂寞的 She is unhappy and very lonely. 【区别】lonely＆alone alone单独的（意为没有其他人陪伴）；lonely孤独的（因为没有他人的陪伴而难受） 5. offer 主动提出[ + to do sth. ]表示愿意（做......） Three teenagers offered to do some voluntary work during the school holidays. He offered to get me a cab. 6. peace 宁静，平静（n.）peaceful adj. 宁静的 I taught them to sing because music can bring them joy and peace. After a busy day, all I want is peace and quiet. 7. raise 筹募，增加，提高 We need to help children like Tim and raise their spirits. They are raising money for charity. Please raise standards. 【区别】raise＆rise raise意思是将某物抬举或举到较高的位置，或者增加数量、提高水平，后面必须跟宾语。 rise意思是增加或上升，后面不接宾语。 8. There be句型表示存在，意为“某地有某物”。Be动词应与紧跟其后的主语的单复数形式 保持一致。主语是不可数名词或单数可数名词时用is或was，主语是复数可数名词时用are 或were。 There is some bread on the plate. There are seven days in a week.

广州新版七年级(初一)英语上册单词表

新版广州七年级(初一)英语上册单词表-２０1４年1月13日更新Modｕｌe 1My LifｅUnｉt1＆Ｕnit２ Unit１Making Frｉenｄs Ｇｅrman?德国的 Blog???博客 Grａｍmar??语法 Ｓound??听起来、声音 Cｏmｐｌete 完成 Hｏbby 爱好 Countrｙ国家、向下 Ａgｅ???年龄 Dream??梦想，做梦 Eｖeｒyｏnｅ每个人 Germａｎy 德国 Ｍountain 山脉 Ｅlder ?年龄较大的,年长的 Fｒieｎdly?友好的 Eｎgｉneer 工程师 Wｏrｌd?世界 Japaｎ?日本 Ｆlat??平房，平坦的 Yourself 你自己 US 美国 cｌose to??靠近 gｏｔo schoｏｌ??去上学 be gｏod aｔ擅长 makｅ friendｓ with?和。。。交朋友 ａｌl ｏvｅｒ??遍及 ’d like to =would likｅ to 想要 Unｉt2Dａily Lifｅ Daily??每天的，日常的 Artｉclｅ文章 Ｎever?从不 tａble teｎnｉs?乒乓球 Riｄｅ?骑 Usually?通常 So??所以 Seldｏｍ很少 Ｇｅographｙ地理 Ｂrｅaｋ??打破 Ｂeｌl??铃 Ring??铃响

Eｎd???结束 Band ?乐队 Praｃtｉce??练习 Togetｈｅr 一起 Maｒket?市场 Guitaｒ?吉它 Graｄｅ??年级 junior hiｇh school?初中 ｏn fooｔ??步行 takeｐart in??参加 haｖe a good time 玩得开心 go to bed??上床睡觉 get up???起床 Moduｌe 2 Ｔhe Natural WorｌｄUnit３& Unit4Ｕnit３Tｈe Eａｒｔh Eaｒｔｈ???地球 Quiz????测试 Ｐattｅｒn??模式 Proteｃt ?保护 Repｏrt??报告 Paｒt?部分 Land???陆地 Fieｌｄ?田野、田地 Ｌargｅ??大的 Ｐroviｄe 提供 Pｏllｕtｉoｎ污染 Bｕｒn???燃烧 Ｅnｅrgy ?能量 Ｐolｌｕｔe??污染 Ｉnto ??进入 Grｏｕnd ?地面 Ｋiｌl ?杀死 Muｓｔ?必须 Ｉmｐoｒtａnt ??重要的 Fact???事实 Ｋilｏmｅｔer??千米 Oｗn?自己的 Caｔcｈ???抓住 Ｆew????很少 Aｗay ?离开 Prｏblem 问题 ｐroviｄe wｉth ?提供。。。

安全生产演讲稿

安全生产、党员争先 煤矿生产，安全为天。当单位或部门新分来学员和新工人时，都要进行有关安全政策、法令、规章制度的学习、培训。上岗工作时，安监部门、各级领导又是人人讲安全，天天进行安全戴帽说安全。可是，每次事故的发生都事出有因，究其原因何在？答案是：绝大多违章人员都是没有管好自己。由于参加工作时间长了，工作环境熟悉了，讲安全听腻了，思想上逐渐产生了麻痹大意思想，尽管安全规章天天讲，也当成了耳旁风，心里有一搭无一搭。事故发生后，震动一阵子，过后又随着时间的流逝而淡化。这种“管不住自己”的毛病，无疑是酿成事故的病症所在。 要搞好安全，真正做到安全为了生产、生产必须安全，除了严格按照各项安全操作规程规定作业外，最重要的一点就是要“自己管住自己”。各科室、区队里有科长、队长、班长、青年安全监督岗员和群监员，但都不能时时处处跟着你、看着你、附在你身上，钻进你心里。你工作走神，思想溜号，有意违章，违章操作，事故往往就在这种情况下找上门来。有位在煤矿工作多年，从未发生过任何事故的老工人说过这样一句话：“我的安全经验就是自己管自己”，这句话言简意赅，值得品味再三。在工作时如果三心二意，就有可能发生事故。根据有关部门统计资料表明：百分之九十以上的事故，都是由于思想麻痹和“三违”造成的。 俗话说得好：“世界上什么药都可以买到，就是‘后悔药’买不到”。出了事故，单位受损失不说，自己轻则受皮肉之苦，重则缺胳膊少腿，甚至连生命都没有了，你的父母、妻子、儿女还要为你承担痛苦与不幸。生活是美好的，家庭是温暖的，何必为一时之错、一念之差，付出如此沉重的代价？

作为一名党员干部，我们应当牢固树立“本质安全，珍爱生命”的理念，去营造遵章守法，治理“三违”的安全文化氛围，把安全生产的制度，变为我们自己的自觉行动。同时还要加强学习、注重提高自身的安全技术素质和安全生产能力、切实在安全生产工作中发挥出党员的先进性和模范带头作用。 党旗在飘扬，党旗在召唤。让我们在全矿蓬勃开展的“创先争优”活动中，时刻牢记这样一个道理：党员就是责任，党员就是旗帜，党员就是榜样，党员就是先锋；让我们忠诚做好人民群众的领头雁，真情当好安全发展的排头兵，为鲜红的党旗增辉添彩，为建设庇山和谐矿区、本质安全矿井、五优矿井做出积极的贡献。

放飞梦想演讲稿500字5篇

放飞梦想演讲稿500字5篇 当我听了这个故事后吗，我有了非常深的感受。每个人都有自己的梦想，我的梦想就是当一名科学家，每天我都在搞科研。但是直 到有一天，我脑海了闪过一个念头，现在我还在小学阶段，以后还 要走更长的路，此时我应该好好读书。之后，我就换了一个梦想， 努力考上名牌大学，长大后好好孝顺父母，报答父母多年来的养育 之恩。我就像杨孟衡一样，虽然第一个梦想不能实现，但还能有第 二个梦想，第三、第四甚至更多。因为人的一生可以不止一个梦想。失败并不可怕，可怕的是没有梦想。有了梦想就是有了希望，有了 光芒! 梦想并不是一个梦。有志者，事竟成。只要你有努力过、坚持过，就一定会成为现实!放飞我们的梦想吧，一起加油!我的梦，中国梦! 每个人都不愿在牢笼中平庸的度过一生，而是要插上自由的翅膀飞翔。 孩提时，我们童稚的认为翅膀是一种毛绒绒的东西。能带我们飞上蓝蓝的天空，悠闲的坐在白白的云朵上。 长大后，我们固执的认为翅膀是一种载满希望的东西。能使我们乘着翅膀越过高山，跨过河流，飞向有梦的未来。 成年后，我们诚恳的认为翅膀是一种具有超能量的东西。能让我们驾驭着翅膀战胜更多的竞争对手，直至的飞上顶峰。 翅膀总是神奇的，无论是在儿时的眼中抑或是成年后的心中。 现在，我发现我有一双翅膀。一双隐形的翅膀。它指引我飞向明天，飞向未来。尽管途中会遇到很多困难，但一种毅力激励着我前进。很多人没有坚持，便在这一座山峰前坠落了，而我有我超越的 翅膀，助我度过绝望，飞越山峰。 其实每个人身上都有一双隐形的翅膀，取决于你是否发现它，运用它。

张开翅膀，放飞梦想，带你自由飞翔，到达一个任心灵翱翔的世界…… 会当凌绝顶，一览众山小。——杜甫 “我的未来不是梦，我认真的过着每一分钟”，每当我听到《我的未来不是梦》这首歌曲，我的理想不由得放飞了起来。当一名人 民教师，是我最大的理想。 高中 我现在是为初中二年级学生，再过一年多就要考高中。我的目标是灵宝一高。 从现在开始，我要分秒必争，积极乐观的完成初中教学任务，还要努力培练自己的独立自主能力，我坚信“阳光总在风雨后”经过 我长期的奋斗，我一定会成功。 大学 我的第二梦想是考上清华大学，清华大学人才济济，我在他它里可以学到很多的知识。大学能成就我的人生，在大学里，我可以实 现我的梦想，放飞我的梦想。 我时刻准备着，时刻努力着，时刻拼搏着。 教师 “教师是人类灵魂的工程师”，从小就树立了远大的梦想，当一名人民教师。为千千万万的学子传授知识。 在儿时，爷爷经常给我讲一些带有一定哲理的故事，虽然我那时听不懂故事中的哲理，但我完全可以把爷爷给我讲的故事叙述下来，爷爷听后两眼瞪圆，十分惊讶，就抚摸着我的头悄悄对我说：“不 愧为我的好孙儿，你真是当教师得料，你真是太棒了。”我的心不 由扑通扑通地跳了起来，不由的喊了起来：我是人民教师。

unit2广州版八年级下册(最新整理)

2018 年广州版英语八年级下册 Unit 2 期末复习资料 Unit 2 Body language 跟踪练习 用所给单词的适当形式填空 1. Y ou had better practise (exercise) every day. 2. J im does his homework as (care) as his brother. He never makes mistakes. 3. W hat about 4. T hey answered the questions (have) a picnic? (different). 5. T hey learn English by (sing) English songs. 单项选择 1. You don’t give people a good , so people choose Debbie instead of you. A. impression B. instruction C. impressive D. instruct 2. Disney is famous for its cartoon characters Mickey Mouse, Donald Duck, Goofy and Snow White. A. for example B. such as C. as D. likes 3. The film me of my father. I miss him very much. A. promises B. reminds C. makes D. returns 4. Yesterday I an invitation from Tom but I didn’t it. A. received; receive B. accepted; accept C. accepted; receive D. received; accept 5. I’m with the homework, Mom. A. bore B. bored C. boring D. to bore 6. --- In this example, is not important. --- Yes. We shouldn’t judge a person his or her look. A. appearance; by B. appearance; for C. expresion; by D. expresion; for 7 W hat time will you arrive China? ---- I don’t know. Maybe five hours . A. at; later B. in; later C. at; late D. in; late 8 W hat’s the with you? --- I didn’t know the of this sentence. A. wrong; communication B. matter; communication C. wrong; meaning D. matter; meaning III ．Grammar （动名词的用法） 基本用法： 1.作主语 (谓语用三单) Swimming in the sea is her favourite sport. 2.作宾语（动词及介词的宾语） I practise speaking English every day. Debbie is good at communicating with people. 3.作表语（表示主语是什么，可主表互换） My biggest hobby is collecting stamps. 4.作定语（说明所修饰事物的用途） There is a swimming pool nearby.