Technical NoteGT-2M TechnologyOlson D. (CMG), Jurist, S., Smoller J. (Gravimetric Technology)This paper has been prepared to bring together the answers to questions about the GT-2M marine gravimeter. It provides some technical material which is not available on the GT-2M Specification Sheet, and it also provides some comparison material perhaps not available elsewhere. The GT-2M is compared with other marine gravimeters and with the very successful GT-1A and GT-2A airborne gravimeters.IntroductionThe GT-2M is a new marine gravimeter which has evolved from the highlysuccessful GT-1A and GT-2A airborne gravimeters. As such, the GT-2M hasinherited the wide dynamic range, the high quality platform, and theadvanced control electronics which allow GT gravimeters to measuregravity successfully in high turbulence in the air, or during very high seastates. The GT-2M can successfully collect high quality data in difficulttransition zones using a very small, shallow-draft survey vessel.Some customers in the market for marine gravimetershave focussed on specifications such as gyro life,operating temperature of the sensor, and platformstabilization. However, the most importantperformance specifications are: dynamic range, spatialresolution, noise level, and production rate, and yetthese specifications are sometimes overlooked whencomparing gravimeters from various manufacturers.GT gravimeters outperform all other commerciallyavailable mobile gravimeters by a wide margin in eachof these very important parameters.GT-2M CommercializationThe GT-2M is the latest marine gravimeter to enter the market after two years of sea trials and several commercial surveys. The first commercial GT-2M was purchased in early 2011 by Geoken, a geophysical contractor based in Kazakhstan.CMG also owns one GT-2M which was developed through a conversion of a GT-1A airborne gravimeter. This instrument can be used for either airborne or marine surveys.GT-2M AdvantagesThe following are some key advantages of the GT-2M marine gravimeter:1.The GT-2M has been developed from an airborne instrument called the GT-1A, whichmeans it has inherited an advanced Schuler-tuned 3-axis platform. The GT-1A and the newer GT-2A have operated in a large number of helicopters and fixed-wing aircraft (Appendix 1) and have collected in excess of 2 million km of high quality airborne gravity data.2.The GT-2M has a vertically-oriented sensor, rather than an angled zero-lengthspring. This minimizes coupling of horizontal accelerations into the vertical channel.3.The GT-2M sensor is an axial system with a reference mass on a spring suspension, aphotoelectric position pickup, and a moving-coil force feedback transducer. This design minimizes the effect of cross-coupling, which is an undesirable effect which contaminates gravity measurements with components of horizontal accelerations induced by aircraft or ship motion. Zero-length spring gravimeters have serious problems with cross-coupling due to their sensor design, which is why the GT-2M outperforms other gravimeters in turbulent conditions, both on the sea and in the air.4.The GT-2M has a very wide dynamic range of ±1 g, which means it can generate highquality data in high sea states. Almost all other marine gravimeters have a dynamic range of around ±0.1 g, or one-tenth that of the GT-2M. This limits their performance in anything other than calm seas.5.The GT-2M has superior data quality and ease of data handling and processing whichmeans less operator training and faster turn-around.6.The GT-2M can produce high quality data in much worse survey conditions and at amuch faster rate than any other marine gravimeter, thus increasing production rates and lowering acquisition costs significantly.7.The GT-2M can collect high quality gravity data in the very shallow waters of transitionzones using small, shallow-draft survey vessels.For these reasons and more, the GT-2M is a far superior instrument to other gravimeters on the market. Clients collecting marine gravity data want high production rates of high quality data, and only the GT-2M can achieve this in all sea conditions in both shallow and deep water.GT-2M Case historiesVietnam 2008The first GT-2M commercial survey outside of Russia was carried out in 2008 off-shore Vietnam.A portion of this 5,000 km survey can be seen in Figure 1 on the GT-2M brochure, which is reproduced below.Fig 1.Free-air marine gravity grid: grid size 90 km x60 km; line spacing 5 km x 15 km; filter 350 sec;spatial resolution 480m; RMS error 0.3 mGal.Show rights courtesy PGS.Kazakhstan multi-meter deep water trials 2010Recent sea trials have been carried out in the Caspian Sea(2010/2011) by Geoken of Kazakhstan using three marinegravimeters (photo at right). The report ‘2011 GT Marine testsurveys in Kazakhstan with GT-2M, L and R, and Chekan.pdf’describes the technology used and the results of thesesurveys. The important point to note is in the final paragraphswhich describe Figure 6 (below left) and survey Line298. During 20 km of stormy sea conditions, the threegravimeters on board the survey vessel showed differentresults along this line.Geoken re-surveyed Line 298 in the spring of 2011 and foundthat the L&R and the Chekan meters generated falseanomalies during the stormy stretch, while the GT-2M gavethe same results as the first pass in stormy conditions.At left is Fig. 6 reproduced from the Geoken article. It showsresults from the three marine gravimeters in L298, with the GT-2M trace marked in blue. This was the only gravimeter whichrepeated the same results during calm sea conditions. Thestormy section of this line can be seen in the middle of thevertical acceleration plot at the bottom.Kazakhstan shallow water trials 2011Geoken completed 500 km of shallow water gravity surveys in the fiords of the Caspian Sea during the first half of 2011. They used a GT-2M, which they purchased in early 2011 after the results of the sea trials mentioned above, mounted on board an 8m long, shallow-draft survey vessel, while they also measured some control stations with a Scintrex CG-5 ground gravimeter on a long tripod for comparison.The results from the GT-2M were excellent at 0.2 mGal despitethe severe turbulence during which the vessel rolled andpitched up to 30 degrees. The vessel also made many headingchanges of up to 90 degrees and greater while navigatingthrough the waves along the survey lines. The GT-2M canaccomplish this because of the instrument’s airborne origins –it has set a new standard for marine gravity surveys, achieving0.2 to 0.3 mGal on intersections in high sea states. Geokenestimated it would have taken at least 30 days to survey all 500km using the CG-5, while the GT-2M achieved this in just 2 days.It should be noted that the GT-2M gravimeter achieved thesame ± 0.2 mGal accuracy as the CG-5.The following are excerpts from Geoken’s brief report on these shallow water surveys which is available on their website :“In August 2011 SPC «Geoken» has successfully completed, started in 2010, marine Gravity and Magnetic Survey at Kazakhstani Sector of the Caspian Sea. Total survey area was 115,000 sq. m and the scope of executed gravity-magnetic measurements were more than 40,000 linear km.Gravity-magnetic survey was carried out using well-known marine ship-borne gravimeters, such as Lacoste & Romberg ZLS, Chekan AM, and most recent marine gravimeter, GT-2M. At northern part of the Caspian Sea the gravity-magnetic survey was carried out using gravimeter Lacoste & Romberg ZLS.During the survey execution at relatively deep-water middle part of the Caspian Sea the gravity-magnetic survey was started, as an experiment, simultaneously using three types of gravimeters that were installed onboard of the same research vessel.On the shallow area, in the reedy transit zone of the North Part of the Caspian Sea, where water depth was from 0 to 2-2.5 m, gravity-magnetic measurements were also executed utilizing gravimeter GT-2M installed on the small-displacement vessel with 0.4 m draft.Measurements were carried out along raceway and channels at the sites of open shallow area. Tracks of the tacks had complex curvilinear form. Turn angles were up to 90 andmore degrees. Repeated measurements using ship-borne gravimeter GT-2M were run on the same tacks as control observations that were conducted by tripod survey method utilizing gravimeter AutoGrav CG-5. The results of these control observations has confirmed high quality of the executed measurements using marine gravimeter GT-2M in transit zone conditions. In the sequel, the gravimetric survey of the transit zones was carried out in uninterrupted logging mode, utilizing marine gravimeter GT-2M. In the areas with difficult access for vessels, due to shoal and riotous vegetation, the gravity-magnetic observations were conducted by gravimeters AutoGrav CG-5 using tripods”.Galway Bay, Ireland 2010CMG carried out GT-2M trials off the west coast of Ireland during 2010. A report of these trials is available separately. The GT-2M was installed on a large research vessel (length 85 m) for part of the survey, and on a smaller vessel (length 12 m) for more passes of the repeat lines and the completion of a 5 km by 5 km grid with 500 m line spacing. The grid was completed with the smaller vessel in fairly rough sea conditions.The map below left shows the 20 survey lines in the grid, 10 of them oriented E-W and 10 oriented N-S. The intersection RMS was 0.06 mGal after first-order levelling. The final free-air gravity map of the 25 square km area is shown on the right.GT-2M Various technical points1.GT-2M data is output in RS-232 format. However, this can be converted for a small costto any of the other standard serial outputs such as RS422, etc.2.Power required is 90 to 240 volts AC, 50 or 60 Hz, or 28 Vdc at 150 W.3.The UPS batteries have a life of 15 to 20 minutes. External batteries can be added tothe UPS to provide additional back-up for many hours.4.The GT-2M platform stabilization time is 5 minutes.5.The GT-2M generates real-time values of corrected relative free-air gravity. The onlyadditional correction which needs to be applied is long-term drift which in any case is normally removed without any special attention during the grid levelling process.6.The GT-2M has multiple simultaneous filters which can be programmed. For example,they can be set to values of 300, 450, 600, and 900 seconds. The four channels of filtered real-time free-air gravity values are each recorded in the GT-2M data files.7.The GT-2M has a variable data recording rate which can be set between 0.1 Hz and 2 Hz.GT-2M Measurement rangeBetween the equator and poles the Earth’s field has a maximum change of < 5,000 mGals; between poles the maximum change is < 10,000 mGals.When Gravimetric Technologies calibrate the GT-2M gravimeter, they measure the scale coefficient (SC). To measure the SC, they tilt the sensing element ±6 degrees which corresponds to 10,000 mGal. Using 6 degrees of tilt, they estimate the SC to 10-4 mGal.GT can make a custom measurement range by using ±7, ±8, ±9 or even ±10 degrees during calibration of the SC. With ±8 degrees of tilt, a 12,000 mGal range can be provided. To achieve a static measurement range of 40,000 mGal, a calibration range of ±10 degrees would be used.GT-2M Platform dampingLet’s consider a simple pendulum with liquid damping. With no damping, the natural oscillation period is T n sec. With damping, we can refer only to the period of damping T d which is different from the natural period of oscillation.The GT-2M platform is a pure Schuler platform with a period of 84.4 minutes. It has a natural oscillation at this period, which is equivalent to the period of a pendulum with length equal to the Earth’s radius. To dampen this Schuler period GT uses GPS velocity. They compare the velocity of the platform computed by integrating accelerometer data and compare this with GPS velocity. If there is a discrepancy between these two velocities they must dampen the platform motion. They use a digital damper which is part of the real-time software used in the GT-2M.The “transition time” of damping is very close to a Butterworth filter. It is more complicated and uses many functions, but the final transition can be described closely by a Butterworth filter. The transition time is the time it takes for the amplitude of a swinging pendulum to reduce to zero due to drag or damping. The time between the first push until zero amplitude is the transition time. This period of transition equals 10 to 12 times the time constant (TC) of the platform. If the TC of the platform is set to 600 seconds, then the period of transition will be 6,000 to 7,200 seconds. For the marine platform, a long transition time is required, which is why 600 seconds is used. A Butterworth filter describes the transition period in the simplest way.The GT-2M platform is damped by GPS with a very long period. Damping takes place through sensors located on the gyro.The platform is not disturbed by ship manoeuvres, so it is referred to as an “undisturbed” platform.GyrosSome manufacturers of marine gravimeters claim to have gyros with a lifespan of 250,000 hours. This large life span puts the gyro into the space and military applications market segment and these gyros cost an enormous amount of money ($500,000). They would also be tightly controlled by the US government and the Wassenaar Arrangement on Export Controls for Conventional Arms and Dual-Use Goods and Technologies, both of which seriously restrict the shipment of any instrument containing such high-specification devices.The GT-2M gravimeter has a Fiber Optic Gyro (FOG) with a life of 14,000 hours, and a Dynamically Tuned Gyro (DTG) with a life of 30,000 hours. Together these two gyros cost about 98 cents per hour to operate. This is a very reasonable number considering the gravimeter is mounted on a ship costing thousands of dollars a day to operate.Judging by the low number of gyros which CMG has replaced since 2003 on the 22 GT gravimeters in service, it would appear that the gyros last longer than their specified lifespan. We conclude that it is highly unlikely that any commercial gravimeter has a gyro with a life of 250,000 hours due to the very high cost (c. $500,000) and the severe restrictions on shipping of such instruments. Gyro life is a misleading specification which keeps attention focussed away from the most important specifications of a mobile gravimeter, which are its dynamic range, spatial resolution, noise level, and data production rate. GT gravimeters outperform all other commercially available mobile gravimeters by a wide margin in each of these very important parameters.There are some interesting facts about the gyros mounted on NASA’s Hubble Space Telescope which can be found at the following site:/mission_pages/hubble/servicing/SM4/main/Gyro_FS_HTML.htmlFor example:•“Gas-bearing gyros are the most accurate in the world, and Hubble uses the best gas-bearing gyros available”.• “Gyros have limited lifetimes and need to be replaced periodically. Currently, three of the six gyros are working”.• “Four new gyros were installed on Hubble in 1993 (Hubble was launched in 1990) and all six gyros were replaced in 1999. During 2008, all six gyros were replaced again”. Please note that the time between gyro replacements on the Hubble varied from 26,000 hoursto 79,000 hours, and these gyros are probably the most expensive gyros ever made.Sensor and platform designPlease note the recent photo below left of the ZLS gravimeter taken from the ZLS website and compare it with the second photo showing a very early Scintrex airborne gravimeter (c. 1959) and with the third photo of a Scintrex AirSea gravimeter (c. 2004).All three photos, taken over a span of 50 years, show the same basic platform mounted in the same rubber cord suspension. The sensor itself has not changed significantly, being a zero-length spring in all L&R meters. It is this sensor which has caused the zero-length spring gravimeters to perform poorly in multiple trials in which it has been compared with a GT-1A or GT-2M gravimeter from CMG.The following two L&R airborne gravimeters are the TAGS Air III (left) and System 6 (right) from Micro-0G Lacoste. They both have the same sensor as the two L&R instruments above, although the System 6 sensor has a much larger reading range. The reading range for the System 6 sensor is +/- 500 Gals, although the Earth’s field has a maximum range of only 5 Gal.The most important point to note for these two gravimeters is that the dynamic range is the same for all L&R meters, at roughly ±100 Gal. This compares to the GT-1A at ±500 Gal and the GT-2A and GT-2M gravimeters at ±1,000 Gal.Accuracy and Dynamic RangeOn the CMG website can be found the specifications of the GT-1A, the GT-2A, and the GT-2M gravimeters, including their relevant dynamic ranges.On the Micro-g LaCoste website, there is no clear mention of the dynamic range of the TAGS, System 6, or AirSea gravimeters.On the ZLS website you will find no mention of the dynamic range of the ZLS Dynamic meter, but you will find the following statement:“Accuracy of the ZLS Dynamic meter is difficult to specify, as it is dependent onthe characteristics of the ship, the sea-state, and accuracy of navigation (typically1 mGal)”.This is a typical statement referring to the accuracy of zero-length spring gravimeters, including the Micro-g LaCoste TAGS, and System 6 gravimeters. Neither Micro-g LaCoste nor ZLS has published a significant amount of data collected in rough sea conditions which supports accuracies below one mGal, while CMG can support accuracies at sea of 0.1 to 0.3 mGal and accuracies of 0.5 to 0.6 mGal in the air. Most manufacturers do not prominently display the dynamic range of their gravimeters because the range of all zero-length spring gravimeters is extremely small, limiting their use to periods of very calm acquisition conditions. This applies to both airborne and marine gravimeters.Appendix 2 shows a document given to customers who purchase a TAGS gravimeter. In one sale it was not presented to the customer until after the Sales Contract was signed (personal communications). It shows clearly (highlighted in yellow) the dynamic range of the TAGS gravimeter to be ±0.1 g. Compare this to the dynamic range of the GT-2A or GT-2M gravimeters which are ±1.0 g, or ten times larger than the TAGS system. Our best guess is that the ZLS meter performs in the same range as the TAGS system, with a dynamic range limitation of around ±0.1 g. The KSS 31 gravimeter has a similar dynamic range as observed in the published specifications.The graph on the following page illustrates the wide performance range of various gravimeters compared with zero-length spring systems such as the TAGS and System 6 gravimeters. The vertical axis is the figure-of-merit (FOM), which is a dimensionless parameter calculated by dividing the dynamic range (in mGal) by the resolution (in mGal). The highly accurate CG-5 ground gravimeter is included for comparison.The GT-2A airborne and the GT-2M marine gravimeters have double the dynamic range of the GT-1A, so they have very high FOMs. The GT-2M has such a large FOM because of the high quality GPS data collected at very low survey speeds.The following table shows the values used in calculating the FOM.Instrument Dynamic range(mGal)Resolution(mGal)Figure of Merit(FOM)GT-2A 2,000,000 0.600 3,333,333 GT-2M 2,000,000 0.200 10,000,000 GT-1A 1,000,000 0.600 1,666,666 CG-5 8,000 0.005 1,600,000 TAGS etc 150,000 1.000 150,000GT-2A GT-2M GT-1A CG-5TAGSCost of dataThe purchase price and operating cost of a gravimeter needs to be compared to the production rate of high quality data. For example, there are industry anecdotes stating that 2,000 or 3,000 km of 2 mGal data can be collected per month with a TAGS airborne gravimeter. Compare this with an average of not less than 10,000 km/month with a GT-1A or 15,000 km/month with a GT-2A. If the GT-1A initially cost five times as much as the TAGS system, then the cost per km of data collected would be the same, and the quality of the data would be much higher.The same comparison applies to marine data. Many geophysicists recite marine surveys using an L&R meter with noise levels of ±2 to ±3 mGal. Bear in mind that PGS and Shell have stated that such low resolution, low quality data is useless for hydrocarbon exploration.On the other hand, high quality data (±0.2 to ±0.3 mGal) with high spatial resolution can be collected with the GT-2M in sea conditions where the L&R, ZLS, or Bodenseewerk meters would be switched off to protect their sensors. This means the client using a GT-2M can save a significant amount of money in the long run by not having to repeat lines. This cost savings needs to be factored into the purchase price, as it is probably the most important point when selecting a gravimeter for either airborne or marine use.References•Shell Oil company in 1999, at the company’s in-house potential fields group international conference, announced (personal communications) that they would no longer accept data from a zero-length spring gravimeter due to the very high noise level and low spatial resolution.•PGS in Houston stated in 2009 (personal communication) that they discontinued collecting marine gravity data on their seismic vessels in the Gulf of Mexico after 10 years of doing so, because the data quality at ±2 to ±3 mGal was unacceptable to their clients and was not useful in helping to interpret their seismic data.•One government agency in China flew the TAGS and GT-1A gravimeters in identical aircraft on a large test survey in 2008. The TAGS system could not measure acceptable gravity during the turbulence experienced, so the agency were pleased to purchase three GT-1A instruments.•Geoken in Kazakhstan collected marine gravity data in 2010 with three marine gravimeters on the same seismic vessel, and bought a GT-2M because of the superior data quality and the ease of data handling and processing.•The world’s largest airborne geophysical contractor bought two TAGS system predecessors (AirSea II) in 2004 but were unable to collect acceptable data with them.They tested a GT-1A and bought one immediately.• A large public airborne survey company in Canada bought two TAGS systems three years ago (personal communications) but they found these systems to be unusable in the turbulence of the tropics. The company is currently bidding on a large gravity survey and have asked CMG to sell them a GT-2A gravimeter in the event that they are successful in obtaining this contract.• A large US company has been unsuccessful in using a TAGS airborne gravimeter on surveys in South America and is looking forward to purchasing a GT-2A.The list of customers who purchased a zero-length spring gravimeter for either marine or airborne use, and who are now looking for a better instrument, is growing.Fixed-wing and rotary-wing aircraft in which the GT-1A or GT-2A have been installed and operatedROTARY WING1 AS50 Eurocopter AS350 Ecureuil (Squirrel, A-Star)2 R44 Robinson R443 S76 Sikorsky S-764 LAMA Eurocopter SA-315 Lama5 ALH Hindustan Aeronautics Limited (HAL) Dhruv6 MI8 Mil Mi-8FIXED WING1 C208 Cessna Caravan C2082 C404 Cessna 404 Titan3 F406 Reims Caravan 2 (Cessna 406)4 PA31 Piper Navajo PA-315 PA31 Piper Chieftain PA-316 BN2T Britten-Norman Turbine Islander BN-2T7 P750 Pacific Aerospace 750XL8 CRES Pacific Aerospace Cresco9 AC90 Rockwell 690 Commander10 AC50 Rockwell 500S Shrike Commander11 DHC6 DeHavilland DHC-6 Twin Otter12 BE20 Beechcraft Super King Air B200SE13 PC6P Pilatus Porter PC-614 AN26 Antonov AN-2615 P68 Partenavia P-6816 GA8 GippsAero GA-8 Airvan17 I114 Ilyushin IL-11418 AN30 Antonov AN-30Scintrex TAGS Performance SpecificationsThe following information was presented to a customer who purchased a TAGS airborne gravimeter. It was not presented to the customer until after the customer had signed the Sales Contract.The most important specification by far is located in the final sentence on the following page and is highlighted in yellow. It shows that the TAGS system has a dynamic range of just one-fifth that of the GT-1A and one-tenth of that of the GT-2A or the GT-2M. It is almost impossible to find such calm flight or cruise conditions anywhere in the world, at any time of day or night, which is why the TAGS system has such a low production rate and such a high cost of data acquisition.Also note that the first specification says the drift should be less than 0.5 mGal per day, or less than 15 mGal per month. On the other hand, the TAGS brochure available on the Scintrex website states that the drift is less than 3 mGal per month or 0.1 mGal per day.Static testingDrift: the meter will show a drift of < 0.5 mGal / day in static mode.Repeatability: the meter will show an error of < 0.5 mGal for repeat occupations of a static test point (after drift and tide corrections).Accuracy: the meter will track tidal gravity to within 0.05 mGal, after drift removal and10 minute low-pass filtering.Dynamic testingThe acceptance criteria will be defined in terms of the standard deviation about the mean of at least four repeats of a single flight line (accuracy), with a specified filter length (resolution). The filter length is defined by a cut-off period, defined as the half-amplitude, half-period point of the filter transfer function. This corresponds with the common, half-amplitude, half-wavelength definition of resolution used by the potential fields community.The performance specifications for TAGS are:• 1.5 mGal accuracy at 80 seconds resolution, and• 1.1 mGal accuracy at 110 seconds resolution.Autopilot: all test lines will be flown under autopilot control.Flight speed: 60 m/s or less. If the flight speed is required to be higher for operational reasons, either the accuracy or the resolution will be scaled accordingly. For example, a speed of 75 m/s will scale to either 1.5 mGal at 100 seconds or 1.88 mGal at 80 seconds. The speed deviations around the mean will not exceed ±1 m/s over any two-minute span.Course: the survey lines will be flown on a constant course. The deviations around the mean will not exceed ±2° over any two-minute span.Altitude: the survey lines will be flown at a constant barometric altitude. The deviations around the mean will not exceed ±4 meters over any two-minute span. Deviations of the mean GPS heights of the repeat lines will agree within ±5 meters.Line length: at least 30 minutes on course, on speed. This will include a 4-minute run-in and a 2-minute run-out.Turbulence: the unfiltered vertical acceleration will not exceed ±100,000 mGal more than four times over any two-minute span, and will not exceed ±200,000 mGal at any time.Appendix 3Airborne and Marine GravimetersComparison of Dynamic RangesThe red traces are indicative of the maximum RMS turbulence, or vertical acceleration, which can be tolerated by various gravimeters. The larger the turbulence which can be tolerated, the better the gravimeter and the higher production rates which can be achieved.± 100 Gal (± 0.1 g) ± 500 Gal (± 0.5 g)± 1,000 Gal (± 1.0 g) Micro-g LaCoste Air Sea IIGT-1A GT-2A GT-2M Micro-g LaCoste TAGSMicro-g LaCoste System 6Chekan Graviton-M ZLS KSS 31。
