continuation:Determining from a set of measurementsof a potential field made over one surface what valuesthe field will have over another surface (specifically atanother elevation).
假频：采样数据所表现出的频率的多值性。当每周期少于两个采样点时，某一频率的输入信号与系统输出端另一频率的信号具有（看起来象是）相同的采样值。采样频率的一半叫作“折叠频率”或“奈奎斯特频率”fN；大于该频率的频率fN+Y与小于该频率的频率fN-Y看起来很象。这两个频率fN+Y和fN-Y相互“混叠”。因此，对于采样间隔为4毫秒的信号，或者说每秒采样250次的信号，其奈奎斯特频率为125hz；例如，设频率为50hz的信号在通频带内，如果不用去假频滤波器，则频率为200hz的信号在输出端也会得到频率为50hz的输出信号。为避免频率混叠现象，在采样之前必须用去假频滤波器将高于奈奎斯特频率的频率成分滤除。由奈奎斯特频率折叠得到的通频带叫作假频通带、旁瓣和副瓣。混叠是所有采样系统的固有特性。适用于数字地震记录、检波器和炮点排列的单个元素采样、和重力测量中大小仅取决于离散点位置的位场的采样等等。
艾里假说：地壳均衡补偿的一种模式，认为地形升高的区域可由它下面扎入更高密度地壳物质形成的“根”来补偿。
alias:An ambiguity in the frequency represented by sampled data. Where there are fewer than two samples per cycle, an input signal at one frequency yields the same sample values as (and hence appears to be ) another frequency at the output of the system. Half of the frequency of sampling is called the “folding” or “Nyquist” frequencyfN; a frequency larger than this,fN+Y, appears to be the smaller frequency,fN-Y. The two frequencies,fN+Y andfN-Y are “aliases” of each other. Thus , for a signal sampled every 4 msec, or 250 times per second, the Nyquist frequency is 125 hz; if, for example, 50 hz is within the passband, then 200 hz will also be passed if an alias filter has not beenused and will appear in the output to have a 50 hz frequency. To avoid aliasing, frequencies above the Nyquist frequency must be removed by an alias filter before the sampling. The passbands obtained by folding about the Nyquist frequency are called alias bands, side lobes, and secondary lobes. Aliasing is an inherent property of all sampling systems. It applies to digital seismic recording, to the sampling which is done by the separate elements of geophone and shotpoint arrays, and to sampling such as done in gravity surveys where the potential field is determined only at discrete stations, etc.
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