masses located at depths ranging from 7 to 12 kilometers beneath the surface 

 of a spherical earth. Amplitudes of the anomalies range from -17 mgls to +83 

 mgls. For purposes of comparing the various models and residual charts to 

 follow, a transparency of Figure 4 is contained in a pocket attached to the 

 rear cover of this report. 



A. Gridding of Survey Data 



To design a simulated survey for obtaining data to test the interpolation 

 procedures, a two-dimensional FFT was computed using all of the model field 

 data (Figure 4) available on a one-minute grid interval. Contours of the 

 amplitude spectrum resulting from this computation are shown in Figure 5. 

 Since only the shape of the spectrum is used to design the simulated survey, 

 the contour values are arbitrarily normalized, and the frequency range is 

 normalized to the units of cycles per data interval. Utilizing the result 

 embodied in equation (28), Figure 5 shows that the trends in the model field 

 are predominately in the NE-SW direction indicating that the preferred survey 

 track direction is NW-SE. With the track direction specified, the flattening 

 of the amplitude spectrum at a frequency of approximately 0. 17 cycles per data 

 interval in both the along-track and cross-track directions indicates that a track 

 spacing and sampling interval of three nautical miles would be sufficient to 

 minimize aliasing of the data. Figure 6 shows the simulated survey, designated 

 as model 1, which was designed from this interpretation of the two-dimensional 

 amplitude spectrum. By overlaying this survey with the transparent copy of 

 Figure 4, it is apparent that no attempt was made to sample the peak values of 

 the anomalies. Simulated deviation of the survey vehicle from the planned 

 survey tracks was included, but measurement errors were excluded by computing 

 the exact value of the gravity field at each survey point by using equation (30). 

 The cubic spline algorithm was utilized with this survey data to compute inter- 

 polated values of the gravity field at one minute grid intervals. The interpolated 



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