In the inverse solution of a gravity problem when s2 Y2 1 z* the direct method of interpretation is not effective, the g2 = YP II{ k dy SI Y1 I I dx' + y2 + 22 ZI interpretation is done by assuming a geometric model of the subsurface mass distribution based on geologic SZ YZ 22 or some other considerations, calculating the gravity =YP I r In (y + 4x2 + y2 + z*) s1 I I I dr. Y, 21 effect of the model, and finally matching with the Let observed anomaly curve. The model is repeatedly changed until there is an agreement of the theoretically I = .f In (y + dx" + y* + z*) dx calculated and observed anomaly curve. Again for =xln(y+~x*+y*+z*)-Il. estimation of terrain effect, it is necessary to com-where pute the gravity from the known mass distribution. So the importance of direct potential problems in gravity I, = X2 should not be overlooked. I ( y + 4x2 + y2 + 2) 4x2 + y2 + 22 dr Talwani and Ewing (1960) developed a method of computation of gravity attraction of 3-D bodies of = I x*(tix2+y2+z*-y) arbitrary shape. An arbitrarily shaped body may be (x' + 2)4x" + y2 + 22 0!x considered to consist of a number of right rectangular X2 prisms, and for calculation of the effect of the whole = I xz + zz dr block, it is sufficient if the gravity effect of an individual prism is evaluated. Nagy (1966) considered the gravitational attraction of a right rectangular -y I X2 (x' + 2)4x* + y2 + 12 ak.prism. Nagy' s formula has certain limitations which we discuss below, and some suggestions are put The first term will eventually become zero when forth to overcome them. limits of y are put. So I, may be written as THEORY X2The vertical component of gravitational attraction at the origin of a right rectangular parallelepiped bounded by planes x = x,, x = x2; y = y,, y = y, and z = zi, z = z2 (Figure 1) is given by 1,=-y I (x' + z*) dx' + yz + zz dx _ dx Y 4x2 + y2 + z= g,(O,O,O) -g, = YP zdxdydz (x' + y* + 2)3' 2 '(1) where y is the universal constant of gravitation, and p is the density of the prism. ,Neglecting sign of g,, we have from (1) where z2 dx (x2' + 22) dx" + yz + 22 1 y [In (x + \/x2 + y2 + z2) -121.

If the conductivity of any one of the layers of a horizontally stratified earth varies exponentially with depth with or without a discontinuity at the interface, the corresponding expressions for apparent resistivity for Wenner‐ and Schlumberger‐sounding arrays can be formulated. The general case has been broadly divided into three categories for mathematical simplicity. All previous discussions of this problem can be regarded as particular cases of the present study.

We appreciate Messrs. Choudhury and Amaravadi’s interest in our paper and thank them for pointing out some typographical errors in the paper. In fact, we were aware of these and some other similar errors. To us, the corrections of these errors are quite obvious to any serious reader of the paper and they do not seem to deserve any special attention of the general readers of Geophysics. However, we take this opportunity to list them below.

The hidden layer problem in seismic refraction work has been studied for three velocity configurations – the intermediate layer having (a) lower, (b) intermediate and (c) higher velocity than the underlying and overlying beds. It has been shown that conventional methods fail to locate the presence of the intermediate layer for the cases (a) and (c) and lead to errors in calculating the depth to the bedrock. For the case (b), it is possible to interpret the first arrival travel time analytically as an alternative to Green's graphical approach. It has been suggested that the hidden layer may be detected in all the three cases if converted S waves are also recorded in the seismogram.

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