RELATIONS OF GEOPHYSICS TO GEOLOGY 7 
Gravity, magnetic, and electric geophysical methods are measure- 
ments of a potential function, but the seismic method is not. From 
the standpoint of geology and geophysics, measurements of the po- 
tential function and its components have the following implications. 
1. Size (that is, mass) and distance cause most of the effect. The 
larger the mass, the greater the effect. The greater the distance, the 
less the effect. 
2. All bodies have an effect, and this effect can become very small 
only if the body is very small, or very far away. 
3. The shape of the body modifies the effect only slightly. 
In reconnaissance geology, we wish to find the existence of large 
bodies, and so potential function methods are valuable in reconnais- 
sance. In detail geology, we wish to construct a contour map of a 
comparatively small segment of a surface of a body, and therefore 
unless we can determine the shape of this segment very accurately, 
we can not construct accurate maps; thus, in this case the potential 
function methods are not easy to apply unless the part of the body 
which we wish to contour is very close to our observation point as 
compared with the rest of the body. 
The seismic method is not a potential function method and it at- 
tempts to map contours on an underground bed, or beds, by determin- 
img surfaces which are the possible loci of the travel of a sound wave, 
the bed sought being the envelope of these loci. The seismic method is 
intended, therefore, to give a result which determines uniquely the 
depth to a bed, or beds, below the point of observation, and is prob- 
ably the only method which gives theoretical possibilities of being the 
equivalent of core drilling; it has theoretical possibilities for construct- 
ing contour maps greater than those of core drilling, because the seis- 
mic method in order to determine the depth below a point of observa- 
tion determines the amount and direction of slope at that point. 
The statements made in the preceding paragraph are general, and, 
like all generalities, are wrong. In certain cases, measurements of the 
potential function can be used by the geologists for contouring be- 
cause the number of possible distributions of matter which might give 
the measurements are limited by known facts regarding the geology. 
For example, a very strong gravitational minimum covering a limited 
area where sedimentary rocks are present to considerable depth must 
be caused by salt or by a large block of diatomaceous shale. There- 
which is associated with the potential; this factor of proportionality depending upon 
the forces under consideration and the system of units which is employed” (MacMillan, 
op. cit., p. 283). 
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