716 EXPLORATION GEOPHYSICS 



stones, limestones, schists and relatively porous igneous rocks. An increase 

 in water content of these rocks seems to be associated with a decrease in 

 velocity; however, this decrease is not as appreciable as the increase of 

 velocity in the case of water-saturated unconsolidated rocks. The effect of 

 water saturation of rocks on seismic velocity is obviously complex. 



Evaluating Effects of Low Velocity Layer. — In general practice the 

 so-called "weathering" problem is handled by one of two methods. In one 

 method, the effect of the low velocity layer is eliminated by actually making 

 all the seismic observations below this layer or zone. The second method 

 may be described as a process of low velocity shooting, in which the velocity 

 and the thickness of the layer or zone are measured. 



In the first method, making the observations below the low velocity 

 layer, the shot-point and the seismometers are placed at or below the 

 contact between the low and the higher velocity layers. f Under these 

 conditions there is no delay due to the presence of the low velocity layer. 

 Somewhat better mechanical coupling between the seismometers and the 

 earth is obtained, and thus more efficient detection of earth displacements 

 may be realized. The most serious difficulties in using this technique are 

 the numerous drill holes necessary for the seismometers, and the retrieving 

 of instruments from the drill holes after the observations have been made. 



In the second method, that of low velocity shooting, results are obtained 

 much more rapidly and economically, but they may not be as precise. The 

 accuracy is usually sufficient for normal commercial operations, and we 

 can express our computations in three general ways, all giving substantially 

 equivalent results. First, the low-speed layer may be theoretically replaced 

 with materials of the same seismic velocity as that which underHes this 

 layer. Second, by calculation, it is possible to reduce the observed times to 

 correspond to a situation in which both the shot-point and the seismometers 

 are placed at the bottom of the low velocity layer. Third, all observations 

 may be reduced to a theoretical reference plane or datum level, as is some- 

 times done in surveying. This reduction is obtained by calculating what 

 the observed times would be if both the detecting seismometers and the 

 shock charge were placed on the datum plane of a given elevation. 



A study of the surface or low-velocity layer reveals that in general 

 three conditions exist: (1) the properties of the zone may be relatively 

 constant over a considerable area; (2) the velocity and thickness of this 

 layer may be approximately the same at the shot hole and along the profile 

 of seismometers, but considerable difference may exist between shot-point 

 locations : i.e., the velocity and thickness may vary considerably in an 

 area to be surveyed but not perceptibly beneath a single instrument set-up ; 

 (3) the variations of the low-speed zone may be perceptible beneath a 

 single instrument set-up. 



t B. McCollum, "Method and Apparatus for Studying Geologic Structure," U. S. Patent 

 1,676,619, July 10, 1928. 



