SEISMIC METHODS 855 



Variable Directional Discrimination 



A method utilizing both the discriminating property of multiple 

 detection and an extension of the overlapping principle is involved in some 

 of the instrumental methods of interpretation.! (See page 825.) 



Multiple Shot-Points 



Beyond a certain maximum charge for a given shot-point the ratio of 

 useful to extraneous response begins to decrease. (Compare p. 849.) 

 Hence if the relative reflected energy of the "maximum" charge is 

 inadequate, more than one shot-point may be utilized, the charges in 

 all shot-holes being fired simultaneously in as far as uniformity in detona- 

 tion is possible. The effect of multiple shot-points is very much the same 

 as that of multiple seismometers. Besides increased energy, therefore, 

 there is also a discriminating efifect which favors waves from an approxi- 

 mately vertical direction. 



Considerable experimental work has been done by the author, with 

 quite interesting results, using a multiplicity of small charges, spaced 

 uniformly within a circular pattern, and placed on the surface of the 

 ground. The charges are fired simultaneously. When the diameter of the 

 disc approximates the depth to the bottom of the low-velocity zone, the 

 wave front is sufficiently flat to give good records under a majority of 

 conditions. This procedure is helpful in glacial fill and similar areas. 



FIELD OF APPLICATION OF SEISMIC METHODS 



Refraction techniques are employed wherever lateral structural studies 

 may be advantageous. They are also used for determining the depths 

 of one or more well-marked shallow boundaries, such as the low velo- 

 city layer, the water table, or the gravel beds in placer mining. Refraction 

 profile methods are also useful in studies of soil dynamics designed to 

 determine the suitability of soil for dam and building purposes, highways, 

 etc. ; the required thickness of concrete highways ; the bearing capacity 

 and depth of upper and lower beds, etc.$ Fan shooting is most useful for 

 finding shallow structures with quite definite velocity anomalies, for 

 example, shallow salt domes and gravel deposits. 



The reflection method has an advantage where depth is a factor, 

 anfl generally is the only practicable method for depths in excess of about 

 5.000 feet. The method gives most accurate results when the low velocity 

 layer is uniform and the structures under investigation have uniform 

 and gentle relief. In broad areas where correlation is good, faults can 

 be determined. The reflection method has not proved satisfactory in 



t F. Rieber, "A New Reflection System with Controlled Directional Sensitivity," Geophysics, 

 Vol. 1. No. 1, Jan. 1936. "Visual Presentation of Elastic Wave Patterns Under Various Structural 

 Conditions," Geophysics, Vol. 1, No. 2, July 1936. "Application of the Geo-Sonograoh to Petroleum 

 Exploration," Petroleum Engineer, Feb. 1937. "Complex Reflection Patterns and their Geologic 

 Sources," Geophysics, Vol. 2, No. 2. March, 1937. 



t Compare R. K. Bernhard, "Geophysical Study of Soil Dynamics," A.T.M.E. Geophysical 

 Prospecting, Tech. Pub. No. 834, Feb. 1938. 



