SEISMIC METHODS 715 



tained when the shot-point is placed in the low velocity layer, at or near the 

 surface of the ground. 



The shot wave traveling downward in all directions will encounter the 

 boundary between low velocity surface la3'er (1) and the underlying 

 higher velocity layer (2). Because of the physical discontinuity between 

 these two layers, a considerable portion of the energy is reflected at the top 

 of the underlying layer. This reflected energy A traveling upward may be 

 again reflected when it hits the surface-air interface of the upper layer, 

 and travels back downward towards the lower interface, where the process 

 is repeated, thus giving rise to an indefinite number of reflections or 

 reverberations A, A' , etc. This reverberation will tend to mask the desired 

 reflection B, and the seismometer will continue to be disturbed for a pro- 

 longed period of time by a series of shallow reflections, depending upon 

 the acoustic properties, form, and dimensions of the low velocity layer. 

 This undesirable condition can be greatly minimized by placing the shot- 

 point below the low- velocity boundary, whereby more of the shot energy 

 will go into the lower layer directly to produce stronger reflections from 

 the deeper strata. 



The source of knovv^ledge on the low velocity layer is confined almost 

 entirely to experiences in seismic prospecting. Research has not given a 

 clear explanation of the low velocities found in this zone in terms of the 

 observed properties of the layer. However, calculations for velocities for 

 soil and air mixtures have demonstrated low velocities of the magnitudes 

 observed in practice. f Velocities as low as 550 to 1600 feet per second 

 have been measured in the uppermost part of this zone. In most cases the 

 bottom of the low velocity layer is closely associated with the top of the 

 ground water table. This layer is also characterized by generally poor 

 seismic wave transmission qualities, and for this reason the shots are usually 

 placed below or near the bottom of the zone. Explosives detonated below 

 the low velocity layer produce more efficient and effective origins for 

 seismic waves than shots placed within the low velocity zone. 



The difference in velocity between the low velocity zone and the rocks 

 below it seems to be related to the porosity or unfilled voids present. An 

 increase in porosity of unconsolidated materials when the pore space is filled 

 with air decreases the modulus of elasticity and the wave velocity of the 

 material. However in the case where the pore space is filled with moisture, 

 the modulus of elasticity of porous material is increased. This is probably 

 the situation at the top of the permanent water table, which usually marks 

 the bottom of the low velocity layer. Associated with the increase of the 

 modulus of elasticity, an improvement in the wave transmission quality is 

 noted. In field procedures, wherever possible, practical use is made of this 

 improvement by placing the explosive charges below the water table. The 

 increase of water content in unconsolidated materials does not have a 

 parallel efifect in the case of consolidated geological units such as sand- 



t O. C. Lester, A.A.P.G. Bull., Vol. 16, No. 12, pp. 1230-1234, Dec, 1932. 



