Subsurface Methods as Applied in Geophysics 1093 



of complex geology and where more accurate control is desired. This 

 technique is known as "continuous profiling." Several variations of this 

 technique are in common use, but, when maximum detail is desired or re- 

 flections are difficult to correlate between records, overlapping continuous 

 spreads are preferred. Dip shooting must be practiced in areas where cor- 

 relation is doubtful or impossible because of the lack of persistent 

 reflecting horizons, a condition which exists in much of the Gulf Coast 

 and in parts of California. It is also indicated in regions where dips in 

 excess of a few degrees are expected. Frequently both dip data and corre- 

 lations are used together in order to give a better indication of subsurface 

 conditions. Dips are determined by utilizing the "step-out" times (the 

 difference in time between two detector positions for a given reflection). 

 All seismic data must be corrected for elevation differences and 

 variations in the thickness of the weathered zone or low-velocity layer. 

 Several correction techniques are available, detailed descriptions of the 

 corrections being found in the text books on geophysical exploration. 

 Frequently extreme variations in the thickness of the weathered layer re- 

 quire a revision of operating procedure. Olson ^° reports unusual varia- 

 tions of the weathered layer due to the irregular distribution of lenses of 

 unconsolidated mud and vegetation in the Tucupita area of Venezuela. 

 By mounting geophones on individual pipes driven ten to fifteen feet into 

 the ground, the effects were eliminated in certain portions of the area. 

 Gaby and Solari ^^ mention a similar situation in San Joaquin County, 

 California, where the effects of the erratic distribution of the surface 

 material were eliminated by burying the geophones to a depth of 28 feet. 



Interpretation Methods 



The object and the fundamental problem in seismic exploration are 

 to obtain an accurate picture of subsurface conditions from data obtained 

 at the surface. A computation method is selected for a given area to give 

 the greatest degree of accuracy with the minimum expenditure of time 

 and labor. The assumption must always be made that there is a conform- 

 able relationship between the geologic strata and discontinuities of the 

 physical properties of tlie geologic section. Although velocity discontinui- 

 ties are generally correlated, West ^^ has shown that reflections may be 

 obtained from density contrasts alone. (See figure 577), 



It is always necessary to make an assumption concerning the attitude 

 of the velocity zones that lie between the surface and the reflecting hori 

 zon. Alcock^^ mentions several possibilities. (See figure 578.) 



1. The velocity zones are all horizontal. This assumption is most 



^^ Olson, W. S., Geophysical History of Tucupita Oil Field, Venezuela in Geophysical Case Histories, 

 vol. 1, p. 615, Tulsa, Soc. Exploration Geophysicists, 1948. 



" Gaby, P. P., and Solari, A. J., Geophysical History of the McDonald Island Gas Field, San 

 Joaquin County, California in Geophysical Case Histories, vol. 1, p. 605, Tulsa, Soc. Exploration 

 Geophysicists, 1948. 



°^ West, S. S., The Effect of Density on Seismic Reflections: Geophysics, vol. 6, no. 1, p. 45, 

 Jan. 1941. 



^^ Alcock, E. D., Selection of Computational Methods for Seismic Paths: Geophysics, vol. 8, no. 3, 

 p. 297, July 1943. 



