Some Problems of Shallow Refraction Investigations*

Abstract

The term shallow refraction, as used in this paper, refers to investigations confined to the superficial layer of rocks, composed primarily of unconsolidated material. To define the scope of the discussion it will be assumed that the term shallow refraction applies to work of which the lower limit is approximately 300 feet and the upper of the order of a few feet. The consideration of the small magnitude of quantities measured (distances and times) determines the perspective of the problems involved, since difficulties encountered in the interpretation, although equally disturbing whether in deep or shallow work, will cause a greater percentage of error in the latter case. The purpose of this paper is to discuss and illustrate these problems. The factors considered include the location of the geophone spread in relation to the topography of the site, influence of the ground conditions in the vicinity of geophones on recorded times, consideration of the shallow uphole shots and problems arising from the repeated use of the same shothole. The rapid variation in the vertical velocity of the overburden and errors due to it are discussed together with the effects of a non homogeneous unconsolidated material and velocity reversal. The effects of the ill-defined solid rock surface are also considered. It appears that as the depth of investigations becomes shallower, the limits of the practical capabilities of the method are approached, because the differences between the theoretical assumptions and the actual conditions become more pronounced.

Geophysical Model Studies of the Tularosa Basin, New Mexico.

“…Travel time curves in the presence of a low velocity layer . Domzalski (1956), Pres s and Dobrin (1956), and Knox (1967), there does appear tD be an approxima te correlation between the length of the time gap observed and the thickness o f the low veloci ty layer as determined from bore hole data. In any case , the correc tion employe d is in the "r igh t direc tion " ; that is, without it (without assuming a low velocity layer), the depth computed for the third interface woul d be an overes timat ion .…”

“…In the cross section of a pçrtion of the Jornada del Muer to in Figure 2 , a volcanic section is seen underlying the lower Santa Fe G'roup; however , an aeromagnetic s tudy o f the Tularos a Basin by Ba th (1977) does not sugges t that and others , 1956 ;McClean , 1970) Without borehole data it is not possible to positively assign a p a r t i c u l a r stratigraphic i d e n t i f i c a t i o n to the in li t1~bgy such as clay overlying sand , as described by Domzalski (1956), could , of course , also produce a low velocity…”

“…- Field examples of similar appearing gaps observed in compressional wave refraction surveys have been given by Burg (1952), Press and Dobrin (1956), Domzalski (1956), and Knox (1967). Figure 10 ).…”

Geomorphology and glacial geology, Wolverton and Crescent Meadow areas and vicinity, Sequoia National Park, California

“…The overlying alluvium probably does not have a uniform seismic velocity, but consists of layers and lenses of gravel, sand, and clay, which all have different seismic velocities. The problems that can result from these variations in the character of the rock materials and their interfaces are described by Domzalski (1956). In particular, if a hidden lowvelocity layer underlies a higher-velocity surface layer, the analysis of the data would give an erroneously great depth to bedrock, and there would be no way of recognizing this fact, solely from seismic refraction studies.…”

Seismic and resistivity exploration of soils and shallow bedrock in forestry research and management

“…Domzalski (1956) recorded an unusually low value, 470 ft/sec on a traverse of 52 f t with an initial geophone spacing of 1.5 ft: in this instance, dry desert sand overlay gravel resting on marl. Pleistocene glacial tills of different ages were found by Johnson (1954) to have different representative seismic velocities.…”