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Title: Critical study of shallow seismic exploration in the limestone areas of the Ozark highlands
Authors: United States. Mississippi River Commission
U.S. Army Engineer Waterways Experiment Station
Keywords: Seismic prospecting
White River Watershed (Ark. and Mo.)
Publisher: U.S. Army Engineer Waterways Experiment Station.
Engineer Research and Development Center (U.S.)
Description: Technical memorandum
Synopsis: The results of a comparative study of foundation exploration by boring and seismic refraction methods at several proposed dam sites in the limestone regions of the Ozark highlands are reported in this technical memorandum. The purpose of this study was to determine the practicability of adopting seismic exploration to the particular region and to determine the extent of applicability of the geophysical method. Numerous proposed dam sites were being investigated by the Little Rock Engineer Office within the White River watershed. Subsurface conditions at six of these sites, all the weathered limestone region of the Ozark highlands, were explored by boring and by seismic refraction methods. The seismic exploration was performed by the Waterways Experiment Station for the Little Rock District. Geological investigation and boring had shown, prior to initiation of the seismic exploration, that at all sites concerned the streams flowed in entrenched meanders and that the limestone bedrock was weathered in a manner typical of such valleys. At some sites, weathering was characterized by deep overburden and thick zones comprised of rock pinnacles and clay-filled fissures at the transition between overburden and sound rock; at other sites the weathering was less deep and the transition zones were relatively thin. At all sites, fresh rock was near the surf ace in the stream bed and the flood plains consisted of alluvial silts, sands, and gravels. Information was required concerning the depth to firm rock at the sites. Seismic exploration was confined principally to spillway, areas and abutment ridges with a few depth profiles located in the flood plains. The seismic refraction method depends upon the elastic properties of subsurface materials. Changes in these properties affect the seismic data in such a manner that the depths to such changes can be computed. The seismic data obtained in the deeply weathered portions of the dam sites indicated, by their irregularity, that the rock surface was irregular. Seismic depths to an apparently rigid material were shown, upon comparison with several adjacent deep borings, to lie within the transition zone of pinnacled rock between soil overburden and firm rock. An exception to the latter case was found where open and clay-filled caverns existed below the seemingly firm rock. No indication of the existence of caverns so situated could be obtained by seismic methods. It is concluded from this study that in the deeply-weathered limestone regions of the Ozark highlands, seismic refraction exploration will not yield definite depth to firm rock. Such data can apparently be obtained only by numerous borings. The seismic depths obtained are of a qualitative nature, yielding only general information concerning the order of magnitude of the depth to rock and, very roughly, the degree of irregularity of the rock surface. Seismic refraction methods cannot identify cavernous conditions below apparently sound rock. In the explored areas where weathering of the limestone is shallow and in the flood plains, seismic exploration is considered to be satisfactory and usable, since the rock depths indicated are in close agreement with those shown by borings.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Technical Memorandum

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