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|Title:||Microgravimetric techniques for geotechnical applications|
|Authors:||United States. Assistant Secretary of the Army (R & D)|
Butler, Dwain K.
|Publisher:||Geotechnical Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||Miscellaneous paper (U.S. Army Engineer Waterways Experiment Station) ; GL-80-13.|
Abstract: Application of high-resolution gravimetry to geotechnical problems is properly termed microgravimetry and is a relatively recent addition to the list of geophysical methods available for geophysical site investigations and other geotechnical applications. This report investigates the applicability of microgravimetry to the wide range of geotechnical problems of interest to the Corps of Engineers. Microgravimetry should be viewed as a complement to other geophysical methods for site investigations. For sites with strata which are nearly horizontal (i .e., a "layer cake" type structure), microgravimetry will be of little use . However, microgravimetry is ideally suited for sites with bedrock irregularities, faults, fracture zones, cavities, buried channels, etc. Use of microgravimetry accompanied by other complementary geophysical methods (such as seismic refraction and/or electrical resistivity) and only selective drilling can achieve adequate site definition at considerably less cost than the comprehensive drilling program alone necessary to achieve the same site definition. Pertinent aspects of gravitational potential theory are reviewed and the concept of gravity anomalies is explored. Model studies are presented which investigate the detectability of subsurface structures by gravity surveys. Using a detection threshold of 10 μGal, based on the sensitivity and accuracy of state-of-the-art microgravimeters, the detectability of spherical, horizontal cylindrical, and truncated horizontal slab models is assessed. As a rule of thumb, compact structures which can be approximated as spherical in shape, can be detected at a depth to center of about two times the effective diameter at the 10-μGal threshold level. A substantial portion of the report is devoted to the practical field survey procedures for microgravity work. The emphasis is on acquiring high-quality data. Topographic survey requirements and gravity data acquisition procedures are discussed in detail. The corrections which must be made to gravity data are discussed and a practical example of the drift correction procedure is presented. Three case histories of the application of microgravimetry to the detection and delineation of subsurface cavities are presented. Also, the results of a full-scale microgravity field investigation at a natural cavity site in Florida are presented and analyzed. With only one exception, all known cavities at the site were delineated by the gravity results. Other negative gravity anomalies were investigated by drilling and found to be due either to cavities or to clay pockets in the top of the limestone. Even very subtle geologic features were correctly expressed in the gravity data. There is considerable fundamental and practical importance in the measurements of horizontal and vertical gravity gradients. In particular, gravity-gradient profiles (A.) have diagnostic properties which make subsurface structure identification more straightforward, and (B.) tend to selectively filter out anomalies caused by deeper-seated structures and hence enhance anomalies caused by shallower structures of interest in geotechnical investigations. Results of a field gravity-gradient study are presented demonstrating successful determination of both vertical and horizontal gravity-gradient profiles over a shallow man-made structure. Finally, two more exotic applications of microgravimetry are discussed. Microgravimetry can be used to study deflections of the crust due to reservoir loading, underground fluid injection or withdrawal, and earthquakes. Elevation changes due to reservoir loading are specifically addressed. Examples of the use of the gravimeter to record earth tides and as a long-period vertical seismometer are presented. The value of theoretical and recorded ear th tide records in the analysis of microgravity field data is also emphasized. Microgravimetry has many varied applications to geotechnical problems. It should be carefully considered for applicat ion to geophysical site investigations as well as other areas discussed in this report.
|Rights:||Approved for public release; distribution is unlimited.|
|Appears in Collections:||Miscellaneous Paper|
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