Microgravimetric Techniques for Geotechnical Applications

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 that the comprehensive drilling program alone necessary to achieve the same site definition. 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 earth tide records in the analysis of microgravity field data is also emphasized.