We've Moved!
Visit SDSU’s new digital collections website at https://digitalcollections.sdsu.edu
Description
Gravity surveys can be an excellent technique for studying relief in the basement underlying sedimentary rocks. Using a LaCoste-Romberg gravity meter for the measurements and a computer modeling program for interpretation, an east-west survey was run along Main Street and Otay Valley Road in San Diego, CA to determine basement depth and to locate possible faults in the La Nacion Fault Zone. This survey line is important because there is an old oil well which bottoms in basement at the west end and the basement outcrops at the surface at the east end, thus providing depth control. Utilizing a computer modeling program it was possible to determine the average density contrast between the sediments and the basement. Originally suspected to be about -0.5 glee, use of the modeling program showed that the average density contrast 1s about -0.32 glee. The modeling also showed two large and a couple of minor steps along the survey line which are believed to be faults. One of the major steps in the basement model is close to the main trace of the La Nacion fault, but the other large step dosen't coincide with any mapped faults. The unnamed "fault" is located 5000 ft west , and the La Nacion strand is about 1000 ft east of Interstate 805. The throw on the western fault is about 1200 ft and it is about 1400 ft on the La Nacion strand. This survey demonstrates the effects of the different corrections involved in completing a gravity survey. Graphs show how each correction effects the theoretical gravity calculations during each step of the data reduction. The different models that were created using the computer program show the ambiguous nature of geophysical data interpretation.
