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Description
A gravity survey which measured minute changes in gravity at the land surface was conducted along University Avenue in San Diego, California. The gravity line consisted of 103 stations at one block intervals (~250 feet) covering a total length of approximately eight miles. This data set is at the northern extent of a gravity survey conducted for the San Diego County Water Authority. The purpose of the survey was to attempt to determine the thickness of the San Diego Formation which acts as an aquifer underlying downtown San Diego. Standard calculations were used in the reduction of the gravity data. Taken into accuont were elevation, latitude, and a surface rock density of 2.3 gm/cc which resulted in values of the Simple Bouguer anomaly. The data show a decrease in gravity (~7 mGal) from the easternmost station, at University Ave. and 69th St. to the base station, at Washington St. and Pacific Hwy. These results coincide with the changes in the depth to basement which crops out one mile east of the gravity line and drops to a depth of 3,000-4,000 feet on the west end of the survey. The data were then evaulated by constructing 2-d computer models which illustrated the subsurface geology based on the changes in gravity, elevation, and distance from the base station. The change of thickness in the San Diego formation, and older sedimentary rocks, is controlled by the various faults that intersect the field area. The La Nacion fault has an apparent throw of 1,000 feet, down-to-the-west. The Texas Street and FloridCanyon faults form a structural graben which has down-dropped approximately 500 feet. And, finally, the Rose Canyon Fault Zone (Old Town fault) resulted in 2,000-3,000 feet of throw, causing a juxtaposition of deeper basement on the west to more shallow basement on the east side of the fault. The San Diego Formation is thickest on the west side of the Rose Canyon Fault Zone, with a modeled thickness of 1,500-1,800 feet.
