Using seismics to locate a buried fault at the Pinnacles National Monument

This case study presents a seismic refraction's usefulness in finding faults. The investigation took place at the Pinnacles National Monument in the central coastal hills of California. The Pinnacles National Monument is about 150 miles south of San Francisco. Majestic rock spires and unusual talus caves attract thousands of visitors to the Pinnacles each year. In the summer of 1993 the park wanted to improve its water supply. The project geologist wanted to drill a new water well in a small valley near the park's visitors center. The valley consisted of stream deposits overlying a fault. The bedrock west of the fault was rhyolite. The bedrock to the east was a fanglomerate. The geologist wanted to drill into the fanglomerate and avoid the rhyolite. The geologist also wanted the well as close to the middle of the stream valley as possible.

Diagram of River Valley

To locate the fault we performed a seismic refraction survey using the generalized reciprocal method (GRM) for data collection. GRM differs from standard refraction in that more shot points and geophones are used and a greater amount of raw data is collected. GRM is a great technique for finding faults when the rock on one side of the fault is harder than the rock on the other. In this case the rhyolite was expected to be harder than the fanglomerate. Below is the seismic profile collected across the fault. It shows a bowl shaped bedrock topography typically found below river valleys. The bedrock topography is relatively smooth and any offset caused by the fault had long since eroded away.

Actual Seismic Profile

The refraction profile is useful in determining the alluvial depth but does not show the fault's location. Simpler refraction surveys would not have solved the geologist's problem. Because GRM data were collected, we could calculate a velocity profile for the bedrock. A velocity profile shows the bedrock's seismic (P) wave velocity and how it changes along a bedrock horizon. The diagram below shows the P-wave velocities measured along the bedrock at the Pinnacles. The P-wave velocity of the rhyolite, 9500 feet per second (fps), is much greater than the P-wave velocity of the fanglomerate, 7400 fps. Typically harder rocks have faster P-wave velocities than weaker rocks. The abrupt change in velocity approximately 200 feet along the line indicates the location of the fault.

Bedrock Velocity Profile

Based on the velocity information a well was drilled into the fanglomerate on the east side of the fault. It was successfully drilled into the fanglomerate and avoided the nonwater bearing rhyolite. This well is now producing water for the thousands of families that visit the park each year.

Seismics is just one of many geophysical techniques offered by J R Associates. Please contact us to discuss how our services could benefit you.

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