Use of Precision Gravity Survey to Determine Bedrock

The density contrast between a deeply weathered schist and a marble which contains a small percentage less than 5% of clay-filled solution channels allows differentiation between the two bedrock units with a gravity survey. Gravity highs lie over the marble which, even with the solution channels, is more dense than the schist. Because of the solution channels, the marble presents a less suitable bedrock foundation for large bearing loads. The bedrock on the campus of the University of California at Santa Cruz consists of marble and schist in complex contact with each other. Application of the gravity method to a building site on the campus indicates that in special cases a gravity survey is an economic and accurate first step in providing information regarding the foundation suitability of bedrock at a building site while in the planning stages.