Bedrock elevation measurement using ambient vibrations and ultra-sonic pulse test.

Abstract

One of the important steps in site specific analysis is finding the elevation of bedrock. Seismic waves in terms of acceleration time histories will be propagated from bedrock elevation to the earth surface. Invasive and non invasive are two methods commonly used for estimating the elevation of bedrock. Invasive methods required drilling into the ground. Non-invasive methods for estimating of bedrock elevation can be performed using ambient vibrations. Ambient vibrations are short period vibrations (0.02 to 50 Hz) result from environment activities such as traffic, wind interaction with vegetation, factory activities etc. A single feedback seismometer can be performed for measuring ambient vibration waves. A simple method by placing seismometer on the earth is required for ambient vibrations measurement. This study presents result of bedrock measurements in south area of Semarang City, Indonesia using a single feedback seismometer. The Horizontal-to-Vertical Spectral Ratio (HVSR) analysis method from Nakamura (1989) was performed for estimating the depth of bedrock elevation. Bedrock elevation map was performed based on 196 locations of ambient vibration measurement. Drillings for finding rock sample for rock elevation less than 30 meter were also performed. To get the information of shear wave velocities of rock sample Ultrasonic Pulse Test of rock tests were performed.