Mapping the underworld multi-sensor device creation, assessment, protocols: Acoustic technologies advancement to support multi-sensor device. An assessment of the use of a scanning laser to measure ground vibration

Abstract

This report concerns experimental work undertaken at ISVR under the EPSRC-funded ‘Mapping the Underworld’ programme, phase 2, EP/F065973/1. In the experimental work reported here, using a scanning laser is compared with using geophones for the measurement of ground vibration at low frequencies (typically <500Hz).
The performance of the sensors was compared on a number of different ground surfaces. For all the surfaces, there was general agreement between the laser data and the geophone data; the laser performed better on some surfaces than others, but the laser data was consistently of poorer quality than the geophone data. Surface velocity was found to be the key factor in determining data quality, rather than the surface texture itself; for most of the tests, the surface velocities were close to the laser system noise floor.
A number of ways to improve data quality were investigated including altering the surface texture, either by removal of surface dust/grit or by applying retroreflective tape, high pass filtering, signal averaging, both spatially and in the time/frequency domain, and using different types of input signal.
Finally, effects of the laser stand-off distance were assessed.

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