Cross-hole seismic measurements for detection of disturbed ground beneath existing structures

Cross-hole seismic surveys are well established for non-invasive investigat ions of geological structure and are well suited for detecting hazards bene ath buildings. Inversion of the seismic data confirms the value of tomograp hic imaging as a means of expressing the results in a form that is of pract ical value to the engineer. Physical property research has identified petro physical factors, which control the attenuation and velocity of compression al and shear waves in reservoir-type rocks. This indicates that there is va lue in assessing changes in amplitude of seismic waves in addition to their velocity. The practical value of such surveys is demonstrated using case h istories. While minimising the losses between the borehole casing and the f ormation (e.g. using bentonite grout) is required for amplitude studies, it is noted that diffraction causes a bias towards higher values in velocity studies. Two surveys are described where cross-hole compressional waves hav e been used to investigate the foundation conditions under domestic dwellin gs in response to subsidence. The results show that a wide range of velocit ies and signal loss is possible above the water table and that consideratio n of amplitudes enables areas of disturbed ground to be identified. Automat ic data acquisition and processing has enabled a 3-D dataset to be generate d using 16 pairs of boreholes and over 5000 individual seismic rays. (C) 20 01 Elsevier Science Ltd. All rights reserved.