Seismic Techniques
Crosshole Seismic Tomography
Detail
Borehole seismic tomography involves the measurement of the travel times of seismic raypaths between two or more boreholes in order to define an image of seismic velocity in the intervening ground.
Data is collected in a similar manner to crosshole seismic surveys by using one hole for the seismic source (normally a sparker) and measuring first-arrival times using strings of hydrophones in the others. However, unlike crosshole, travel times are collected at regular intervals (usually 0.5m to 2m) all the way down the hole(s) for each shot position.
Measurement of arrival times for each shot, at each position in the receiver borehole, results in a network of overlapping raypaths which can then be used to model the velocity profile (see figure). The plane separating the source and receiver holes is divided into a mesh of grid cells known as finite elements. Each element in the mesh is assigned a starting velocity and the synthetic travel time for the portion of each raypath passing through it is calculated. In this way the total travel time for each raypath is built up and then compared to the measured travel time. The velocities assigned to the various elements are then adjusted iteratively until the calculated and measured travel times for the raypaths are the same. As many of the cells are intersected by a number of raypaths the process can result in very accurate estimates of the velocity for each cell.
The resulting velocity image is termed a tomogram and enables identification of anomalous velocity zones lying between the boreholes as well as imaging individual velocity layers. The primary application of borehole seismic tomography is in engineering studies for the identification of features such as fault zones and voids. When combined with an S-wave survey, the data can additionally be used to provide information on material stiffness properties (see crosshole seismic surveys).
