Cross-hole Seismic Tomography

OUTLINE

Borehole seismic tomography involves the measurement of the travel times of seismic ray paths between two or more boreholes in order to derive an image of seismic velocity in the intervening ground. Data is collected using one hole for the seismic source (normally a sparker) and measuring first-arrival times using strings of hydrophones in the others. Travel times are collected at regular intervals (usually 0.5m to 2m) all the way down the hole(s) for each shot position. This results in a network of overlapping ray paths that can then be used to model the velocity profile. 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.

DETAIL

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 ray paths 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 ray path passing through it is calculated. In this way the total travel time for each ray path 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 ray paths are the same. As many of the cells are intersected by a number of ray paths 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).

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RESULTS
During data acquisition individual shot records are displayed as variable area wiggle traces indicating travel time against downhole distance for each shot position. Following acquisition wiggle traces are used to pick the first-arrivals for each source/receiver pair. The image displays a shot gather for 40 receiver locations down a hole.image058

Following picking the raw travel-time data is input into the modeling software and the ray coverage between displayed as a ray density profile (right). This provides an initial indication of the amount of data within different sections of the profile and helps illustrate the resolution of the final model.