Numerical Example: Friendswood Crosswell Traveltime Tomography

The traveltime tomography method is now tested on the Friendswood crosswell data set collected by Exxon (Chen et al., 1990) near Friendswood, Texas.

The crosswell experiment geometry (see Figure 10) consists of two boreholes 600 feet apart that extend from the surface to a depth of 1000 feet. There are 96 sources in the cased source well and 98 receivers in the cased receiver well, with source and receiver intervals of approximately 10 feet. However, problems with near-surface velocity anomalies required the elimination of a few near-surface sources and receivers so that only traveltime data from 93 receivers and 97 sources were used for the tomographic reconstructions. The source consisted of a few grams of dynamite at each shot location and the receivers consisted of a string of pressure sensitive hydrophones.

Figure 10 depicts a typical shot gather from the field records having first arrivals with a high signal/noise ratio. Traveltime picking of the 97 shot gathers yielded about 93x97=9021 first-arrival traveltimes.

 

Figure 2.10: (Top) Friendswood crosswell geometry and (Bottom) A shot gather from the Friendswood field experiment after bandpass filtering. Depth is along the horizontal axis and goes from 0 feet to 1000 feet on the right. The upgoing reflections are denoted by B, C, D, and E, and G denotes the direct arrivals recorded across from the shot.

Figure 11 depicts the result of applying the traveltime tomography algorithms to the Friendswood data. The tomogram also contain layer boundaries that are consistent with those shown in the surface reflection record denoted by the traces overlain on the tomogram.

 

Figure 2.11: Velocity tomogram on left and reflection image obtained from CDP data on right (from Chen et al., 1990).