When searching for fossils, it is important not to damage or break apart the fossilized bones while excavating. Sometimes it is difficult to know how to safely dig around the bones. In 1990, when Jurassic Park was published, computer-assisted technology for various functions was still in its infancy. The CAST system described in the novel works similarly to a sonar system:
The question was how large a trench the velocipede fossil required. To decide this, they were using computer-assisted sonic tomography, or CAST. This was a new procedure, in which Thumper fired a soft lead slug into the ground, setting up shock waves that were read by the computer and assembled into a kind of X-ray image of the hillside.
(Crichton, Jurassic Park, Google Books)
By reading sonic shock-waves and using digital computer processing instead of analog receivers to read and interpret the output, it is possible to see how large the fossil is without causing any damage. In fact, similar technology is used every day in the medical field, as computer-assisted tomography, or CAT scanning, which uses electromagnetic waves instead of sonic waves. Since Crichton was a medical doctor, this is likely where he got the idea to incorporate it into fossil-hunting.