Detecting Cavities With Seismic Refraction Tomography: Can It Be Done?

Conventional seismic refraction analyses (e.g. delay-time or generalized reciprocal method)<br>generally perform poorly for imaging many karst-related features because first-arriving seismic waves<br>can generally circumvent the low-velocity target of interest without a major impact on travel time,<br>particularly where the target is three-dimensional. We are using synthetic models and field data to<br>evaluate the performance of seismic refraction tomography codes for karst detection. Two-dimensional<br>karst models have been created and used to generate synthetic travel time data for tomography analysis.<br>The results of this analysis suggest that the presence of a feature can sometimes be determined, but the<br>velocity is not accurately represented. In no synthetic case that we have tested has the velocity of the<br>cavity been an accurate approximation to the true low velocity. In contrast to the inconclusive synthetic<br>results, we have refraction tomography data that clearly indicate karst features. A refraction tomography<br>data set collected on the Oak Ridge Reservation shows a well-defined low velocity feature that crosses<br>three parallel lines along an axis that extends more than 200 meters. The feature that was detected has<br>velocities of about 1500-2000 m/s in a matrix of 3000-4000 m/s, reasonable velocities for a mud filled<br>void in saprolite at these depths, rather than having a velocity slightly lower than the matrix velocity, as<br>would be expected from our synthetic study. This suggests that the synthetic models are incomplete or<br>inaccurate. In one case, matrix smoothing of the synthetic model before generation of synthetic<br>traveltimes generates synthetic results that better match field results.