VOID DETECTION USING SEISMIC REFRACTION TOMOGRAPHY

We use synthetic models and field data to evaluate the performance of seismic refraction tomography codes detecting voids. The results obtained from two-dimensional modeling suggest that the presence of a cavity can sometimes be determined, but the inversion yields a feature with velocities only slightly lower than that of the surrounding volume.

A refraction tomography data set collected on the Oak Ridge Reservation shows a well-defined low velocity feature that crosses three parallel lines along an axis that extends more than 200 meters. This feature has velocities of about 1500-2000 m/s in a matrix of 3000-4000 m/s, reasonable velocities for a mud filled void in saprolite at these depths. These results suggest that the synthetic models are incomplete or inaccurate.

Two groups of data are examined in this study. The first is a set of synthetic traveltimes generated from 2-D models using the refraction tomography code GeoCT-II (version 2.3) (GeoTomo, LLC). The synthetic models allow us to have a “reference” model with which to compare the results generated by SRT using another refraction tomography code Rayfract™ (version 2.51) (from Intelligent Resources Inc).

No synthetic model will ever be a completely accurate depiction of the real subsurface because it is comprised of discrete units, which are further broken down into small constant velocity grids. This means that however carefully constructed and applied, numerical analysis is based upon simplified and digitized representations of physical laws and models. Field testing complements the synthetic modeling by providing a real life basis for determining model validity. For this we used a set of refraction tomography profiles collected in support of the NABIR Field Research Center (FRC) in Oak Ridge, TN.