Azimuthal Seismic First-Arrival Tomography as a Proxy for Hydraulically Conductive Subsurface Fracture Networks

The Oak Ridge Field Research Center (ORFRC) was established by the Environmental Sciences Division (ESD) at the Oak Ridge National Laboratory (ORNL) in Oak Ridge, TN, in order to study the various biogeochemical processes involved in the remediation as well as natural attenuation of a large contaminant plume that is extant in the vicinity of the ORIFRC. A part of this work has been to characterize the movement of this groundwater/contaminant plume with the use of azimuthal seismic first-arrival tomography (ASFT).

Within the general area of the ORIFRC, a 0-2 m layer of generally isotropic anthropogenic fill and unconsolidated soil overlies the deeper structural elements caused by the folding that formed the Valley and Ridge region of East Tennessee. Beneath this layer of fill, a fractured shale transition zone from saprolite to competent bedrock exists. It is suspected that this fracture network forms anisotropic flow conditions where contaminants exist beneath the surface layers.

In an effort detect fracture-driven hydrologic anisotropy, we have collected surface SFT profiles at 10° intervals around a central point at the NT-2 site at ORNL. Each seismic survey consisted of a 96-channel survey with a 0.5 m offset, and shot points located at every fourth receiver along the line. The resultant tomograms were converted from XZ plane cross-sections to XY plane cross-sections. The resultant map-view velocity profiles showed a dramatic decrease in seismic isotropy with depth, as well as delineating the saprolite/bedrock transition zone at the NT-2 site. Two additional datasets have been collected approximately one half kilometer (Km-1 site) and one kilometer (Km-2 site) down valley from NT-2. Both of these datasets agree with the direction and degree of anisotropy present at the NT-2 site, and both were able delineate the transition between saprolite and competent bedrock, underscoring the efficacy and replicability of this experimental method. All of these datasets were compared to measured fracture set orientations in trenched saprolitic shale as well as measured hydrologic anisotropy with positive results in order to establish the accuracy of ASFT relative to conventional methods of hydrologic testing.