Abstract

Surface and borehole seismic imaging techniques delineate the subsurface expression of an active sinkhole above the former salt mine at Weeks Island, Louisiana, which was converted for use by the United States Department of Energy’s Strategic Petroleum Reserve. The sinkhole, which originally was ~12 m wide and 11 m deep, is directly over the edge of the upper storage chamber and in an area where the salt dome top is 60 m below ground surface. Surface seismic reflections detect a dramatic bowl-shaped depression in a 28-m-deep reflector directly adjacent to and centered on the sinkhole. A vertical seismic profile (VSP) provides time-to-depth relationships and assists correlation of reflections with drill-confirmed geologic contacts. Two reflectors (28 and 60 m) interpreted on multichannel VSP data, represent the only velocity and/or density contrasts detected between ground surface and just beneath the salt dome top. The 28-m reflector identified on both VSP and surface seismic reflection data is depth consistent with the piezometric surface. However, because of the high measured permeability and the relative severity of depression in the reflector, it is questionable whether the 28-m reflection on surface seismic data originates at the water table. The 60-m salt reflection, evident on VSP, can be interpreted on selected processed surface seismic shot gathers but is difficult to identify confidently on common depth point stacked sections. The sinkhole lies along a northeast-trending acoustic lineament, possibly related to or associated with salt dissolution. The acoustic expression of the sinkhole suggests a localized, predominantly vertical feature. No evidence was discovered to ascertain confidently the mechanism (i.e., fractures from mine activities, shear zone associated with uplift, etc.) responsible for exposuring the salt to unsaturated meteoric water.