Intergrated Well‐Log, VSP, and Surface Seismic Analysis of Near‐Surface Glacial Sediments: Red Lodge, Montana

Abstract

We conducted a series of geophysical surveys to characterize a glacial bench deposit and underlying strata near Red Lodge, Montana. Well logs and VSP data were acquired in a PVC-cased, 115m deep borehole. the multi-offset VSP was undertaken using surface sources (an accelerated weight drop and sledge hammer) with a hydrophone string and downhole, wall-clamping, 3-component geophone. the well logs included measurements of conductivity, radioactivity (gamma ray), temperature, and sonic velocity. Sonic and VSP velocities range from 1500m/s in the very near surface to 3000m/s at 85m depth. A distinct black clay (with high conductivity, high gamma ray, and low velocity) is penetrated at 85m. High-resolution 2D and 3D seismic surveys, using a sledge hammer source, show a number of reflectors to about 150ms two-way traveltime. An 80ms seismic reflection correlates to the 85m interface (via the composite or L-plot with well log, synthetic seismograms, and VSP processed to a corridor stack). Various other reflections in the VSP and surface seismic data are interpreted to represent glacial deposit layers and water zones (from the perforation logs).