High Resolution Seismic Reflection Profiling At Aberdeen Proving Grounds, Maryland

Abstract

The effectiveness of shallow high resolution ‘seismic reflection (i.e., resolution potential) to image geologic interfaces between about 70 and 750 ft at the Aberdeen Proving Grounds, Maryland (APG), appears to vary locally with the geometric complexity of the unconsolidated sediments that overlay crystalline bedrock. The bedrock surface (which represents the primary geologic target of this study) was imaged at each of three test areas on walkaway noise tests and CDP (common depth point) stacked data. Proven high resolution techniques were used to design and acquire data on this survey. Feasibility of the technique and minimum acquisition requirements were determined through evaluation and correlation of walkaway noise tests, CDP survey lines, and a downhole velocity check shot survey. Data processing and analysis revealed several critical attributes of shallow seismic data from APG that need careful consideration and compensation on reflection data sets. This survey determined: 1) the feasibility of the technique, 2) the resolution potential (both horizontal and vertical) of the technique, 3) the optimum source for this site, 4) the optimum acquisition geometries, 5) general processing flow, and 6) a basic idea of the acoustic variability across this site. Source testing involved an accelerated weight drop, land air gun, downhole black powder charge, sledge hammer/plate, and high frequency vibrator. Shallow seismic reflection profiles provided for a more detailed picture of the geometric complexity and variability of the distinct clay sequences (aquatards), previously inferred from drilling to be present, based on sparse drill holes and basewide conceptual models. The seismic data also reveal a clear explanation for the difficulties previously noted in correlating individual, borehole-identified sand or clay units over even short distances. Geologic cross-sections derived from CDP stacked data and borehole logs suggest locally complex geometries and horizontally variable geologic contacts.