High Resolution Seismic Reflection Profiling
at Aberdeen Proving Grounds, Maryland

Richard D. Miller Jianghai Xia Joe M. Anderson David R. Laflen Sara Marcus

SUMMARY
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. Optimum acquisition equipment and parameters were determined through analysis of walkaway noise tests performed at each of the three test areas. Source testing involved an accelerated weight drop, land air gun, downhole black powder charge, sledge hammer/plate, and high frequency vibrator. Proven high resolution techniques (Steeples and Miller, 1990) were used to design and acquire data on this survey. Production data were collected in a standard CDP format (Mayne, 1962) using roll-along acquisition techniques similar to conventional petroleum exploration data acquisition. Shallow seismic reflection enhanced the resolution, accuracy, and reliability of the horizontal extrapolation of onedimensional borehole geology to two-dimensional geologic cross-sections. Interpretations of reflections from unconsolidated lithologic interfaces between the bedrock and ground surface possess varying degrees of resolution and confidence. Reflectors from 70 ft to over 750 ft of depth (depending on the line location) were imaged on data with average practical vertical bed resolution limit of about 15 to 20 ft. CDP stacked sections were correlated with available borehole geology using a combination of stacking velocities and borehole-determined average and interval velocities. Most major clay and sand sequences (i.e., high percentage of clay/sand over at least 20 ft [1/2 wavelength]) identified in boreholes were confidently correlated to coherent reflections on both walkaway data and CDP stacked sections. Shallow seismic reflection profiles provided for a more detailed and realistic picture of the geometric complexity and variability of the distinct clay sequences (aquatards) previously only inferred from drilling to be present based on sparse drill holes and basewide conceptual models (Swartzel and Miller, 1992). 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 (Vroblesky and Fleck, 1991). Geologic cross-sections derived from CDP stacked data and borehole logs suggest locally complex geometries and horizontally variable geologic contacts near the western boundary fence west of the three-mile test track while a much more consistent, uniform subsurface is suggested south of Phillips Airstrip and on Spesutie Island. This seismic reflection feasibility study included three sites, each with uniquely different primary target depths and surface conditions (Figure 1). Feasibility of the technique and minimum acquisition requirements were determined through evaluation and correlation of walkaway noise tests, CDP survey lines (1.2, 0.5, and 0.3 miles), 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. The goals of this survey were to determine: 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. Shallow seismic reflection effectively imaged the subsurface, to varying degrees, at all three test sites at APG. The extensive series of source comparisons performed at each of the three sites generally agree—when considering the non-invasive, total energy, spectral requirements, and imaging goals of the area—a high frequency vibratory source seems most effective in the 70 to 750 ft depth range. With the structural complexity and the short wavelength geometric variability observed along the western boundary it seems prudent to acquire multi-fold CDP data using an end-on source/ receiver orientation. End-on acquisition generally allows more recording channels to be evenly distributed across a given spread length, thus improving trace-to-trace coherency of rapidly changing interfaces as well as improving accuracy of the velocity function. For target depths between 30 and 70 ft, source and geophone spacing should not exceed 4 ft with a minimum source-to-farthest-offset distance of around 100 ft. To image reflectors between 70 and 300 ft of depth the source and receiver station spacing should not exceed 10 ft, with a source-to-farthest-receiver offset of at least 250 ft. For deeper targets, on Spesutie Island for example, source and receiver station spacing should be on the order of 15 ft with a farthest receiver offset of at least 600 ft. The processing flow that optimizes the signal-to-noise and resolution is fairly similar to those used in petroleum exploration, except for the high level of time and effort required for editing and the lack of wavelet suppression. From a basewide perspective, the technique proved successful in imaging the bedrock surface and primary sedimentary contacts between 70 and 750 ft at APG. Site-specific capabilities and specifications of the technique at APG must include consideration of near-surface conditions, geologic target depths, and resolutionrequirements (vertical and horizontal). At the sites tested in this study the theoretical vertical bed resolution limit (frequency and velocity dependent) was about 10 ft with a practical bed resolution limit on the order of 15 or so feet. Horizontal resolution (depth, frequency, and velocity dependent) is proportional to the radius of the first Fresnel zone, which at a depth of around 150 ft is approximately 50 ft. If interpreted reflections from the three sites are representative of acoustic signatures and reflector geometries at APG, seismic reflection programs, if properly planned and executed, could provide extremely useful information to complement existing geologic data and guide placement of future boreholes and monitor wells. Shallow seismic reflection greatly improved the detail and resolution of geologic crosssections as well as substantiated speculative well-to-well correlations on crosssections derived from drilling alone.

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