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.
Full Paper Aberdeen.pdf 19.5 MB