High Resolution Seismic Reflection Survey to Map Bedrock and
Glacial/Fluvial Layers at the U.S. Navy Northern Ordnance Plant (NIROP) in Fridley,
Minnesota
Richard D. Miller Jianghai Xia
Abstract
Shallow seismic reflection in conjunction with downhole velocity profiles and
local borehole data allowed delineation of discrete layering within the approximately
120 ft thick glacial drift that overlays the St. Peters Sandstone and/or the
Prairie du Chien dolomites beneath the Northern Ordnance Plant, currently the
Naval Industrial Reserve Ordnance Plant (NIROP) in Fridley, Minnesota. The primary
goals of this study were to determine the feasibility and limitations of the
technique and to develop a continuous subsurface image of major acoustic impedance
contrasts related to geologic/hydrologic features. Establishing feasibility
includes determining the horizontal and vertical resolution potential, optimum
acquisition geometries and parameters, best suited equipment for surface and
subsurface conditions, level of effort necessary to delineate the geologic/hydrologic
features of interest, and to establish a reasonable set of expectations for
the technique across the entire facility. Shallow seismic reflection profiles
allowed delineation of discontinuous confining units within the glacial drift
at this site. A high confidence reflection profile should greatly improve the
accuracy of hydrologic flow models in areas with significant lateral variability.
The glacial drift that covers the entire site consists of till, outwash, valley
train, and lake deposits. The goals of this survey necessitated the acquisition,
processing, and interpretation of a walkaway VSP, three walkaway noise tests,
and 1,441 shotpoints of 24-fold CDP data on three different lines. The uphole
survey provided excellent ground truth as well as improved event identification
and verification through curve modeling. The three walkaway noise tests permitted
selection of optimum acquisition geometry and equipment for the CDP profiles
and provided a measure of the strengths and limitations of shallow reflection
in the various near-surface settings around the site. The CDP profiles collected
along the western boundary fence of NIROP, southern extreme of the North 40,
and southwest corner of the enclosed facility all possess sufficient resolution
potential to image layers as shallow as 20 ft, discontinuities in the upper-drift
confining unit, the bedrock (sandstone), and dolomite/sandstone contact. The
dramatic change in interval velocities at the water table provided unique challenges
to the generation of an accurate time section. The data were collected during
two field outings: one between the 19th and 22nd of April and the other on September
28th and 29th. The walkaway noise testing and check shot surveys were designed
and executed to allow evaluation of acoustic signature, optimum acquisition
equipment and parameters, near-surface velocity structure, horizontal consistency
in reflection character, general resolution potential, signal-to-noise ratio,
and impact of cultural noise (i.e., jet aircraft, industrial facility, vehicle
traffic, etc.). The walkaway noise test guided the definitive selection of equipment
and parameters as well as optimum station spacing and recording geometries.
The check shot survey (downhole velocity/one-way travel time) established an
approximate velocity structure for the upper 100 ft of sediments in well 13-D.
The check shot surveys were necessary to correlate drill/log defined geology
with reflections interpreted on CDP stacked sections and allow reflection verification
on shot gathers. Several of the shot gathers from site #1 suggest practical
vertical bed resolution on the order of 5 ft. It is also clear from the walkaway
noise tests that the confining layer(s) and bedrock vary several feet vertically
across the length of a spread (192 ft). This evaluation/ feasibility study was
designed to allow analysis of acoustic characteristics, and more generally,
the reflection method, which in turn permitted accurate estimations of resolution
and optimization of acquisition equipment and parameters. With the primary focus
of the CDP production survey being delineation of the upper-drift confining
unit and bedrock surface, resolution potential is a critical characteristic.
Most shallow reflection wavelets recorded during this study possess dominant
frequencies in excess of 200 Hz with some upwards of 240 Hz. Based on the downhole
survey in well D-13, the top and bottom of the borehole encountered confining
units can be separated by as little as 1 msec (4 ft). Theoretically, resolving
top and bottom of a bed this thin requires dominant frequencies in excess of
250 Hz, however in practice the reflection frequency band would need an upper
corner of around 400 Hz to permit a confident interpretation. Simply identifying
a particular layer as present, however, is easily possible with frequencies
as low as 100 Hz. Delineating the top and bottom of a bed separated by at least
10 ft is well within the resolution potential of this data set. Using existing
drill data around the NIROP site and the walkaway and downhole data as guides,
detection of the confining layer and bedrock, when present, is possible across
the entire site with the exception of areas contaminated by facility noise and/or
less than 50 ft from surface structures. The 24-fold CDP profiles contained
in this report possess a horizontal resolution on the order of 15 ft at 50 ft
and a practical vertical resolution potential of around 5 ft.
Full Paper KGS-97-12 9.88 MB