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

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.

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