Title

Geohydrologic and Stratigraphic Applications of High-Resolution Seismic Reflection Profiling in Southeast Boise, Idaho

Publication Date

12-1989

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geophysics

Department

Geosciences

Major Advisor

John R. Pelton

Abstract

A high-resolution seismic reflection study was conducted on the Boise River flood plain in southeast Boise in an attempt to laterally map a major sand aquifer and to determine the thickness of the water bearing sedimentary section. Two 12-fold common depth point seismic lines (approximately 350 m of total subsurface coverage) were obtained using a 12-gauge buffalo gun source. Placement of the source beneath the water table (at 2 m depth during data acquisition) provided sufficient energy to recover reflections at two-way times up to 395 ms; the frequency content of reflections recorded on a fixed-source noise test exceeded 100 Hz for travel times up to 360 ms. The aquifer is a sand unit within the late Tertiary Idaho Group sediments and is overlain by a clay/silt unit and underlain by a clay unit as determined from a lithologic log of a nearby water well. The two-way travel-times to the top and bottom of the aquifer as estimated from a travel-time survey of the same water well are 110 and 128 ms; these two-way times correspond to depths of 105 m and 123 m below seismic datum, respectively. The aquifer top is represented by a reflection peak showing moderate continuity, whereas the aquifer bottom is not clearly defined suggesting a low impedance contrast at this boundary. A prominent reflection at approximately 245 ms two-way time probably marks the base of the water-bearing sedimentary section where it overlies a relatively impermeable basalt unit. Based on a stacking velocity range of 1830-2260 m/s, the depth to the top of the basalt is 224-276 m below seismic datum; this depth represents the thickness of the sedimentary section that is viable for cold groundwater exploration in the immediate area of the seismic lines. A deeper reflection at about 325 ms two-way time may represent a sedimentary basalt interbed or the top of the thick rhyolite sequence that is the principle geothermal aquifer of Boise. Two other reflections at approximately 370 ms and 395 ms two-way times may represent alternating sedimentary and silicic volcanic layers. Fourteen Generalized Reciprocal Method refraction profiles were shot along one line to determine the average thickness (2.7 m) and average velocity (305 m/s) of the low-velocity layer for use in the initial static correction routine. These data were also used to analyze the relationship between variability in the low-velocity layer and an observed long-wavelength (about 60 m) static shift (approximately 4 ms). The results indicate that the static shift could be caused by a combination of lateral velocity variations within the low-velocity layer and an irregular low-velocity layer thickness.

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