High-Resolution Seismic Imaging of Shallow Sedimentary/Volcanic Interfaces Beneath the Western Snake River Plain Near Boise, Idaho
Type of Culminating Activity
Master of Science in Geophysics
John R. Pelton
High-resolution seismic methods were used in two separate studies to map shallow sedimentary/volcanic interfaces beneath the western Snake River Plain (WSRP). The first investigation took place near Orchard, Idaho at a site known locally as Orchard Ranch. Severe attenuation of seismic energy in dry semi-consolidated sediments is characteristic of this part of the WSRP. An experimental north-south refraction line 920-m long consisting of nine geophone spreads was completed at Orchard Ranch. Data were acquired using a Bison Instruments model 8024 fixed-gain, 8-bit, 24-channel portable engineering seismograph with 8 Hz geophones. The seismic source consisted of two pounds of Kinestik explosive buried in 1.8-m auger holes and detonated with an Atlas Staticmaster blasting cap. Shotholes were drilled with a hand-held auger at 0 m, 50 m, and 100 m in-line offsets at both ends of each spread. Thus, reversed data coverage was acquired for Reciprocal Method (RM) and Generalized Reciprocal Method (GRM) analyses.
One well at each end of the 920-m long profile provided vertical control for the refraction survey and allowed examination of errors in the calculated results. The Orchard Ranch well located at the south end of the line is open to 98 m depth, and the PVC-cased well at the north end is open to 41 m depth. Natural gamma ray logging in the two wells resulted in precise identification of the sediment/basalt interface which underlies the area at about 40 m depth and represents the target refractor for the study.
RM analysis was applied to phantomed first-arrival data with forced depth ties between adjacent spreads. Reliance on first layer velocity determinations was eliminated by calibrating the depth conversion factors for the two end spreads with the known basalt depths. Four spreads (1 through 4) at the south end of the line were processed from south to north, and four spreads (5 through 8) at the north end were processed from north to south. The depth discrepancy created at the middle of the line by the bi-directional analysis was 4.2 m or 13% of the total average depth. Spread 9 was shot as a southern extension of the original line to help explain anomalous refraction data observed south of the Orchard Ranch well; these data suggest a sudden truncation in the basalt mass or a sudden increase in its depth immediately south of the Orchard Ranch well. The first-arrival data were also processed using the GRM with inferred reciprocal times for far offsets. The GRM analysis was done independently for each spread without forced depth ties between adjacent spreads and without calibration of the depth conversion factors. A personal computer software package called GREMIX was used to complete the analysis.
Both the RM and GRM profiles show a slightly irregular basalt surface with a gentle dip from north to south. The depth to basalt given by the GRM is deeper than the true depth obtained from gamma ray logging. The GRM-calculated depths at the Orchard Ranch and PVC wells are 19% and 13% greater than true depth, respectively. The GRM-calculated depths are also consistently greater than the RM-calculated depths with a typical discrepancy of less than 4 m. This may be the result of inaccurate (too large) first-layer apparent velocities used in the GRM analysis. Undulations in the RM and GRM profiles agree very well although there is no independent confirmation that the undulations are real.
An additional refraction experiment was conducted at Orchard Ranch using a trailer-mounted drill rig to obtain data from larger charges in deeper shotholes. One experimental spread lying approximately parallel to the original line was shot with shothole depths ranging from 4.1 to 6.7 m and with in-line offsets of 0 m and 100 m. Four to twelve pounds of Kinestik were buried in the drill holes and tamped with soil and water. Data acquired in this manner showed significant improvement over the previous data, but the drawback was a considerable cost increase to provide adequate drilling capabilities and explosives.
The second seismic study involved testing the use of high-resolution common-depth-point (CDP) reflection profiling to map fault offsets in the Boise Warm Springs geothermal aquifer. The study site was Quarry View Park in northeast Boise, Idaho which lies at the northeastern margin of the WSRP. A zone of northwest-trending normal faults strongly influences the location and production of deep geothermal wells in the area. The geothermal aquifer consists of fractured rhyolitic volcanic rocks which are overlain by basaltic tuffs or flows. The volcanic sequence is then overlain by lacustrine and fluvial Idaho Group sediments. Some depth information and limited correlation between subsurface units is provided by lithologic interpretations (from driller's notes) and natural gamma ray logging in the Quarry View Park and Boise Warm Springs Water District #3 deep geothermal wells. The two wells are open to 221 m and 179 m depth, respectively. A significant seismic impedance contrast between the overlying sediments and underlying volcanics was expected to provide strong seismic reflections.
CDP reflection data were acquired along two northeast- trending lines running perpendicular to the fault zone. Each line was approximately 200 m in length with shot and geophone stations spaced at 2.5 m intervals, a CDP interval of 1.25 m, and a 12-fold horizontal stacking redundancy. The same Bison Instruments model 8024, 24-channel seismograph used at Orchard Ranch was used in this study. A 12-gauge buffalo gun source was used along with high- pass, low-cut filtering and 100 Hz geophones to help preserve the critical high-frequency energy above 100 Hz. Standard CDP processing was applied to the data.
The results of the study show that subsurface structural information in the depth range of 60 to 260 m can be obtained by using CDP reflection profiling in the Quarry View Park area. Two normal growth faults with an estimated total offset of 130 m were observed on the seismic sections from line 1. A strong reflection estimated to be 252 m deep based on stacking velocities is a prominent feature on the seismic sections and is clearly offset by the two normal growth faults. Precise correlation between the reflection and a specific subsurface interface is hindered by the lack of a reliable well travel-time survey. It is inferred, however, that the reflection most likely represents the top of a basalt unit which is thick relative to an overlying basalt unit detected by natural gamma logging in the Quarry View Park well. Another possibility is that the reflection represents the top of the rhyolitic geothermal aquifer. In either case, the seismic reflection data support the hypothesis that hydraulic continuity of the geothermal aquifer is disrupted by faulting.
It was not possible to place the buffalo gun shots beneath the 10-m deep water table at Quarry View Park with our existing drilling equipment. Saturated conditions in the swamp north of the park and on the park lawn greatly improved the quality of the reflection data. Shotholes placed on the dry hillside north of the swamp resulted in very severe high-frequency seismic wave attenuation (most evident on the line 2 data).
Howarth, Robert Brett, "High-Resolution Seismic Imaging of Shallow Sedimentary/Volcanic Interfaces Beneath the Western Snake River Plain Near Boise, Idaho" (1989). Boise State University Theses and Dissertations. 462.