Geophysical Measurements of Basalt Intraflow Structures

Publication Date

12-1997

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geophysics

Department

Geosciences

Supervisory Committee Chair

Paul Michaels

Abstract

Basalt is a common constituent in the subsurface throughout much of the Pacific Northwest. As such, the physical properties of basalt are relevant to engineering and hydrogeology in the region. The physical properties of basalt can change dramatically within a single flow and may be associated with changes in intraflow structure. The purpose of this thesis was to make geophysical measurements of a basalt flow and interpret them in terms of available geologic information. It is hoped that these results will be of value in future research to characterize basalt intraflow structures, such as efforts to remotely determine fracture densities and infilling materials.

The basalt of Lucky Peak is a thick flow that exhibits common intraflow structures in outcrop. Geophysical data were collected near the outcrop, and the results of data analysis were compared to the outcrop. The primary geophysical methods used were vertical seismic profiling and cross-borehole ground penetrating radar. Secondary geophysical methods used were transient electromagnetism (TDEM), Schlumberger resistivity, and the measurement of hand sample dielectric permittivity. In addition, petrographic descriptions and measurement of fracture characteristics were performed on core samples, and a magnetic field experiment was conducted to confirm the extent of basalt beneath the study site.

The basalt of Lucky Peak, which originated along the south fork of the Boise River near Smith Prairie, filled the then active river channel and buried an adjacent terrace 20 meters above the active flood plain. Magnetic field strength data show the basalt extends more than 200 meters back from the cliff face just downstream from the Lucky Peak Dam. Visible in outcrop at this location are well developed intraflow structures common to basalt flows throughout the region. Relative elevations of these intraflow structures (flow top, upper colonnade, entablature, and lower colonnade) were measured using a total station survey instrument and converted to absolute elevation above sea level. Elevations of intraflow structures in the subsurface near the outcrop were assumed equal to the elevations in outcrop. Electrical resistivity profiles made using TDEM and Schlumberger methods clearly show the contact between the overlying sediments and the basalt, but they do not show contrasts in resistivity within the basalt flow.

Core was taken from the basalt of Lucky Peak approximately 35 meters back from the cliff face. Three additional boreholes were also drilled at this location. Fracture characteristics determined from core do not show strong correlation with fractures seen in outcrop. Overall fracture density measured on core is greatest in the upper colonnade and lowest in the lower colonnade. Thin sections made from core show increased glass content in the entablature compared to the upper and lower colonnade. Dielectric permittivity measurements on hand samples from core do not show strong correlations with intraflow structure.

Cross-borehole ground penetrating radar data show that the entablature of the basalt of Lucky Peak has lower velocity than does the upper or lower colonnade, although the difference cannot be called statistically significant. Attenuation analysis of the radar data yields contradictory information. Data records reveal lower signal to noise ratios in the entablature compared to the upper and lower colonnade. Amplitude spectra from the records show a nearly complete loss of signal above -50 MHz in the entablature which is not seen in the other intraflow structures. However, the spectral ratio method used for calculating radar attenuation yields results indicating less attenuation in the entablature compared to the upper and lower colonnade. This apparently contradictory result is attributed to the loss of coherent signal within the entablature. The resulting ratio of noise amplitudes is misleading, and the validity of the spectral ratio results is discounted. Based on visual inspection of data records and amplitude spectra, radar wave attenuation is highest in the entablature.

Vertical seismic profile data show that the entablature and lower colonnade have higher seismic velocity (both P- and S-wave) and lower P-wave attenuation than the upper colonnade and flow top (S-wave attenuation calculations were not done). These results correlate with a general decrease in fracture density measured in core from the upper colonnade through the entablature to the lower colonnade. Although the results were not what was expected based on fracture characteristics observed in outcrop, they are consistent in showing high velocity in zones of low attenuation, and low velocity in zones of high attenuation.

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