Electronic Theses and Dissertations

Identifier

126

Date

2010

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Earth Sciences

Concentration

Geophysics

Committee Chair

Charles Langston

Committee Member

Robert Smalley

Committee Member

Mitch Withers

Committee Member

Arch Johnston

Abstract

Calibration coefficients yielding the borehole coupling parameters and corrections to misalignment errors have been calculated for seven borehole Gladwin Tensor Strainmeter (GTSM) instruments, in southern California, associated with the Plate Boundary Observatory (PBO). The calibration coefficients are derived from a linear relationship between the strain field measured in the borehole and the reference strain field derived at the surface. In this study, we derived the reference formation strains (i.e., areal strain, differential normal strain, and shear strain) at the surface from broadband data collected by the Anza Seismic Network in southern California. We find reasonable agreement between the calibration coefficients derived here using seismic data and the calibration coefficients derived elsewhere using theoretical earth tide measurements. We tested whether each strainmeter required different calibration coefficients over time and found no time dependency over a ~2 year period. We also investigated potential instrument orientation errors during the calibration process and determined that the seven borehole GTSM instruments in this study required correction factors ranging from 3 to 50 degrees.Accurate reference formation strains are necessary to ensure accurate calibration of the borehole GTSM instruments. In order to derive accurate measurements of reference strain, we evaluate four different methods for estimating the spatial displacement gradient using broadband, teleseismic data. During these experiments we identified two methods, one derived from spatial gradient analysis in two-dimensions (2D) and one based on plane-wave polynomial interpolation, that appeared to perform well consistently. During the calibration process we combined these two methods in order to further increase the accuracy of our spatial displacement gradient estimates, and therefore the accuracy of our reference strain estimates.Three interesting applications utilizing data collected by a calibrated borehole GTSM instrument co-located with a broadband seismic station and by a dense array of calibrated borehole GTSM instruments are discussed. We find that important characteristics of the seismic wavefield, including the change in geometrical spreading, horizontal phase velocity, and propagation direction, can all be derived at a single location on the surface of the earth using techniques derived from spatial gradient analysis in 1D and 2D.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to the local University of Memphis Electronic Theses & dissertation (ETD) Repository.

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