Doctor of Philosophy
Chris H Cramer
Mitchell M Withers
Randel T Cox
We examined the utility of an experimental gradiometer array within the context of a phased array using earthquakes and explosion data. These two arrays were fielded during the IRIS Community Wavefields Experiment that was conducted in northern Oklahoma during the summer of 2016. The central idea behind this assessment is to generate wave attribute estimates from the same event using gradiometry and phased array techniques, then appraise the precision of the gradiometer-derived attributes relative to those attribute estimates from the phased array method. The wave attributes estimated from gradiometry are derived from the wavefield and its spatial and temporal gradients; recording instrument and station site errors can introduce uncertainties into the recorded ground motion and its derivatives, and these uncertainties can propagate into the resulting wave attribute estimates from gradiometry. Therefore, it was necessary to calibrate the gradiometer sensors to mitigate the effects of these uncertainties before conducting gradiometry. The instrument calibration exercise premised on the assumption that a teleseismic wavefield should be identical over the compact gradiometer elements, and that the deviations from this assumption in the teleseismic data are due to instrument and site errors. We used teleseismic P and S waves recorded by the experimental gradiometer to calibrate its sensors. We observed that the estimated correction factors were stable over frequency bands where the teleseismic signals are coherent and/or have high signal-to-noise ratio. We also saw notable improvement in the accuracy of the wave attributes estimates from a local earthquake after calibration. Prior to gradiometry (and phased array) analysis, we applied two novel denoising and signal partitioning techniques to both gradiometer and phased array records to remove noise and extract distinct energy packets in the continuous wavelet domain. Following denoising and signal partitioning, we observed improved signal coherence across array elements and we also saw a clearer P wave onset on the horizontal component records. Ultimately, we found that the denosing and signal partitioning exercises are not detrimental to phased array and gradiometry analyses. To appraise the effectiveness of the gradiometer in characterizing seismic signals, we examined gradiometer and phased array records of three local earthquakes that happened during the 2016 Wavefields Experiment. We carried out gradiometry and phased array analyses on the body and surface wave data from these events and compared gradiometry results with their corresponding phased array equivalents. We found back azimuths estimates from gradiometry that disperse by as little as one degree from associated estimates from the phased array considering the same earthquake. Moreover, in some cases, we observed identical phase velocity estimates from using the two array methods to characterize an event. Overall, we saw a good correspondence between the wave attributes estimated from gradiometry and the phased array, which suggests that the gradiometer can be used to detect, locate, and characterize seismic events.
Dissertation or thesis originally submitted to ProQuest
Bolarinwa, Oluwaseyi J., "ASSESSING THE PERFORMANCE OF AN EXPERIMENTAL GRADIOMETER ARRAY" (2022). Electronic Theses and Dissertations. 3204.