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Papers by Gregg Nordquist

We developed a Bayesian model to invert magnetotelluric (MT) data using a 2D sharp boundary param... more We developed a Bayesian model to invert magnetotelluric (MT) data using a 2D sharp boundary parameterization. We divided the 2D cross section into layers and considered the locations of interfaces and resistivity of the regions formed by the interfaces as random variables. We assumed that those variables are independent in the vertical direction and dependent along the lateral direction, whose spatial dependence is described by either pairwise difference or multivariate Gaussian priors. We used a parallel, adaptive finite-element algorithm to rapidly forward simulate frequency-domain MT responses of the 2D resistivity structure and used Markov chain Monte Carlo methods to draw many samples from the joint posterior probability distribution. We applied the Bayesian model to a synthetic case that mimics a geothermal exploration scenario. Our results demonstrated that the developed method is effective in estimating the resistivity and depths to interfaces and in quantifying uncertainty on the estimates. We also applied the developed method to the field MT data collected from the Darajat geothermal site in Indonesia. We compared our inversion results with those obtained from a deterministic inversion of 3D MT data; they are consistent even if the two inversion methods are very different and the amount of information used for inversion is different.

SEG Technical Program Expanded Abstracts 2010, 2010

Stochastic approaches for inverting geophysical data have many advantages over deterministic inve... more Stochastic approaches for inverting geophysical data have many advantages over deterministic inversion methods in terms of finding global solutions and quantifying their associated uncertainty. Since stochastic methods often need to run forward models more times than deterministic algorithms, their applications currently are limited to 1D inverse problems, where forward models can be run very fast. However, due to the rapid growth in computing power, especially parallel computing techniques, and the recent development of new numerical simulation methods, stochastic inversion of 2D geophysical data has become feasible. In this study, we explore the use of Markov chain Monte Carlo based Bayesian models for inverting 2D magnetotelluric (MT) data. To minimize the number of parameters, we adopt sharp boundary parameterization, in which we consider the locations and the resistivity of regions formed by the interfaces as unknowns. We use a parallel, adaptive finite-element algorithm to forward simulate frequency-domain MT responses of 2D conductivity structure. Synthetic case studies show the developed stochastic model is effective for estimating the interface locations and resistivity; most importantly, it provides detailed uncertainty information on each unknown parameter.

SEG Technical Program Expanded Abstracts 2010, 2010

An extensive magnetotelluric (MT) survey comprised of 85 sites has been acquired over the Darajat... more An extensive magnetotelluric (MT) survey comprised of 85 sites has been acquired over the Darajat geothermal field in Indonesia to map the geothermal reservoir and the overlying clay cap. The rouged topography and the geometry of the margin of the clay cap makes the ...

SEG Technical Program Expanded Abstracts 2000, 2000

Geophysical exploration for geothermal resources: An application for combined MT‐TDEM. [SEG Techn... more Geophysical exploration for geothermal resources: An application for combined MT‐TDEM. [SEG Technical Program Expanded Abstracts 19, 1071 (2000)].William Cumming, Gregg Nordquist, Doddy Astra.

Geothermics, 2004

Sixteen precision gravity surveys carried out since 1979 provide an additional constraint for num... more Sixteen precision gravity surveys carried out since 1979 provide an additional constraint for numerical simulation of the Bulalo system. The gravity surveys were all run using the same LaCoste and Romberg gravity meter (D-33). Leveling surveys collected in conjunction with the gravity surveys used Wild N3 spirit levels. The leveling surveys were used to correct the gravity data for elevation changes. Since production began in 1979, the Bulalo field has experienced a gravity change of up to À600 microgal. The decline in gravity is interpreted to result from density changes in the reservoir, primarily due to the development of a two-phase zone where liquid saturation is lower in both the rock matrix and its fracture network. An accurate match of the trends and magnitudes of observed and simulated gravity changes provides a constraint for the mass balance of the numerical model beyond the matching of pressure and enthalpy evolution. Because of internal flow effects in most of the wells, pressure and temperature measurements do not necessarily represent static formation pressure and temperatures. Gravity data, therefore, became particularly important for calibrating the net depletion of the numerical model and for constraining reservoir mass replacement from reinjection and natural inflow from aquifers outside the production zone. Early comparisons of the observed and simulated gravity showed a too rapid gravity decrease prior to 1992, and a too slow gravity decrease after that time. Subsequent changes made to the numerical model, including modifying permeability structure and in-flow direction, resulted in an improved match to the gravity and available pressure data. The result has been a numerical model that accurately matches not only the quantity of net mass withdrawal, but also the location of the mass withdrawal including recharge from outside the producing reservoir.

Geothermics, 2008

The Awibengkok (Salak) geothermal system is a liquid-dominated, fracture-controlled reservoir wit... more The Awibengkok (Salak) geothermal system is a liquid-dominated, fracture-controlled reservoir with benign chemistry and low-to-moderate non-condensable gas content. The geothermal system is hosted mainly by andesitic-to-rhyodacitic rocks, and floored by Miocene marine sedimentary rocks cut by igneous intrusions. The volcanic sequence is capped by an 8400-year-old phreatic explosion breccia, rhyolite fallout tuff (>8400 years and <40,000 years), rhyolite lavas, domes

Geophysics, 2012

We developed a Bayesian model to invert magnetotelluric (MT) data using a 2D sharp boundary param... more We developed a Bayesian model to invert magnetotelluric (MT) data using a 2D sharp boundary parameterization. We divided the 2D cross section into layers and considered the locations of interfaces and resistivity of the regions formed by the interfaces as random variables. We assumed that those variables are independent in the vertical direction and dependent along the lateral direction, whose spatial dependence is described by either pairwise difference or multivariate Gaussian priors. We used a parallel, adaptive finite-element algorithm to rapidly forward simulate frequency-domain MT responses of the 2D resistivity structure and used Markov chain Monte Carlo methods to draw many samples from the joint posterior probability distribution. We applied the Bayesian model to a synthetic case that mimics a geothermal exploration scenario. Our results demonstrated that the developed method is effective in estimating the resistivity and depths to interfaces and in quantifying uncertainty on the estimates. We also applied the developed method to the field MT data collected from the Darajat geothermal site in Indonesia. We compared our inversion results with those obtained from a deterministic inversion of 3D MT data; they are consistent even if the two inversion methods are very different and the amount of information used for inversion is different.

Journal of Volcanology and Geothermal Research, 1991

Sorey, M.L., Suemnicht, G.A., Sturchio, N.C. and Nordquist, G.A., 1991. New evidence on the hydro... more Sorey, M.L., Suemnicht, G.A., Sturchio, N.C. and Nordquist, G.A., 1991. New evidence on the hydrothermal system in Long Valley caldera, California, from wells, fluid sampling, electrical geophysics, and age determinations of hot-spring deposits. J. Volcanol. Geotherm. Res., 48: 229-263.

We developed a Bayesian model to invert magnetotelluric (MT) data using a 2D sharp boundary param... more We developed a Bayesian model to invert magnetotelluric (MT) data using a 2D sharp boundary parameterization. We divided the 2D cross section into layers and considered the locations of interfaces and resistivity of the regions formed by the interfaces as random variables. We assumed that those variables are independent in the vertical direction and dependent along the lateral direction, whose spatial dependence is described by either pairwise difference or multivariate Gaussian priors. We used a parallel, adaptive finite-element algorithm to rapidly forward simulate frequency-domain MT responses of the 2D resistivity structure and used Markov chain Monte Carlo methods to draw many samples from the joint posterior probability distribution. We applied the Bayesian model to a synthetic case that mimics a geothermal exploration scenario. Our results demonstrated that the developed method is effective in estimating the resistivity and depths to interfaces and in quantifying uncertainty on the estimates. We also applied the developed method to the field MT data collected from the Darajat geothermal site in Indonesia. We compared our inversion results with those obtained from a deterministic inversion of 3D MT data; they are consistent even if the two inversion methods are very different and the amount of information used for inversion is different.

SEG Technical Program Expanded Abstracts 2010, 2010

Stochastic approaches for inverting geophysical data have many advantages over deterministic inve... more Stochastic approaches for inverting geophysical data have many advantages over deterministic inversion methods in terms of finding global solutions and quantifying their associated uncertainty. Since stochastic methods often need to run forward models more times than deterministic algorithms, their applications currently are limited to 1D inverse problems, where forward models can be run very fast. However, due to the rapid growth in computing power, especially parallel computing techniques, and the recent development of new numerical simulation methods, stochastic inversion of 2D geophysical data has become feasible. In this study, we explore the use of Markov chain Monte Carlo based Bayesian models for inverting 2D magnetotelluric (MT) data. To minimize the number of parameters, we adopt sharp boundary parameterization, in which we consider the locations and the resistivity of regions formed by the interfaces as unknowns. We use a parallel, adaptive finite-element algorithm to forward simulate frequency-domain MT responses of 2D conductivity structure. Synthetic case studies show the developed stochastic model is effective for estimating the interface locations and resistivity; most importantly, it provides detailed uncertainty information on each unknown parameter.

SEG Technical Program Expanded Abstracts 2010, 2010

An extensive magnetotelluric (MT) survey comprised of 85 sites has been acquired over the Darajat... more An extensive magnetotelluric (MT) survey comprised of 85 sites has been acquired over the Darajat geothermal field in Indonesia to map the geothermal reservoir and the overlying clay cap. The rouged topography and the geometry of the margin of the clay cap makes the ...

SEG Technical Program Expanded Abstracts 2000, 2000

Geophysical exploration for geothermal resources: An application for combined MT‐TDEM. [SEG Techn... more Geophysical exploration for geothermal resources: An application for combined MT‐TDEM. [SEG Technical Program Expanded Abstracts 19, 1071 (2000)].William Cumming, Gregg Nordquist, Doddy Astra.

Geothermics, 2004

Sixteen precision gravity surveys carried out since 1979 provide an additional constraint for num... more Sixteen precision gravity surveys carried out since 1979 provide an additional constraint for numerical simulation of the Bulalo system. The gravity surveys were all run using the same LaCoste and Romberg gravity meter (D-33). Leveling surveys collected in conjunction with the gravity surveys used Wild N3 spirit levels. The leveling surveys were used to correct the gravity data for elevation changes. Since production began in 1979, the Bulalo field has experienced a gravity change of up to À600 microgal. The decline in gravity is interpreted to result from density changes in the reservoir, primarily due to the development of a two-phase zone where liquid saturation is lower in both the rock matrix and its fracture network. An accurate match of the trends and magnitudes of observed and simulated gravity changes provides a constraint for the mass balance of the numerical model beyond the matching of pressure and enthalpy evolution. Because of internal flow effects in most of the wells, pressure and temperature measurements do not necessarily represent static formation pressure and temperatures. Gravity data, therefore, became particularly important for calibrating the net depletion of the numerical model and for constraining reservoir mass replacement from reinjection and natural inflow from aquifers outside the production zone. Early comparisons of the observed and simulated gravity showed a too rapid gravity decrease prior to 1992, and a too slow gravity decrease after that time. Subsequent changes made to the numerical model, including modifying permeability structure and in-flow direction, resulted in an improved match to the gravity and available pressure data. The result has been a numerical model that accurately matches not only the quantity of net mass withdrawal, but also the location of the mass withdrawal including recharge from outside the producing reservoir.

Geothermics, 2008

The Awibengkok (Salak) geothermal system is a liquid-dominated, fracture-controlled reservoir wit... more The Awibengkok (Salak) geothermal system is a liquid-dominated, fracture-controlled reservoir with benign chemistry and low-to-moderate non-condensable gas content. The geothermal system is hosted mainly by andesitic-to-rhyodacitic rocks, and floored by Miocene marine sedimentary rocks cut by igneous intrusions. The volcanic sequence is capped by an 8400-year-old phreatic explosion breccia, rhyolite fallout tuff (>8400 years and <40,000 years), rhyolite lavas, domes

Geophysics, 2012

We developed a Bayesian model to invert magnetotelluric (MT) data using a 2D sharp boundary param... more We developed a Bayesian model to invert magnetotelluric (MT) data using a 2D sharp boundary parameterization. We divided the 2D cross section into layers and considered the locations of interfaces and resistivity of the regions formed by the interfaces as random variables. We assumed that those variables are independent in the vertical direction and dependent along the lateral direction, whose spatial dependence is described by either pairwise difference or multivariate Gaussian priors. We used a parallel, adaptive finite-element algorithm to rapidly forward simulate frequency-domain MT responses of the 2D resistivity structure and used Markov chain Monte Carlo methods to draw many samples from the joint posterior probability distribution. We applied the Bayesian model to a synthetic case that mimics a geothermal exploration scenario. Our results demonstrated that the developed method is effective in estimating the resistivity and depths to interfaces and in quantifying uncertainty on the estimates. We also applied the developed method to the field MT data collected from the Darajat geothermal site in Indonesia. We compared our inversion results with those obtained from a deterministic inversion of 3D MT data; they are consistent even if the two inversion methods are very different and the amount of information used for inversion is different.

Journal of Volcanology and Geothermal Research, 1991

Sorey, M.L., Suemnicht, G.A., Sturchio, N.C. and Nordquist, G.A., 1991. New evidence on the hydro... more Sorey, M.L., Suemnicht, G.A., Sturchio, N.C. and Nordquist, G.A., 1991. New evidence on the hydrothermal system in Long Valley caldera, California, from wells, fluid sampling, electrical geophysics, and age determinations of hot-spring deposits. J. Volcanol. Geotherm. Res., 48: 229-263.