Joint 1D inversion of TEM and MT data and 3D inversion of MT data in the Hengill area, SW Iceland

Árnason, Knútur; Eysteinsson, Hjálmar; Hersir, Gylfi Páll

doi:10.1016/j.geothermics.2010.01.002

Cited by 130 publications

(69 citation statements)

References 31 publications

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“…At both Krafla and Hengill, auxiliary TEM data sets have been measured in close proximity to the MT soundings, enabling a static shift correction. These TEM data have been used to correct MT data for 3D inversions in Hengill (Árnason et al, 2010;Rosenkjaer, 2011) and for 1D inversion in Krafla (Friðleifsson et al, 2014). Rosenkjaer (2011) discusses the use of TEM-static-shift-corrected data, both for a synthetic geothermal system with near surface resistivity inhomogenities and for MT data from the Hengill area.…”

Section: Magnetotelluric Methodsmentioning

confidence: 99%

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Comparison of 3D MT inversions for geothermal exploration: Case studies for Krafla and Hengill geothermal systems in Iceland

et al. 2015

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a b s t r a c tThe magnetotelluric (MT) method is important for exploration of geothermal systems. The information on the Earth's resistivity obtained with MT methods has been valuable in imaging the hydrothermal alteration of such systems. Given its ability to recover complex resistivity models for the Earth, threedimensional (3D) MT inversion has become a common practice in geothermal exploration. However, 3D inversion is a time-consuming and complicated procedure that relies on computer algorithms to search for a model that can explain the measured data to a sufficient level. Furthermore, many elements of inversion require input from the practitioner, which can easily bias the results. Consequently, final 3D MT results depend on various factors, including the inversion code, the model mesh used to represent the Earth, data quality and processing, and constraints imposed during the inversion procedure.In this paper, to explore how this variability in 3D MT modeling impacts the final model, we invert MT data sets from the Krafla and Hengill geothermal areas in Iceland, using three different inversion codes. In each case, the modelers had the freedom to select a subset of the data and implement the inversion for the respective code in an optimized way. We compare the results from all the inversion codes, as well as consider the setup and assumptions made during the inversion process, all of which helps enhance the robustness and quality of the results. The comparison is done in multiple ways, using visual comparison of the recovered resistivity models, as well as comparing the structural similarities of the models by employing a structural correlation metric based on cross-gradients and other types of metrics for structural correlation. This approach highlights structures that are common in all three models, and implies that these structures are independent of the inversion code and necessary to fit the data.All modeling results from both Krafla and Hengill are consistent to first order, recovering a conductive layer on top of a resistive core typical of high temperature geothermal systems. For Hengill, the models show strong structural agreement, with all inversions recovering a moderately layered resistivity model but adding detail to previous work done in the area. Major differences are found in areas with coarse data coverage and hence questionable model resolution. Where the recovered structures in different models coincide, our confidence that these structures are well-constrained by the data is elevated, in spite of the different setup and assumptions in the codes these structures are required; so they can be interpreted in terms of geology with more certainty. Results from Krafla are not as consistent as results for Hengill, related in part to the Krafla data being nosier than the Hengill data. The models from Krafla have coinciding larger structures, but small-scale structures there are less coherent. One of the consistent structures in all the models is a conductive zone reaching from a depth of 5 km to...

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Section: Magnetotelluric Methodsmentioning

confidence: 99%

“…MT imaging of the geological structures and hydrothermal alteration zones that characterize the geothermal system (Árnason et al, 2010;Heise et al, 2008;Newman et al, 2008).…”

Section: Introductionmentioning

confidence: 99%

Comparison of 3D MT inversions for geothermal exploration: Case studies for Krafla and Hengill geothermal systems in Iceland

et al. 2015

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“…MT stations can only be sited within a broad corridor about the profile line, and must be projected onto the profile (e.g., Ingham et al 2009;Á rnason et al 2010;Heise et al 2010). 2-D inversion is possible only if the data are projected on the same profile (e.g., Hill et al 2009;Ingham et al 2009).…”

Section: When Is 3-d Inversion Useful? (Advantages Of 3-d Inversion Omentioning

confidence: 99%

“…3-D MT inversions have been used in various studies such as ore deposit exploration (e.g., Tuncer et al 2006;Farquharson and Craven 2008;Türkoglu et al 2009;Xiao et al 2010;Goldak and Kosteniuk 2010), waste characterization (e.g., Newman et al 2003), tectonic studies (e.g., Uyeshima 2007;Patro and Egbert 2008), volcano studies (e.g., Spichak et al 2007;Heise et al 2008Heise et al , 2010Jones et al 2008;Hill et al 2009;Ingham et al 2009), hydrocarbon exploration (e.g., Mackie and Watts 2007;Hautot and Tarits 2009;Zhanxiang et al 2010) and geothermal studies (e.g., Han et al 2008;Newman et al 2008;Á rnason et al 2010;Cumming and Mackie 2010). Improvements in 3-D inversion are likely to continue as long as MT data are still being acquired.…”

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confidence: 99%

Three-Dimensional Magnetotelluric Inversion: An Introductory Guide for Developers and Users

Siripunvaraporn

2011

Surv Geophys

116

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In the last few decades, the demand for three-dimensional (3-D) inversions for magnetotelluric data has significantly driven the progress of 3-D codes. There are currently a lot of new 3-D inversion and forward modeling codes. Some, such as the WSINV3DMT code of the author, are available to the academic community. The goal of this paper is to summarize all the important issues involving 3-D inversions. It aims to show how inversion works and how to use it properly. In this paper, I start by describing several good reasons for doing 3-D inversion instead of 2-D inversion. The main algorithms for 3-D inversion are reviewed along with some comparisons of their advantages and disadvantages. These algorithms are the classical Occam's inversion, the data space Occam's inversion, the Gauss-Newton method, the Gauss-Newton with the conjugate gradient method, the non-linear conjugate gradient method, and the quasi-Newton method. Other variants are based on these main algorithms. Forward modeling, sensitivity calculations, model covariance and its parallel implementation are all necessary components of inversions and are reviewed here. Rules of thumb for performing 3-D inversion are proposed for the benefit of the 3-D inversion novice. Problems regarding 3-D inversions are discussed along with suggested topics for future research for the developers of the next decades.

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“…Árnason et al, 2010;Carbajal et al, 2012;Wamalwa and Serpa, 2013) the methodology we introduce in this study helps to understand the complex and site-specific behavior of electromagnetic fields in a 3D geological environment. Therefore, our virtual EM experiments allow the evaluation of the sensitivity pattern for an individual scenario and, consequently, facilitate the optimization of the field setup prior to the actual field campaign.…”

Section: Introductionmentioning

confidence: 99%

Electromagnetic methods for exploration and monitoring of enhanced geothermal systems – A virtual experiment

2015

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Joint 1D inversion of TEM and MT data and 3D inversion of MT data in the Hengill area, SW Iceland

Cited by 130 publications

References 31 publications

Comparison of 3D MT inversions for geothermal exploration: Case studies for Krafla and Hengill geothermal systems in Iceland

Comparison of 3D MT inversions for geothermal exploration: Case studies for Krafla and Hengill geothermal systems in Iceland

Three-Dimensional Magnetotelluric Inversion: An Introductory Guide for Developers and Users

Electromagnetic methods for exploration and monitoring of enhanced geothermal systems – A virtual experiment

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