Interpretation of long‐offset transient electromagnetic data from Mount Merapi, Indonesia, using a three‐dimensional optimization approach

Commer, Michael; Helwig, Stefan L.; Hördt, Andreas; Tezkan, B.

doi:10.1029/2004jb003206

Cited by 17 publications

(20 citation statements)

References 18 publications

(29 reference statements)

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“…After Angenheister [1982], porosity values up to 15% are realistic values for volcanic regions. Porosity estimations of the low resistivity below the summit result in values of about 5(10)% by anticipating a bulk resistivity of 0.7 Ωm and a NaCl concentration of 25(10) equivalent weight%, Commer et al [2005]. 10(20)% are also necessary to explain the fluid resistivity values of 0.2(1) Ωm, measured by LOTEM [ Müller et al , 2002].…”

Section: Geodynamic Framework At Merapimentioning

confidence: 99%

Modeling the density at Merapi volcano area, Indonesia, via the inverse gravimetric problem

Tiede

Camacho²,

Gerstenecker

et al. 2005

Geochem Geophys Geosyst

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[1] Merapi is a high-risk andesitic volcano in Central Java, Indonesia. Very little information is known about the detailed regional density structure around Merapi and its neighbor volcano Merbabu. We compute a subsurface three-dimensional (3-D) model of anomalous density for the volcanoes Merapi and Merbabu in Central Java, Indonesia, by inversion of the gravity field. As input for the inversion methodology, gravity observations from 443 points, whose 3-D coordinates are determined by GPS, are used. The inversion algorithm is based on an explorative approach to fit a least squares condition, including a balancing factor between the minimization of the residuals and the anomalous mass. A mean density about 2242 kg/m 3 for the region of Merapi and Merbabu has been computed by least squares adjustment. Results of the inversion show major low-density contrasts up to À242 kg/m 3 and positive structures about +264 kg/m 3 , referred to the determined mean density. A density anomaly (relative high) with densities up to +264 kg/m 3 is connecting the volcanoes in a 152°course from NW to SE and might be built of older basaltic lava. Low-density contrasts about À242 kg/m could be found in agreement with magnetotelluric and electromagnetic results. Generally, the identified highand low-density bodies are in agreement with the results of other geophysical methods such as electromagnetic and magnetotelluric prospecting or geological formations and structures. A porosity about 21% is derived for the largest negative density bodies about À242 kg/m 3 . Furthermore, the density model gives some new information about the controversial origin of a hill formation near Merapi and is also used to discuss the possible existence of a shallow magma chamber, which is also a controversial subject. Generally, the density model serves as basic information for the interpretation of geodetic and geophysical observations and confirms existing results from magnetotellurics, electromagnetics, and seismic data interpretation.Components: 6265 words, 8 figures.

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Section: Geodynamic Framework At Merapimentioning

confidence: 99%

Modeling the density at Merapi volcano area, Indonesia, via the inverse gravimetric problem

Tiede

Camacho²,

Gerstenecker

et al. 2005

Geochem Geophys Geosyst

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“…The regional geology is generally well studied (e.g., Förster et al, 2006), rendering it well-suited for testing newly developed hardware and imaging tools such as 3D CSEM. Altitude differences throughout the survey area do not exceed 30 m. Therefore, we can reasonably assume flat topography in our inversions and avoid computationally expensive and numerically difficult handling of topography (Commer et al, 2005). Unfortunately, the site is located in a populated region with abundant infrastructure.…”

Section: Field Experimentsmentioning

confidence: 99%

3D inversion and resolution analysis of land-based CSEM data from the Ketzin CO2 storage formation

2014

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We evaluated 3D inversion of land controlled-source electromagnetic (CSEM) data collected across the Ketzin CO 2 storage formation. A newly developed, parallel and distributed 3D inversion code, which is based on a direct forward solver, has been used. This inversion scheme allowed us to calculate the Jacobian matrix explicitly within a reasonable time and use it to calculate regularization parameters, inspect survey coverage, and carry out resolution analysis. After demonstrating that the magnetic field components are sensitive to conductors only, whereas the electric field components are sensitive to all features of interest, we continued to work with electric field data only. Estimates of data uncertainty obtained from robust processing were used for automated data preselection and weighting during inversion. We tested different regularization techniques and a range of starting models to explore the model space and assess the influence of regularization on the inversion images. We further demonstrated an approach for handling numerical singularities due to sources located inside the inversion domain. We estimated survey coverage, horizontal and vertical resolution, and depth penetration using cumulative sensitivity, point spread functions, and depth-resolution plots. Based on data fit analysis, we determined a preferred subsurface conductivity model, which we compared to an independent regional structural geologic model, and we provided an interpretation for the structures resolved. The inversion approach we used provides robust results in good agreement with known geology, offers new possibilities for model assessment, and should be transferable to other CSEM data sets.

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“…For example, Tiede et al (2005) use gravimetric inversion to explore edifice density at Gunung Merapi, identifying a relatively low-density unit on the western flank. These authors calculate an average porosity of 0.21 for this unit: a high value compared to the average edifice porosity of around 0.15 determined by Setiawan (2002) and the range of 0.05 to 0.10 estimated by Commer et al (2005) for the region directly below the Merapi summit. These values are generally consistent with measured laboratory values of porosity for Merapi samples (Le Pennec et al, 2001;Kushnir et al, 2016).…”

Section: Implications For Volcanologymentioning

confidence: 96%

Inelastic compaction and permeability evolution in volcanic rock

2017

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Abstract. Active volcanoes are mechanically dynamic environments, and edifice-forming material may often be subjected to significant amounts of stress and strain. It is understood that porous volcanic rock can compact inelastically under a wide range of in situ conditions. In this contribution, we explore the evolution of porosity and permeability -critical properties influencing the style and magnitude of volcanic activity -as a function of inelastic compaction of porous andesite under triaxial conditions. Progressive axial strain accumulation is associated with progressive porosity loss. The efficiency of compaction was found to be related to the effective confining pressure under which deformation occurred: at higher effective pressure, more porosity was lost for any given amount of axial strain. Permeability evolution is more complex, with small amounts of stress-induced compaction ( < 0.05, i.e. less than 5 % reduction in sample length) yielding an increase in permeability under all effective pressures tested, occasionally by almost 1 order of magnitude. This phenomenon is considered here to be the result of improved connectivity of formerly isolated porosity during triaxial loading. This effect is then overshadowed by a decrease in permeability with further inelastic strain accumulation, especially notable at high axial strains (> 0.20) where samples may undergo a reduction in permeability by 2 orders of magnitude relative to their initial values. A physical limit to compaction is discussed, which we suggest is echoed in a limit to the potential for permeability reduction in compacting volcanic rock. Compiled literature data illustrate that at high axial strain (both in the brittle and ductile regimes), porosity φ and permeability k tend to converge towards intermediate values (i.e. 0.10 ≤ φ ≤ 0.20; 10 −14 ≤ k ≤10 −13 m 2 ). These results are discussed in light of their potential ramifications for impacting edifice outgassing -and in turn, eruptive activity -in active volcanoes.

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Interpretation of long‐offset transient electromagnetic data from Mount Merapi, Indonesia, using a three‐dimensional optimization approach

Cited by 17 publications

References 18 publications

Modeling the density at Merapi volcano area, Indonesia, via the inverse gravimetric problem

Modeling the density at Merapi volcano area, Indonesia, via the inverse gravimetric problem

3D inversion and resolution analysis of land-based CSEM data from the Ketzin CO2 storage formation

Inelastic compaction and permeability evolution in volcanic rock

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