Deep electrical resistivity tomography for the prospection of low- to medium-enthalpy geothermal resources

SUMMARY The growth of the geothermal industry sector requires innovative methods to reduce exploration costs whilst minimizing uncertainty during subsurface exploration. Until now geoelectrical prospection had to trade between logistically complex cabled technologies reaching a few hundreds meters deep versus shallow-reaching prospecting methods commonly used in hydro-geophysical studies. We present a recent technology for geoelectrical prospection, and show how geoelectrical methods may allow the investigation of medium-enthalpy geothermal resources until about 1 km depth. The use of the new acquisition system, which is made of a distributed set of independent electrical potential recorders, enabled us to tackle logistics and noise data issues typical of urbanized areas. We acquired a 4.5-km-long 2-D geoelectrical survey in an industrial area to investigate the subsurface structure of a sedimentary sequence that was the target of a ∼700 m geothermal exploration well (Geo-01, Satigny) in the Greater Geneva Basin, Western Switzerland. To show the reliability of this new method we compared the acquired resistivity data against reflection seismic and gravimetric data and well logs. The processed resistivity model is consistent with the interpretation of the active-seismic data and density variations computed from the inversion of the residual Bouguer anomaly. The combination of the resistivity and gravity models suggest the presence of a low resistivity and low density body crossing Mesozoic geological units up to Palaeogene–Neogene units that can be used for medium-enthalpy geothermal exploitation. Our work points out how new geoelectrical methods may be used to identify thermal groundwater at depth. This new cost-efficient technology may become an effective and reliable exploration method for the imaging of shallow geothermal resources.

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Current knowledge on the crustal properties of Italy

Annals of Geophysics ◽

10.4401/ag-4160 ◽

1994 ◽

Vol 37 (5 Sup.) ◽

Author(s):

C. Morelli

Keyword(s):

Adriatic Sea ◽

Current Knowledge ◽

Gravity Data ◽

Maximum Depth ◽

Gravity Models ◽

Tyrrhenian Sea ◽

Po Plain ◽

Reflection Seismic ◽

Adriatic Plate ◽

The North

The recent advances in experimental petrography together with the information derived from the super-deep drilling projects have provided additional constraints for the interpretation of refraction and reflection seismic data. These constraints can also be used in the interpretation of magnetic and gravity data to resolve nonuniqueness. In this study, we re-interpret the magnetic and gravity data of the Italian peninsula and neighbouring areas. In view of the constraints mentioned above, it is now possible to find an agreement between the seismic and gravity models of the Central Alps. By taking into account the overall crustal thickness, we have recognized the existence of three types of Moho: 1) European which extends to the north and west of the peninsula and in the Corsican-Sardinian block. Its margin was the foreland in the Alpine Orogeny and it was the ramp on which European and Adriatic mantle and crustal slices were overthrusted. This additional load caused bending and deepening and the Moho which now lies beneath the Adriatic plate reaching a maximum depth of approximately 75 km. 2) Adriatic (or African) which lies beneath the Po plain, the Apennines and the Adriatic Sea. The average depth of the Moho is about 30-35 km below the Po plain and the Adriatic Sea and it increases toward the Alps and the Tyrrhenian Sea (acting as foreland along this margin). The maximum depth (50 km) is reached in Calabria. 3) Pery-Tyrrhenian. This is an oceanic or thinned continental crust type of Moho. It borders the oceanic Moho of the Tyrrhenian Sea and it acquires a transitional character in the Ligurian and Provençal basins (<15 km thickness) while further thickening occurs toward the East where the Adriatic plate is overthrusted. In addition, the interpretation of the heat flow data appears to confirm the origin of this Moho and its geodynamic allocation.

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Geothermal resource exploration by using Numerical simulation and comprehensive geophysical method

10.21203/rs.3.rs-828808/v1 ◽

2021 ◽

Author(s):

Yang Yang ◽

Bin Xiong ◽

Sanxi Peng ◽

Ibrar Iqbal ◽

Tianyu Zhang

Keyword(s):

Detection Efficiency ◽

Energy Resource ◽

Clean Energy ◽

Shanxi Province ◽

Geothermal System ◽

Exploratory Research ◽

Geothermal Resource ◽

Geothermal Resources ◽

Transient Electromagnetic ◽

Resistivity Model

Abstract Geothermal energy is an important renewable clean energy resource with high development and usage potential. Geothermal resources, on the other hand, are buried deep below, and mining hazards are significant. Geophysical investigation is frequently required to determine the depth and location of geothermal resources. The Transient Electromagnetic Method (TEM) and the Controlled Source Audio Frequency Magnetotellurics (CSAMT) have the highest detection efficiency and accuracy of all electromagnetic exploration methods. This article initially explains the algorithm theory of the finite difference technique before establishing a simplified geothermal system resistivity model. Established on the simplified resistivity model, a simulation analysis of the ability of CSAMT and TEM to distinguish target body faults at different resistivities and dip angles was performed, and the effectiveness and difference of the two methods in detecting typical geothermal resource targets was verified. A complete exploratory research of CSAMT and TEM was conducted in Huairen County, Shuozhou City, Shanxi Province, China, based on theoretical analysis. Both approaches can reflect the geoelectric structure of the survey region, demonstrating the efficacy of the two methods in detecting genuine geothermal resources.

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2-D and 3-D resistivity image reconstruction using crosshole data

Geophysics ◽

10.1190/1.1443195 ◽

1992 ◽

Vol 57 (10) ◽

pp. 1270-1281 ◽

Cited By ~ 49

Author(s):

Hiromasa Shima

Keyword(s):

Field Test ◽

Numerical Models ◽

Electrical Potential ◽

Nonlinear Least Squares ◽

Three Dimensions ◽

Inversion Method ◽

Good Resolution ◽

Inversion Algorithm ◽

Linear Inversion ◽

Resistivity Model

Theoretical changes in the distribution of electrical potential near subsurface resistivity anomalies have been studied using two resistivity models. The results suggest that the greatest response from such anomalies can be observed with buried electrodes, and that the resistivity model of a volume between boreholes can be accurately reconstructed by using crosshole data. The distributive properties of crosshole electrical potential data obtained by the pole‐pole array method have also been examined using the calculated partial derivative of the observed apparent resistivity with respect to a small cell within a given volume. The results show that for optimum two‐dimensional (2-D) and three‐dimensional (3-D) target imaging, in‐line data and crossline data should be combined, and an area outside the zone of exploration should be included in the analysis. In this paper, the 2-D and 3-D resistivity images presented are reconstructed from crosshole data by the combination of two inversion algorithms. The first algorithm uses the alpha center method for forward modeling and reconstructs a resistivity model by a nonlinear least‐squares inversion. Alpha centers express a continuously varying resistivity model, and the distribution of the electrical potential from the model can be calculated quickly. An initial general model is determined by the resistivity backprojection technique (RBPT) prior to the first inversion step. The second process uses finite elements and a linear inversion algorithm to improve the resolution of the resistivity model created by the first step. Simple 2-D and 3-D numerical models are discussed to illustrate the inversion method used in processing. Data from several field studies are also presented to demonstrate the capabilities of using crosshole resistivity exploration techniques. The numerical experiments show that by using the combined reconstruction algorithm, thin conductive layers can be imaged with good resolution for 2-D and 3-D cases. The integration of finite‐element computations is shown to improve the image obtained by the alpha center inversion process for 3-D applications. The first field test uses horizontal galleries to evaluate complex 2-D features of a zinc mine. The second field test illustrates the use of three boreholes at a dam site to investigate base rock features and define the distribution of an altered zone in three dimensions.

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Structure of the Séchilienne unstable slope from large-scale three-dimensional electrical tomography using a Resistivity Distributed Automated System (R-DAS)

Geophysical Journal International ◽

10.1093/gji/ggz259 ◽

2019 ◽

Vol 219 (1) ◽

pp. 129-147 ◽

Cited By ~ 2

Author(s):

M Lajaunie ◽

J Gance ◽

P Nevers ◽

J-P Malet ◽

C Bertrand ◽

...

Keyword(s):

Large Scale ◽

Electrical Potential ◽

Three Dimensional ◽

Automated System ◽

Data Uncertainty ◽

Complex Data ◽

Electrical Tomography ◽

Data Set ◽

Resistivity Model ◽

Unstable Slope

SUMMARY This work presents a 3-D resistivity model of the Séchilienne unstable slope acquired with a network of portable resistivimeters in summer 2017. The instrumentation consisted in distributed measuring systems (IRIS Instruments FullWaver) to measure the spatial variations of electrical potential. 23 V-FullWaver receivers with two 50 m dipoles have been deployed over an area of circa 2 km2; the current was injected between a fixed remote electrode and a mobile electrode grounded successively at 30 locations. The data uncertainty has been evaluated in relation to the accuracy of electrodes positioning. The software package BERT (Boundless Electrical Resistivity Tomography) is used to invert the apparent resistivity and model the complex data set providing the first 3-D resistivity model of the slope. Stability tests and synthetic tests are realized to assess the interpretability of the inverted models. The 3-D resistivity model is interpreted up to a depth of 500 m; it allows identifying resistive and conductive anomalies related to the main geological and hydrogeological structures shaping the slope. The high fracturation of the rock in the most active zone of the landslide appears as a resistive anomaly where the highest resistivity values are located close to the faults. A major drain formed by a fault in the unaltered micaschist is identified through the discharge of a perched aquifer along the conductive zone producing an important conductive anomaly contrasting with the unaltered micaschist.

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Structural cross sections based on a gravity survey of parts of the Quetico and Wawa subprovinces near Thunder Bay, Ontario

Canadian Journal of Earth Sciences ◽

10.1139/e90-019 ◽

1990 ◽

Vol 27 (2) ◽

pp. 187-199 ◽

Cited By ~ 3

Author(s):

M. M. Kehlenbeck ◽

S. P. Cheadle

Keyword(s):

Cross Sections ◽

Gravity Data ◽

Bouguer Anomaly ◽

Gravity Models ◽

Low Grade ◽

Thunder Bay ◽

Metavolcanic Rocks ◽

Dimensional Gravity ◽

Anomaly Map ◽

Bouguer Anomaly Map

In this study, gravity data from 350 new gravity stations are combined with those from 50 previously surveyed stations in a detailed Bouguer anomaly map of a portion of the Quetico and Wawa subprovinces north and west of Thunder Bay, Ontario.In general, high gravity values characterize the southern and southwestern part of the area where metavolcanic rocks of the Wawa subprovince dominate. Much of the Quetico subprovince forms a broad gravitational low, reflecting extensive exposures of gneisses, schists, and migmatites. Well-defined gravity lows are associated with several granitic intrusive bodies.Three- and [Formula: see text]-dimensional gravity models of subsurface configuration of the density contrasts, representative of major rock units, indicate a trough-like structure for the metavolcanic rocks of the Wawa subprovince. This trough-like structure is flanked by a domical feature in the granitoid rocks to the south. North of the metavolcanic rocks, a succession of low-grade greywackes and slates occupies a basinal structure. These structures form the principal subsurface elements of the Wawa subprovince in this area.The gneisses, schists, and migmatites of the Quetico subprovince form a thick, southward-dipping, wedge-shaped structure that may extend under the structures of the Wawa subprovince. This wedge-shaped structure is underlain by a model unit of greater density representative of mafic gneisses and amphibolites. The denser substratum is modelled with local abrupt changes in dip corresponding in position with the Quetico and Hawkeye Lake faults.

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Crustal thickness under the Gulf of St. Lawrence, northern Appalachians, from gravity and deep seismic data

Canadian Journal of Earth Sciences ◽

10.1139/e89-130 ◽

1989 ◽

Vol 26 (8) ◽

pp. 1517-1532 ◽

Cited By ~ 36

Author(s):

F. Marillier ◽

J. Verhoef

Keyword(s):

Seismic Data ◽

Bouguer Anomaly ◽

Crustal Thickness ◽

Moho Depth ◽

Crustal Layer ◽

Reflection Seismic ◽

Middle Paleozoic ◽

Lateral Extent ◽

Well Data ◽

Lower Crustal

We have determined crustal thickness in the Gulf of St. Lawrence, an area that corresponds to an offset of the main northern Appalachians units. A "complete" Bouguer anomaly was calculated from recent depth-to-basement compilations and sediment densities from well data. The Moho surface was obtained by inverting the Bouguer anomaly, assuming a single density contrast at depth, and using an average depth provided by deep reflection seismic data. The resulting crustal model shows a Moho depth of 42–44 km beneath the Grenville Craton, north of the Appalachian deformation front. South of this front, the depth to Moho displays a pronounced thinning of the crust beneath the Carboniferous Magdalen Basin. This is in striking contrast to the deep seismic data, which give a Moho depth of about 43 km. The modelling of the Bouguer anomaly in the Magdalen Basin, taking into account the seismic reflection and refraction data, reconciles these different results and suggests that a 43 km deep Moho beneath the basin is associated with a lower crustal layer about 13 km thick, with high velocity (7.35 km/s) and density (3.05 g/cm3). The Bouguer anomaly suggests that the lateral extent of this high-density layer is confined roughly to the Magdalen Basin. We suggest that this layer is due to mantle underplating of the crust as a result of the Carboniferous-age formation of the Magdalen Basin, and that it is not a feature related to the early to middle Paleozoic development of the Appalachian Orogen.

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Integration of controlled-source and radio magnetotellurics, electric resistivity tomography, and reflection seismics to delineate 3D structures of a quick-clay landslide site in southwest of Sweden

Geophysics ◽

10.1190/geo2014-0386.1 ◽

2016 ◽

Vol 81 (1) ◽

pp. B13-B29 ◽

Cited By ~ 9

Author(s):

Chunling Shan ◽

Mehrdad Bastani ◽

Alireza Malehmir ◽

Lena Persson ◽

Emil Lundberg

Keyword(s):

Seismic Data ◽

Electric Resistivity ◽

Coarse Grained ◽

Resistivity Tomography ◽

Entire Study ◽

Controlled Source ◽

Reflection Seismic ◽

Reflection Seismics ◽

Quick Clay ◽

Resistivity Model

Quick clay, which is the main cause of landslides that occur in the northern countries, liquefies easily, and its presence implies an increased risk of landslide. Geophysical methods have been increasingly used in landslide investigations. Three-dimensional electric resistivity tomography, radio magnetotelluric (RMT), controlled-source RMT (CSRMT), and high-resolution reflection seismic data were acquired at a quick-clay landslide site in the southwest of Sweden. The main objectives were to evaluate the capability of each method in delineating different subsurface geologic structures that controlled a peculiar and hazardous retrogressive-type landslide in the study area. A 3D resistivity model from the inversion of CSRMT data showed the best correlation with the reflection seismic data and borehole information, thanks to the broad frequency range of the data set. It better imaged the resistive crystalline bedrock underlying the marine conductive clays and showed considerable correlations with the 3D reflection seismic data in resolving a coarse-grained layer that was interpreted to act as a conduit directing freshwater into the clays under a confined pressure, leaching their salt and forming quick clays. The 3D CSRMT resistivity model and 3D reflection seismic data showed that the coarse-grained layer has a varying thickness. At some locations, it was too thin to be resolved by the methods used here. Combination of the CSRMT model, reflection seismic data, and the borehole data suggested that a layer with thickness of approximately 5 m and resistivity between [Formula: see text] was potentially quick clay, which probably extended laterally in the entire study area. These observations suggested that future developments should focus on joint inversion of such 3D data sets incorporating sharp boundaries as constraints in the inversion and particularly when quick clays were studied.

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Integrated seismic and gravimetric model of Jocolí Basin, Argentina

Interpretation ◽

10.1190/int-2012-0022.1 ◽

2014 ◽

Vol 2 (2) ◽

pp. T57-T68

Author(s):

Patricia Martinez ◽

Mario Gimenez ◽

Andres Folguera ◽

Federico Lince Klinger

Keyword(s):

Oil And Gas ◽

Deep Structure ◽

Bouguer Anomaly ◽

Gravity Models ◽

Oil And Gas Exploration ◽

Fold And Thrust Belt ◽

Gravity Measurements ◽

Anomaly Map ◽

Bouguer Anomaly Map ◽

Well Data

Gravity measurements and reinterpretations of previously released seismic lines were made, focusing on the provincial border between neighboring provinces San Juan and Mendoza. A Bouguer anomaly map was obtained after the processing of gravimetric data, which were previously filtered, to obtain the Bouguer residual anomalies used for studying the geologic structures located on the upper crust. The analysis of these Bouguer residual anomalies allowed identification of the Jocolí Basin in a foreland position within a triangle zone at the boundary of the Precordillera fold-and-thrust belt with the Sierras Pampeanas thick-skinned foreland province. The seismic images allowed interpretation of three horizons: Paleozoic, Triassic, and Tertiary-Quaternary ages. The authors have reinterpreted the seismic and well data and reconstructed gravity models for the area under study aiming at unraveling the deep structure of the region and identifying features with potential for oil and gas exploration.

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Conductivity models for Archie rocks

Geophysics ◽

10.1190/geo2011-0297.1 ◽

2012 ◽

Vol 77 (3) ◽

pp. WA109-WA128 ◽

Cited By ~ 19

Author(s):

W. David Kennedy ◽

David C. Herrick

Keyword(s):

Predictive Power ◽

A Priori ◽

Geometrical Factor ◽

Data Set ◽

Archie’S Law ◽

Resistivity Model ◽

Noise Data ◽

Brine Volume ◽

Do So ◽

Conductivity Models

The petroleum industry’s standard porosity-resistivity model (i.e., Archie’s law), although it is fit for its purpose, remains poorly understood after seven decades of use. This results from the choice of the graphical display and trend formula used to analyze Archie’s seminal porosity-resistivity data, taken in the Nacatoch sandstone, a petroliferous clastic formation in the Gulf of Mexico coastal area. Archie’s model accurately predicts the conductivity-brine volume trend for this sandstone. Not all rocks follow the same porosity-resistivity trends observed in the Nacatoch sandstone, but those that do are defined as Archie rocks. Archie’s Nacatoch sandstone data set has significant irreducible scatter, or noise. Data with significant scatter cannot be used to uniquely define a trend. Alternative graphical analyses of Archie’s Nacatoch sandstone data indicates that Archie could have analyzed these data differently had it occurred to him to do so. A physics-based porosity-conductivity model, a “geometrical factor theory” (GFT), is preferred as an alternative to the Archie model because it has a physical interpretation. In this model, the bulk conductivity of an Archie rock is the product of three factors: brine conductivity, fractional brine volume, and an explicit geometrical factor. The model is offered in the form of a theorem, proved in three steps, to make our arguments as explicit and transparent as possible. The model is developed through its culmination as a saturation equation to illustrate that it is a complete theory for Archie rocks. The predictive power of the Archie model and GFT are similar, but unlike the adjustable parameters of the Archie model ([Formula: see text], [Formula: see text], and [Formula: see text]), all of the parameters of GFT have a priori physical interpretations. Through a connection to site percolation theory, GFT has promise to connect porosity-conductivity interpretation to circuit theory first principles.

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Gravity interpretation of Raguba field, Sirte basin, Libya

Geophysics ◽

10.1190/1.1441114 ◽

1980 ◽

Vol 45 (7) ◽

pp. 1153-1163 ◽

Cited By ~ 9

Author(s):

Salah I. El‐Batroukh ◽

Ahmed S. Zentani

Keyword(s):

Bouguer Anomaly ◽

Oil Field ◽

Gravity Models ◽

The North ◽

Anomaly Map ◽

Bouguer Anomaly Map ◽

Sirte Basin ◽

Surrounding Areas ◽

Gravity Interpretation ◽

Field Area

A Bouguer anomaly map of Raguba oil field and the surrounding areas is presented and interpreted. The main features of the map are: (1) A belt of positive anomalies approaching Bouguer values of 3 mgal in the field area then increasing up to 9 mgal toward the northwest. (2) Negative Bouguer values of −20 mgal on the east and west sides and a negative value of −13 mgal on the northeast side of the field. (3) Steep anomaly gradients trending north‐south on both sides of the field. To the north, the trend takes a northwest direction. (4) All these anomalies are superimposed on a regional trend of 0.16 mgal/km negative toward the south. The positive belt is interpreted as a horst structure characterized by crystalline basement at shallow depths. The negative anomalies are due to the density contrast between the sediments and the basement. Structural sections along certain profiles are presented and used for constructing gravity models calculated by computer.

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