Electromagnetic Sounding Data Research Articles

Quasi-2D inversion of surface large fixed-loop transient electromagnetic sounding data

Abstract In many cases, 1D inversion is still an important step in transient electromagnetic data processing. Potential issues may arise in the calculation of apparent resistivity using induced electromotive force (EMF) due to overshoot and the presence of multi-valued functions. Obtaining reliable and consistent inversion results using a uniform half-space as the initial model is challenging, especially when aiming for efficient inversion. Focusing on these problems, we use the land-based transient electromagnetic (TEM) sounding data, which was acquired by using a large fixed-loop transmitter, and adopt a quasi-2D inversion scheme to generate improved images of the subsurface resistivity structure. First, we have considered directly using magnetic field data or converting induced EMF into magnetic field, and then calculating the apparent resistivity over the whole zone. Next, a resistivity profile that varies with depth is obtained through fast smoke ring imaging. This profile serves as the initial model for the subsequent optimal inversion. The inversion scheme uses a nonlinear least-squares method, incorporating lateral and vertical constraints, to produce a quasi-2D subsurface image. The potentiality of the proposed methodology has been exemplified through the interpretation of synthetic data derived from a 3D intricate resistivity model, as well as field data obtained from a TEM survey conducted in a coalmine field. In both cases, the inversion process yields quasi-2D subsurface images that exhibit a reasonable level of accuracy. These images appear to be less moulded by 3D effects and demonstrate a satisfactory level of agreement with the known target area.

Electromagnetic prediction of rock thermal properties beyond boreholes: Soultz-sous-Forets (France) case study

Estimation of deep temperature, thermo-hydro-dynamic and basin simulation, heat exchange and geothermal resource assessment require knowledge of the rock thermal properties beyond boreholes. At the absence of suitable methods and instruments for such estimates we propose a new approach to prediction of thermal conductivity, specific heat capacity and heat flow density based on results of electromagnetic sounding of the study area. Artificial neural network used to this end is taught by correspondence between laboratory measurements on cores from the drilled boreholes and adjacent electrical conductivity profiles determined by inversion of the electromagnetic sounding data collected in their vicinity. This approach is used to estimate rock thermal properties along NW-SE magnetotelluric profile crossing Soultz-sous-Forêts geothermal area, Alsace, France. We demonstrate that accuracy of such prediction both in depths below the boreholes and in the interwell space is comparable with that usually achievable in the borehole scale. In particular, it is shown that relative errors of thermal conductivity prediction at depths twice as large as the borehole depth are as small as 2%, and the accuracy of cross-well thermal conductivity prediction is 6% and 10% for dry and wet rocks, respectively. On the other hand, the prediction accuracy for matrix thermal conductivity of a rock from the known profile of dry thermal conductivity is about 8%. Based on the results of our studies, we constructed dry and wet thermal conductivity sections from the electrical conductivity model determined earlier by 2-D inversion of magnetotelluric data. The temperature model of the study area built along the same profile by means of electromagnetic geothermometer and the obtained thermal conductivity sections enabled estimating the profiles of the vertical component of heat flow density at the surface from dry and wet thermal conductivity sections. They could be considered as minimum and maximum constraints, respectively, bounding the area of its possible values, which, in turn, depend on in situ thermal conductivity of fluids filling the rock pores. The range of possible vertical heat flow density variations (100–150 ± 10% mW·m− 2) agrees well with heat flow estimates in the Upper Rhine graben and, in particular, in Soultz-sous-Forêts geothermal area roughly estimated earlier from borehole measurements. Using the artificial neural network trained on the correspondence between the specific heat capacity determined in the borehole and adjacent electrical conductivity profile, we built the model of specific heat capacity. Against the background, a zone of enhanced specific heat capacity ranging between 2.05 and 2.15 MJ·m− 3·K− 1 was observed in the domain located at the base of the sedimentary cover and upper part of the granitic basement.

Porosity and water saturation predicting beyond boreholes from electromagnetic sounding and core sample data: Soultz-sous-Forêts (France) case study

Porosity predicting beyond boreholes is carried out by means of neural network taught by correspondence of the mercury porosity values determined from core samples and electrical resistivity estimated by magnetotelluric sounding in its vicinity. Testing of the artificial neuronet porosity prediction beyond boreholes indicated that the total relative error of the forecasts is about 7% that is slightly less than the prediction errors (∼10%) obtained in laboratory using seismic attributes. Comparative analysis of the prediction accuracy in boreholes with porosity values determined by direct and indirect ways showed that building of the porosity model of the section using simultaneously porosity data determined by direct and indirect techniques is inexpedient. At the same time, accuracy of electromagnetic porosity predicting using neural networks calibrated with data measured by the mercury porosimetry turns to be about three times higher than the accuracy of porosity estimation by indirect methods (for example, neutron gamma logging). A new approach to building of a water saturation section is proposed. It is based on a comparison of total open porosity determined using porosity values estimated by mercury porosimetry in a reference well and fluid volume fraction determined from electromagnetic sounding data. The latter is estimated taking into account the dependence of the brine electrical resistivity on the temperature determined using electromagnetic geothermometry. The errors of predicting water saturation beyond boreholes are around 20–22%, which are comparable with its estimation from seismic data in laboratory conditions.

Estimating Thermophysical Rock Properties from Electromagnetic Sounding Data and Laboratory Measurements

Abstract—Possibilities to construct the sections of thermal conductivity, specific heat capacity, as well as heat flow density at the surface, using electromagnetic sounding data and laboratory measurements on core samples, have been studied. It has been shown that the neural network prediction of thermophysical properties from electromagnetic sounding data and laboratory measurements enables estimating these properties not only below boreholes but also in the interwell space, with relative accuracy around 6–10%. Sections of “dry” and “wet” thermal conductivity, specific heat capacity and a profile of the heat flow density at the surface of the study area, along magnetotelluric sounding profile, have been built.

The Electromagnetic Prediction of the Open Porosity beyond Boreholes

Abstract —Prediction of the open porosity factor below boreholes and in the interwell space, from electromagnetic sounding and electrical logging data, has been studied. Modeling porosity data are synthesized from laboratory studies on the samples of two boreholes drilled in the Bishkek geodynamic site. The porosity prediction is carried out using specific electrical resistivity data obtained from 1D inversion of magnetotelluric sounding data collected in the vicinities of these wells. A new approach to predict porosity is suggested, based on the constructing of electromagnetic resistivity pseudo-logs at the target location. The comparison of this technique with other options indicates that its application results in substantial improvement of the predicted accuracy (in particular, relative errors of prediction in double depth of the borehole and in the interwell space could be 2 and 8%, respectively). In general, the porosity predictions, based on Archie formula, give worse results.

Features of high-density transient electromagnetic sounding data inversion under oil and gas prospecting in the Nepa-Botuoba anteclise area

The data of near-field transient electromagnetic sounding, which are widely used for the purposes of oil and gas exploration within the Nepa-Botuoba anteclise, are typically interpreted in terms of quasi-horizontally-layered models of media. The purpose of this study is to develop an approach to the inversion of transient electromagnetic sounding curves obtained via high-density 3D observation networks. The study was based on mathematical modeling, whose results allowed to estimate the lateral spatial parameters of a non-stationary electromagnetic field as well as to understand the differences between the results of one-dimensional inversion of the transient electromagnetic sounding data and the true parameters of the target horizon. As a result, the characteristic of the electromagnetic field described by an exponential function and used in the lateral-constraint inversion of high-density electromagnetic sounding was obtained. The proposed approach was tested on the practical data within the site under investigation located on the slope of the Nepa-Botuobа anteclise. It is shown that the use of spatial discrepancy under TEM sounding data inversion makes it possible to obtain geoelectric models characterized by lateral consistency of section geoelectric parameters. The application of the spatial stacking-based approach in the inversion process allows to increase the solution stability of the inverse problem of the near-field TEM sounding data.

Application of the Electromagnetic Geothermometer in Geothermics and Geothermal Exploration

Abstract —The latest results on electromagnetic geothermometry and its use in geothermics and geothermal exploration have been covered. Temperature models for the well-known geothermal areas, Larderello-Travale (Italy), Soultz-sous-Forêts (France) and Hengill (Iceland), constructed from electromagnetic sounding data and temperature well logs have been analyzed. 2D temperature model built along the magnetotelluric profile crossing the Travale (Italy) geothermal system allowed to locate a potential supercritical reservoir at the depth between two isotherms, TSCF ~ 400 °C and TBDT ~ 600 °C, coinciding with the seismic reflectors, detected earlier. 3D temperature model of the Hengill (Iceland) geothermal area built up to the depth 20 km, using electromagnetic sounding data, allowed us to locate high-temperature magma pockets at shallow depth (2–5 km); these magma pockets can be regarded as targets for geothermal exploration. Joint analysis of the specific resistivity and temperature models, taking into account gravity anomalies, allowed us to locate the zones of different thermal regimes within the crust; this, in turn, explained the observed seismicity pattern by thermo-mechanical properties, rather than by crust spreading. Deep temperature model, built for the Soultz-sous-Forêts (France) geothermal area from magnetotelluric data and temperature well logs, used to locate deep heat sources and predict heat transfer mechanism at depth up to 8 km.

Inversion of electromagnetic induction data using a novel wavelet-based and scale-dependent regularization term

SUMMARY The inversion of electromagnetic induction data to a conductivity profile is an ill-posed problem. Regularization improves the stability of the inversion and a smoothing constraint is typically used. However, the conductivity profiles are not always expected to be smooth. Here, we develop a new inversion scheme in which we transform the model to the wavelet space and impose a sparsity constraint. This sparsity constrained inversion scheme will minimize an objective function with a least-squares data misfit and a sparsity measure of the model in the wavelet domain. A model transform to the wavelet domain allows to investigate the temporal resolution (periodicities at different frequencies) and spatial resolution (location of the peaks) characteristics of the model, and penalizing small-scale coefficients effectively reduces the complexity of the model. The novel scale-dependent regularization term can be used to favour either blocky or smooth structures, as well as high-amplitude models in globally smooth structures in the inversion. Depending on the expected conductivity profile, a suitable wavelet basis function can be chosen. The scheme supports multiple types of regularization with the same algorithm and is thus flexible. Finally, we apply this new scheme on a frequency domain electromagnetic sounding data set, but the scheme could equally apply to any other 1-D geophysical method.

Identifying gas chimneys in gas fields of the West Siberian petroleum basin via geophysical and drilling data

There are many push-down seismic anomalies (gas chimneys or pipes with a diameter of 12–15 km) in natural gas fields in the northern part of the West Siberian basin (Urengoy, Yamburg, Zapolyarnoye, Kharasavey, Yurkharovskoye, etc.). This effect is common in gas-saturated sands. The amplitude of push-downs in West Siberian fields is approximately 300–400 ms from the horizontal surface. Using well data, we have measured how much the velocity of seismic waves decreases and study the hydrocarbon presence, fluid pressure, and other information to gain a better understanding of the cause of this effect. Several wells were drilled in three different gas chimneys. All of the wells in the gas chimneys discovered vertical columnar gas deposits in Jurassic and Cretaceous terrigenous formations. Well data from the Yurkharovskoye gas field show that seismic wave velocity in the gas chimney is 40% less than that outside this object. The wells in the gas chimneys show that formation pressure in the Neocomian and Jurassic formations is 80–90 MPa, which is approximately lithostatic pressure, so high-pressured gas in the pores begins to form cracks. There are gravity anomalies above many gas chimneys identified, indicating a density deficit. Transient electromagnetic sounding data from the Yurkharov gas chimney show a columnar zone of increased electrical resistance, as in gas-saturated formations. To explain all of these observations, we suppose that the West Siberian gas chimneys (pipes) are currently active channels for the vertical migration of high-pressured hydrocarbon gases (mainly methane), forming columnar gas-saturated zones. We think that the crucial cause of push-down seismic velocity anomalies is overpressure (approximately lithostatic). We predict that some new gas fields can be related to the gas chimneys at Gydan Peninsula and in Kara Sea. Many new deep gas deposits can be found in old gas fields with gas chimneys such as Urengoy and Yamburg.

ГЛУБИННОЕ СТРОЕНИЕ РАЗЛОМНОЙ ЗОНЫ НА УЧАСТКЕ МУХОР-ТАРХАТА ЧУЙСКОЙ ВПАДИНЫ ПО ДАННЫМ НЕСТАЦИОНАРНЫХ ЭЛЕКТРОМАГНИТНЫХ ЗОНДИРОВАНИЙ С ИСПОЛЬЗОВАНИЕМ ТРЕХМЕРНОГО МОДЕЛИРОВАНИЯ

The paper presents the results of three-dimensional modeling of the fault structure in the central part of the Chuya depression in Gornyi Altai within the Mukhor-Tarkhata plot. On this site, from 2004 to the present, researchers of the IPGG SB RAS have been conducting regular annual observations by the method of nearfield time-domain electromagnetic sounding (TSB) to observe the process of restoration of the geological environment after the catastrophic Chuya earthquake with a magnitude of 7.3 in 2003. One of the aftereffects of the destructive earthquake is fracture zones expressed on the surface. Numerous deformations of the surface and industrial objects are observed in the area of the Mukhor-Tarkhata village. The fault zone with sub-vertical fracturing, identified at the site by geological and electromagnetic data, was activated after the earthquake. Detailed data on the geoelectric structure are required for the interpretation and analysis of monitoring data. Based on the interpretation of the near-field time-domain electromagnetic sounding data, the deep structure of the fault zone was obtained. To verify and clarify structural features of the geoelectric model, three-dimensional modeling was performed.

Electrical and seismological structure of the martian mantle and the detectability of impact-generated anomalies

We derive synthetic electrical conductivity, seismic velocity, and density distributions from the results of martian mantle convection models affected by basin-forming meteorite impacts. The electrical conductivity features an intermediate minimum in the strongly depleted topmost mantle, sandwiched between higher conductivities in the lower crust and a smooth increase toward almost constant high values at depths greater than 400km. The bulk sound speed increases mostly smoothly throughout the mantle, with only one marked change at the appearance of β-olivine near 1100km depth. An assessment of the detectability of the subsurface traces of an impact suggests that its signature would be visible in both observables at least if efficient melt extraction from the shock-molten target occurs, but it will not always be particularly conspicuous even for large basins; observations with extensive spatial and temporal coverage would improve their detectability. Electromagnetic sounding may offer another possibility to investigate the properties of the mantle, especially in regions of impact structures. In comparison with seismology and gravimetry, its application to the martian interior has received little consideration so far. Of particular interest is its potential for constraining the water content of the mantle. By comparing electromagnetic sounding data of an impact structure with model predictions, it might also be possible to answer the open question of the efficiency of extraction of impact-generated melt.

A comparative study of different stabilizers for retrieving geoelectric structure based on a unified framework

Quantitative inversions of electromagnetic sounding data typically implement regularization to overcome the ill-posedness and obtain the geoelectric models in accordance with the prior information. The prior information is included by introducing the corresponding stabilizing functional (or a stabilizer) as a penalty term in the objective functional. However, there are a variety of stabilizers that can be chosen and some of them share similar properties. The choice of the appropriate stabilizer is still a problem in practical application. Therefore, we develop a stabilizer decomposition (SD) method to provide a better understanding of how stabilizers differ from each other and enable inverse problems with different stabilizers to be solved in a unified framework. The SD method decomposes the stabilizer into a weight function and a kernel function. The kernel function represents the core of minimization of the specific stabilizer and determines the fundamental characteristic. The weight function imposes extra tuning effect. The similarities and differences between different stabilizers can be explained from the perspective of the kernel function and weight function. Through the inversions of various geoelectric models, the performance of similar stabilizers under different inversion parameter settings are compared and analyzed in terms of accuracy, stability and structural characteristics. We further discuss the reasons for their distinctive performance based on the SD method. It is hoped that this study would enable a reasoned selection of the stabilizer and more flexible tests with different stabilizers for retrieving geoelectric structure.

Интерпретация данных зондирования методом становления поля в ближней зоне с учетом индукционно-вызванной поляризации при площадных нефтегазопоисковых исследованиях на юге Сибирской платформы

Интерпретация данных зондирования методом становления поля в ближней зоне с учетом индукционно-вызванной поляризации при площадных нефтегазопоисковых исследованиях на юге Сибирской платформы

Erratum

Geophysical ProspectingVolume 67, Issue 7 p. 1977-1978 ErratumFree Access Erratum This article corrects the following: Analysing two-dimensional effects in central loop transient electromagnetic sounding data using a semi-synthetic tipper approach P. Yogeshwar, B. Tezkan, Volume 66Issue 2Geophysical Prospecting pages: 444-456 First Published online: May 5, 2017 First published: 17 May 2019 https://doi.org/10.1111/1365-2478.12807AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat In Yogeshwar and Tezkan (2018), the following error was published on page 452. Figure 8: The color bars are not filled in Fig. 8 a) and d). The correct figure with proper color bars is as follows: We apologize for this error. REFERENCE Yogeshwar P. and Tezkan B. 2018. Analysing two-dimensional effects in central loop transient electromagnetic sounding data using a semi-synthetic tipper approach. Geophysical Prospecting 66, 444– 456. Volume67, Issue7September 2019Pages 1977-1978 ReferencesRelatedInformation

К вопросу автоматизированной комплексной интерпретации данных электрических и электромагнитных зондирований в скважине

К вопросу автоматизированной комплексной интерпретации данных электрических и электромагнитных зондирований в скважине

Advances in electromagnetic techniques for exploration, prospecting, and monitoring of hydrocarbon deposits

Traditional exploration and prospecting for hydrocarbons (HC) has traditionally been carried out using seismic techniques. At the same time, it is well known that seismic techniques are inefficient in the presence of high-velocity layers (which reduce resolution at great depth), igneous rocks, thrusts within the crystalline basement, and tight limestone. Being sensitive to geological structure, seismic techniques are characterized by low resolution at the level of micro-parameters such as fluid type, porosity/ fracture, and degree of pores HC-saturation. Moreover, technical complications, e.g., highly rugged topography, dense vegetation and object remoteness may make seismic surveys difficult, expensive, or even impossible. Therefore, non-seismic methods are increasingly used in HC exploration and prospecting. In particular, electrical and electromagnetic (EM) methods (magnetotelluric sounding, direct current, time-domain EM (TDEM), induced polarization (IP), controlled source EM, etc.) complement seismic techniques and increasingly replace them (Johansen, 2008; Key, 2012; Zhang et al., 2014; Barsukov and Fainberg, 2015; Berdichevsky et al., 2015, among others). In parallel with EM, the efficient technologies for 3D modelling and inversion (see, for instance, a review paper by Siripunvaraporn (2012) and references therein) and integrated analysis of EM and other geophysical data (see, for instance, a review paper by Bedrosian (2007) has been created. Application of these methods in solving problems of exploration geophysics enabled progression in exploration, prospecting, and devel¬opment of HC deposits (see, for instance, a review paper by Strack (2014) and references therein). Meanwhile, very recent advances in indirect estimating of rock geophysical properties of lithologic reservoirs from electromagnetic sounding data (Spichak and Goidina, 2016; Spichak and Zakharova, 2015, 2016; Spichak, 2017) open up new possibilities related to development of more sophisticated approaches to estimation of the reservoir properties and its potential assessment. The purpose of this paper is to demonstrate the advanced capabilities of electromagnetic techniques in solving a wide range of prob-lems, especially those which seismic surveys are not effective in solving.

The technique of joint interpretation of vertical electrical sounding (VES) and transient electromagnetic sounding (TEM) data in the study of Ugra paleovalley

The technique of joint interpretation of vertical electrical sounding (VES) and transient electromagnetic sounding (TEM) data in the study of Ugra paleovalley

The Determination of the Burial Depth of Coal Measure Strata Using Electromagnetic Data

The water-rich zones in the coal fields of Northern China have seriously threatened the safe production of coal mines. It is necessary to accurately locate the deep seated and complex water bodies which are usually located in Ordovician limestone. Due to the relatively high cost of seismic exploration, the development effective electromagnetic detection methods and corresponding data processing techniques for the detection of these water-rich areas becomes a topic for the EM geophysicist. In this study, a method of rapidly estimating the burial depth and undulating morphology of coal bed foundations by employing frequency domain electromagnetic sounding data was proposed. The method uses the overshoot/undershoot phenomenon of the apparent resistivity for the frequency domain electromagnetic measurement to quantitatively determine the burial depth of the northern China coal structure bases. First, the featured frequency points were confirmed, where the overshoot and undershoot effects appeared in the electromagnetic sounding curves beside the borehole drilling sites. Then, the correlation coefficient between the confirmed frequency points and the burial depth of coal seam bed was obtained according to the drilling data. Finally, the burial depth of the coal seam beds in the entire survey area was evaluated based on the correlation coefficient and electromagnetic data. This method was applied in the northern areas of Taiyuan City in Shanxi Province, and achieved good results. [Figure: see text]

Analysing two‐dimensional effects in central loop transient electromagnetic sounding data using a semi‐synthetic tipper approach

ABSTRACTWe present a simple and feasible approach to analyse and identify two‐dimensional effects in central loop transient electromagnetic sounding data and the correspondingly derived quasi two‐dimensional conductivity models. The proposed strategy is particularly useful in minimising interpretation errors. It is based on the calculation of a semi‐synthetic transient electromagnetic tipper at each sounding and for each observational transient time point. The semi‐synthetic transient electromagnetic tipper is derived from the measured vertical component of the induced voltage and the synthetically calculated horizontal component. The approach is computationally inexpensive and involves one two‐dimensional forward calculation of an obtained quasi two‐dimensional conductivity section. Based on a synthetic example, we demonstrate that the transient electromagnetic tipper approach is applicable in identifying which transient data points and which corresponding zones in a derived quasi two‐dimensional subsurface model are affected by two‐dimensional inhomogeneities. The one‐dimensional inversion of such data leads to false models. An application of the semi‐synthetic transient electromagnetic tipper to field data from the Azraq basin in Jordan reveals that, in total, eight of 80 investigated soundings are affected by two‐dimensional structures although the field data can be fitted optimally using one‐dimensional inversion techniques. The largest semi‐synthetic tipper response occurs in a 300 m‐wide region around a strong lateral resistivity contrast. The approach is useful for analysing structural features in derived quasi two‐dimensional sections and for qualitatively investigating how these features affect the transient response. To avoid misinterpretation, these identified zones corresponding to large tipper values are excluded from the interpretation of a quasi two‐dimensional conductivity model. Based on the semi‐synthetic study, we also demonstrate that a quantitative interpretation of the horizontal voltage response (e.g. by inversion) is usually not feasible as it requires the exact sensor position to be known. Although a tipper derived purely from field data is useful as a qualitative tool for identifying two‐dimensional distortion effects, it is only feasible if the sensor setup is sufficiently accurate. Our proposed semi‐synthetic transient electromagnetic tipper approach is particularly feasible as an a posteriori approach if no horizontal components are recorded or if the sensor setup in the field is not sufficiently accurate.

Numerical Differentiation of Electroprospecting Data for Resolution Enhancement

Differential electroprospecting methods provide an efficient technique to search for mineral deposits, but technical difficulties have so far prevented their widespread use. A stable method is available for numerical differentiation of functions defined by tabular data with errors. The method relies on integral splines. The present article applies the integral spline method to process electromagnetic sounding data. The efficiency of the numerical method is analyzed by modeling magnetotelluric fields in twodimensional nonhomogeneous media. The results show that numerical differentiation of profile curves can be efficiently applied for resolution enhancement in magnetotelluric sounding with the purpose of identifying structural features of the medium longitudinally to the Earth's surface.