Electromagnetic Prospecting Methods Research Articles

Natural Source Electromagnetic Component Exploration of Coalbed Methane Reservoirs

As an environmentally friendly and high-calorific natural gas, coalbed methane (CBM) has become one of the world’s most crucial unconventional energy sources. Undoubtedly, it is necessary to conduct in-depth research on reservoir exploration methods to ensure high and stable CBM production in the development stage. However, current methods have disadvantages such as high cost, complex devices, and poor terrain adaptability, and therefore they are unsuitable for reasonable monitoring of CBM reservoirs. In contrast, electromagnetic prospecting methods are increasingly widely employed in the rapid delineation of conductive distributions, contributing a lot to in-situ reservoir interpretation. Furthermore, a natural source Super-Low Frequency electromagnetic component method (i.e., the SLF method for short) has been proposed and applied with high potential in a CBM enrichment area, Qinshui Basin, China. In this paper, this method is thoroughly discussed. The magnetic component responses of the SLF method can be used as the characteristic responses of subsurface layers, and the forward modeling algorithms using the finite element method have been successfully developed and verified. On this basis, the direct depth transformation and one-dimensional nonlinear regularization inversion algorithms of the magnetic component responses are proposed for geo-object interpretation. With the help of the empirical mode decomposition (EMD), an SLF data processing workflow is demonstrated theoretically and practically, which is integrated into a portable instrument. The instrument’s ability to identify the low-resistivity reservoirs and their surrounding rocks has been proved by field survey. The extraction of electromagnetic radiation (EMR) anomalies also helps to refine the reservoir interpretation with higher accuracy. A joint comparative inversion test between the SLF method and the audio-magnetotelluric method (AMT) is also addressed, demonstrating that the SLF method is reliably applicable in the field survey of CBM reservoirs. A preliminary statistical analysis shows that the depth resolution of CBM reservoirs can reach the order of tens of meters. Therefore, the SLF method is expected to become one of the most potential options for in-situ CBM exploration with a cost-effective interpretation capability.

A new transient electromagnetic prospecting method in TBM tunnel environment

Tunnel boring machine (TBM) construction is a safe and efficient tunnel construction method at this stage, which contributes to the essential role of ahead prospecting methods, an essential prerequisite for ensuring the safety of TBM construction. Due to the high interference in the TBM tunnel environment, it is difficult to effectively apply multiple ahead prospecting methods. The existing geophysical methods are restricted because of the extremely small prospecting space in the TBM tunnel environment. Also, the TBM tunnel environment is required to introduce high-resolution detection methods. This paper proposes an ahead prospecting method suitable for TBM tunnel, using the simulation of three-dimensional vector finite element. Specifically, this prospecting method uses transient electromagnetic electrical radiation sources to emit and receives the Ex. Besides, the full field apparent resistivity imaging method is used to perform imaging interpretation of the adverse geological structures in front of the tunnel face in a semi-quantitative manner, to obtain the prospecting results required for the ahead prospecting in TBM tunnel.

Transforming a time-domain electromagnetic signal to a frequency-domain electromagnetic response using Gauss elimination method

Time-domain electromagnetic method (TEM) is an electromagnetic prospecting method that has developed rapidly and widely used in recently years. The inversion of this method uses frequency domain electromagnetic method, so the key is how to transform electromagnetic signals from time domain to frequency domain. Aiming at this problem, this paper proposes an inversion algorithm for transforming a time-domain electromagnetic signal to a frequency-domain electromagnetic response. Assuming that the time domain electromagnetic signal is transformed by cosine or sine and fitted by polynomial under the condition of turn-off current, the linear equations composed of these time signals are solved by Gauss elimination method, and then the frequency domain response can be obtained. When this algorithm is applied to the synthesis of the perpendicular magnetic field of the magnetic dipole, the relative error between the inversion frequency domain response and the theoretical result is less than 1%. Thus, this method can maintain a relatively high accuracy when transform the time domain response to the frequency domain response.

Response characteristics of gradient data from the frequency-domain controlled-source electromagnetic method

Studying how to improve the boundary recognition ability and resolution of anomalies is of great significance in the electromagnetic prospecting method. This paper analyzes the response characteristics of gradient data from the frequency-domain controlled-source electromagnetic method. A survey geometry that has one transmitter and many receiver locations, either along a line or over a grid, is used to obtain the electromagnetic field components. This kind of relatively dense coverage of receivers is needed in order to be able to determine the gradients. Spatial gradients in the two horizontal directions when measuring over a grid or in one horizontal direction when measuring along a line, and a frequency gradient which can give depth resolution, are considered in this paper. Forward modelling was carried out on a number of 3-D earth models to assess the capabilities and usefulness of the gradient data. The source–receiver geometry with survey lines arranged parallel to the direction of the grounded electric line source with measurements of the along-line component of the electric field, which is similar to the broadside geometry of marine CSEM, was found to be the best. The spatial gradients are able to indicate accurately the horizontal extents of the target, with the boundary identification ability of the spatial gradients being stronger than that of the original electromagnetic field components. Also, the frequency gradient is sensitive to the vertical extents of a target, with the main influence on the frequency gradient being the source–receiver distance, and the burial depth, resistivity and thickness of target. It is clear that the spatial and frequency gradients of electromagnetic data can provide a quick and reasonably accurate indication of the location and lateral extent of a conductivity anomaly in the subsurface, as well as an approximate depth and an indication of whether the anomaly is more or less conductive than the background.

Investigation of Archeological Caves in Israel Using High-Frequency Impulse Electric Prospecting

The archeological site of Machpelah Temple (Cave of the Patriarchs), Israel, was studied. At the moment, the first stage of investigations is finished: it included the study of a certain part inside the temple proper. New high-frequency impulse sounding hardware was applied, and its depth and spatial resolution of physical inhomogeneities exceeded the capabilities of existing electromagnetic prospecting methods by an order of magnitude.

The continuous wavelet transform in processing data of high frequency electromagnetic prospecting

Ground Penetrating Radar (GPR), a high frequency electromagnetic prospecting method (10 to 3000 MHz) has been rapidly developed in recent decades. With many advantages such as non-destructive, fast data collection, high precision and resolution, this method is a useful means to detect underground targets. It is currently used in the research of studying the shallow structure for examples: forecast landslide, subsidence, mapping urban underground works, traffic, construction, archaeology and other various fields of engineering, GPR data processing is becoming increasingly urgent. Wavelet transform is one of the new signal analysis tools, plays a vital role in numerous domains like image processing, graphics, data compression, gravitational, electromagnetic and geomagnetic data processing, GPR and some others. In this study, we used the continuous wavelet transform (CWT) and multiscale edge detection (MED) with the wavelet functions which were appropriately selected to determine underground targets. The accuracy of this technique was tested on some theoretical models before being applied on experimental data. The obtained results showed that this was a feasible method that could be used to detect the size and position of the anomaly objects.

ANOMALY IDENTIFICATION FROM SUPER-LOW FREQUENCY ELECTROMAGNETIC DATA FOR THE COALBED METHANE DETECTION

Abstract. Natural source Super Low Frequency(SLF) electromagnetic prospecting methods have become an increasingly promising way in the resource detection. The capacity estimation of the reservoirs is of great importance to evaluate their exploitation potency. In this paper, we built a signal-estimate model for SLF electromagnetic signal and processed the monitored data with adaptive filter. The non-normal distribution test showed that the distribution of the signal was obviously different from Gaussian probability distribution, and Class B instantaneous amplitude probability model can well describe the statistical properties of SLF electromagnetic data. The Class B model parameter estimation is very complicated because its kernel function is confluent hypergeometric function. The parameters of the model were estimated based on property spectral function using Least Square Gradient Method(LSGM). The simulation of this estimation method was carried out, and the results of simulation demonstrated that the LGSM estimation method can reflect important information of the Class B signal model, of which the Gaussian component was considered to be the systematic noise and random noise, and the Intermediate Event Component was considered to be the background ground and human activity noise. Then the observation data was processed using adaptive noise cancellation filter. With the noise components subtracted out adaptively, the remaining part is the signal of interest, i.e., the anomaly information. It was considered to be relevant to the reservoir position of the coalbed methane stratum.

ANOMALY IDENTIFICATION FROM SUPER-LOW FREQUENCY ELECTROMAGNETIC DATA FOR THE COALBED METHANE DETECTION

Natural source Super Low Frequency(SLF) electromagnetic prospecting methods have become an increasingly promising way in the resource detection. The capacity estimation of the reservoirs is of great importance to evaluate their exploitation potency. In this paper, we built a signal-estimate model for SLF electromagnetic signal and processed the monitored data with adaptive filter. The non-normal distribution test showed that the distribution of the signal was obviously different from Gaussian probability distribution, and Class B instantaneous amplitude probability model can well describe the statistical properties of SLF electromagnetic data. The Class B model parameter estimation is very complicated because its kernel function is confluent hypergeometric function. The parameters of the model were estimated based on property spectral function using Least Square Gradient Method(LSGM). The simulation of this estimation method was carried out, and the results of simulation demonstrated that the LGSM estimation method can reflect important information of the Class B signal model, of which the Gaussian component was considered to be the systematic noise and random noise, and the Intermediate Event Component was considered to be the background ground and human activity noise. Then the observation data was processed using adaptive noise cancellation filter. With the noise components subtracted out adaptively, the remaining part is the signal of interest, i.e., the anomaly information. It was considered to be relevant to the reservoir position of the coalbed methane stratum.

Numerical modeling of borehole-surface electromagnetic responses with 3-D finite difference method and comparison with physical simulations

Borehole-surface electromagnetic method (BSEM) is a high-accuracy electromagnetic prospecting method that uses AC-powered vertical finite line source as the excitation source. The observed surface electromagnetic field is inverted for the subsurface conductivity structure. In this paper, the total field is separated into a primary part, due to a horizontally layered host medium, and a secondary part due to 3-D heterogeneities. After solving for the primary field with an analytical method, the frequency-domain second-order differential equation for the secondary field is discretized by finite differences. A sparse matrix storage scheme is employed and a BiCGSSTAB(m) method with a diagonal matrix preconditioner is used to obtain the secondary field as well as the 3-D BSEM response of the model. We compare the result of anomalous responses of three-layered medium derived by 3-D forward modeling with the result of semi-analytical solution. We also perform physical simulation and 3-D numerical forward modeling based on similarity criterion. As a result, the shape of both anomalous response curves are the same, which validates the 3-D numerical simulation method. The anomalous fields of 3-D numerical forward and physical simulation share similar anomalous feature of symmetric bimodal structure that is consistent with its harmonic response curve. It demonstrates that borehole-surface electromagnetic method can be used not only for prospection by employing a multi-frequency response, but also can provide multi-angle information about subsurface anomaly by varying relative depths of vertical finite line source in the borehole.

Dynamic monitoring of coalbed methane reservoirs using Super-Low Frequency electromagnetic prospecting

Dynamic monitoring of coalbed methane (CBM) reservoirs plays an important role in reservoir evaluation, production estimation, exploitation and development planning in order to efficiently operate producing wells. This paper proposes a passive Super-Low Frequency (SLF) electromagnetic prospecting and monitoring method, which helps us derive electromagnetic radiation (EMR) anomalies from reservoirs to directly identify and dynamically analyze CBM reservoirs. The modeling study shows that the SLF magnetic responses are sensitive to high resistivity layers. These responses turn out to be approximately stationary and can be seen as simply a component of the background field. This stationary background field can be clearly distinguished and then dynamic anomaly extraction would be completed. In order to suppress cultural noise and high frequency (HF) random noise, the methods of empirical mode decomposition (EMD) and wavelet transform are used in data processing. The reconstructed curves are employed to identify EMR anomalies at corresponding depths of reservoirs, and subsequently help directly interpret and dynamically monitor reservoirs. The SLF prospecting method is validated using the field data observed from CBM wells in the years from 2007 to 2013 in Qinshui Basin, China. The results present that the high EMR “wave packets” contribute conceivably to the CBM reservoir identification. Compared with audio-magnetotelluric (AMT) inversion results, the SLF identification resolution is greatly improved. The dynamic characteristics of producing reservoirs are revealed using EMR anomalies, and agree with production histories and other surveys.

A very low frequency-electromagnetic study of the geo-environmental hazardous areas in Taiwan

This study utilized the very low frequency-electromagnetic (VLF-EM) technique, a passive electromagnetic prospecting method working in the very low frequency range (15–30 KHz) to investigate the geo-environmental problems of shallow, low conductivity sedimentary layers in Taiwan. Field examples successfully demonstrate the advantages of using this method in locating non-mineralized shallow fault zones. The zero-crossings of in-phase and quadrature measurements in 2-D contour maps clearly locate the position of subsurface anomalous source bodies. Further analysis of the measured VLF single profile peaks reveals that this method is useful in determining subsurface structures and conductivity. The advantages of nondestructive, noninvasive, and low consumption of power make this method extremely friendly to the environment. The authors anticipate this method will have more profound impacts on the interactions between prospecting technology and the earth.

In search of the Royal Ptolemaic Cemetery in central Alexandria, Egypt—the first contact

AbstractAt Chatby in central Alexandria, Egypt, a team from the University of Patras conducted a detailed geophysical study as part of an investigation for locating the Royal Ptolemaic Cemetery. The exploration site is located in the Latin and Greek cemeteries in the southeast corner of the modern cemeteries of Alexandria. An area of 10 000 m2 was investigated down to a depth of 10 m. Gravity, electromagnetic prospecting, electrical, ground‐penetrating radar and seismic methods were applied. The use of various methods allowed as much information from the subsurface as possible to be obtained and comparison of the data in order to enhance interpretation. Despite the increased geophysical noise present in such a highly urbanized environment, intelligent selection of field parameters, use of advanced processing techniques and specialized software made it possible to reveal important information from the subsurface data. Interpreted geophysical features may be related to buried archaeological structures at some locations of the area explored. Copyright © 2003 John Wiley & Sons, Ltd.

Simultaneous use of geochemical and geophysical methods to characterise abandoned landfills

Groundwater sampling and geophysical methods determined a serious contamination problem associated with refilled exploitation sites at the Cal Dimoni area, Llobregat delta, Barcelona, Spain. To characterise this process, hydrogeochemical analyses were performed and showed the following modifications to groundwater chemical composition: increasing pH values, changing redox conditions, significant increases in total organic carbon (TOC) and certain trace elements, and high groundwater conductivity values. Major ion content accumulations were found under the refilled area. In contrast, elements involved in the oxidation–reduction processes, such as iron, manganese and nitrates, clearly diminished. Electromagnetic prospecting methods were also performed and delineated the contamination plume extent. These methods also showed separate sources of contamination, one clearly related to the groundwater–refilled zone leachate interaction, another as a consequence of the manure–accumulation surface site. Geochemical and geophysical methods have shown similar results for locating groundwater contamination sources, and for determining leachate generation mechanisms and flow paths.

Very low frequency electromagnetic prospecting method—A few field applications in India

The very low frequency electromagnetic prospecting equipment measures different space and phase components of the electromagnetic field in the frequency range 15–25 kHz emanating from distant naval radio stations. The field application of the equipment designed and constructed for this purpose has demonstrated the usefulness of the investigations in groundwater and geological mapping. The amplitude measurements necessitate correction of the diurnal variations of the electromagnetic field at the observation point.

Numerical modeling for electromagnetic remote sensing of inhomogeneities in the ground

After a brief description of the mathematical formulation, we show some of the benefits that can be derived from numerical models. One of these benefits is to furnish an estimation of the suitability of electromagnetic prospecting methods with regard to specific sounding tasks. Using as an example the detection of a magma pocket with a line-source of current or an electric antenna as excitation, we also call attention to the limit of validity of two-dimensional models. Then, we consider the remote sensing, from the earth's surface, of cavities embedded at small depths. We point out the disturbing effect of a conducting overburden on the efficiency of electromagnetic methods. A three-loop system is proposed and the various prospecting parameters of this system are conveniently optimized with the help of the numerical model. This three-loop system is then shown to be far superior to the standard two-loop method.

EARTH CONDUCTIVITY DETERMINATIONS EMPLOYING A SINGLE SUPERCONDUCTING COIL

A low‐frequency airborne electromagnetic prospecting method has been developed which exploits the inherent low resistance of a superconducting coil. Changes in the input resistance of this coil are monitored in the presence of the conducting earth. The response of the system, the change in the input resistance, is proportional to the quadrature secondary magnetic field at the transmitter, although unlike two‐coil systems, the response does not decrease with increasing frequency. This research has demonstrated that superconducting wires, large scale nonmetallic cryostats, the requisite measurement circuitry, and an appropriate data acquisition system are realizable in a practical flight configuration. The unicoil presents the following significant advantages in electromagnetic prospecting: 1) The measurement sensitivity is not limited by the relative coil motion experienced by two‐coil systems. 2) Ample field strength may be supplied to override ambient noise. 3) Optimum frequencies for specific geologic sections are easily implemented in the range of 10 to 2000 Hz. 4) Maps of ground conductivity may be obtained because precise thermal stability is maintained and the measurement, therefore, is absolute. 5) The point source observation minimizes analytic complexity. 6) The combination of the foregoing features with multiple frequency operation, yields a system of potentially high sensitivity and unprecedented depth of exploration. The unicoil system also possesses some disadvantages: 1) An operational complexity results from the cryogenic procedures required in the field, and 2) the heavy sensor requires a large helicopter.

Induction methods in prospecting for hot water

It is commonly observed that in areas where abnormally heated water is rising to the surface, the electrical conductivity of the ground is significantly higher than in adjacent rocks. This has been the basis for the use of direct-current electrical prospecting methods in the search for an evaluation of geothermal power prospects. However, it can be argued theoretically that induction electromagnetic prospecting methods should be more effective in searching for conductive zones than direct current methods. We have used four electromagnetic methods in the evaluation of geothermal prospects: the audio-frequency magneto-telluric method, the two-coil profiling method, the two-coil frequency-domain sounding method, and the coil/wire time-domain sounding method. The audio-frequency magneto-telluric method is a very simple means for reconnaissance to depths of a few hundred meters. The two-loop profiling method provides high resolution for conductive zones within the first 20 or 30 meters of the surface, and may be used in detailing the surface expression of hyperthermal manifestations. The two-loop sounding method may be used to measure conductivities to a depth of 1 or 2 kilometers, even when the surface resistivity is too high to allow the use of direct-current methods. The wire/coil method may be used to map the source of anomalous heat into the lower crust and upper mantle.

ELECTROMAGNETIC PROSPECTING METHODS IN ARCHAEOLOGY

ArchaeometryVolume 5, Issue 1 p. 146-153 ELECTROMAGNETIC PROSPECTING METHODS IN ARCHAEOLOGY I. SCOLLAR, I. SCOLLAR Rheinisches Landesmuseum, BonnSearch for more papers by this author I. SCOLLAR, I. SCOLLAR Rheinisches Landesmuseum, BonnSearch for more papers by this author First published: June 1962 https://doi.org/10.1111/j.1475-4754.1962.tb00566.xCitations: 20AboutPDF 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 Citing Literature Volume5, Issue1June 1962Pages 146-153 RelatedInformation