Resistive Overburden Research Articles

3D forward modeling and response characteristics of low-resistivity overburden of the CFS-PML absorbing boundary for ground-well transient electromagnetic method

This study used the stable and convergent Dufort-Frankel method to differentially discretize the diffusion equation of the ground-well transient electromagnetic secondary field. The absorption boundary condition of complex frequency-shifted perfectly matched layer (CFS-PML) was used for truncation so that the low-frequency electromagnetic wave can be better absorbed at the model boundary. A typical three-dimensional (3D) homogeneous half-space model was established and a low-resistivity cube model was analyzed under the half-space condition. The response patterns and drivers of the low-resistivity cube model were discussed under the influence of a low-resistivity overburden. The absorption boundary conditions of CFS-PML significantly affected the low-frequency electromagnetic waves. For a low-resistivity cube around the borehole, its response curve exhibited a single-peak, and the extreme point of the curve corresponded to the center of the low-resistivity body. When the low-resistivity cube was directly below the borehole, the response curve showed three extreme values (two high and one low), with the low corresponding to the center of the low-resistivity body. The total field response of the low-resistivity overburden was stronger than that of the uniform half-space model due to the low-resistivity shielding effect of electromagnetic waves. When the receiving-transmitting distance gradually increased, the effect of the low-resistivity overburden was gradually weakened, and the response of the low-resistivity cube was strengthened. It was affected by the ratio of the overburden resistivity to the resistivity of the low-resistivity body.

Augmented electrical resistivity tomography magnetotelluric response function

With the magnetotelluric (MT) method, conductive layers can mask deeper resistors. An analytical relationship is derived between electrical resistivity tomography (ERT) and MT data to calculate an augmented ERT-MT response function. The augmented ERT-MT response function adds additional high-frequency content in the MT response function. This allows us to model the near-surface conductive layer from the ERT data and resolve deeper resistive structures with the MT data. In general, for the 1D case, the augmented ERT-MT response function results in a 1D inverse model that demonstrates an improvement of up to approximately 91% in recovering the true resistivity model. The augmented ERT-MT response function also is tested on theoretical 2D models. Two electrical resistivity models are used for the 2D tests. For both 2D models, the resistive dike and conductive overburden that were initially concealed are recovered when using the augmented ERT-MT response function. The augmented ERT-MT response function also is applied to field data, allowing us to map a resistive dike below a conductive overburden that was absent in the standard MT inverse model. The recovered dike and fault zone in the augmented ERT-MT response function resistivity inverse model correlate with the same features observed in a magnetic susceptibility inverse model. The augmented ERT-MT response function technique improves the resolution of MT to recover resistors beneath a conductive layer for the theoretical and experimental cases.

A comparison of regularized, sharp boundary and tear zone inversions along an MT profile in Sabalan geothermal field, Iran

This paper investigates magnetotelluric (MT) data recorded along a profile in the Sabalan geothermal region, NW of Iran. To find the range of relevant models consistent with the data, this study employed the so-called regularized, tear zone, and sharp boundary inversions. This study could effectively derive three alternative classes of models. Although the models show stable common resistive and conductive features there are some inconsistent details. Unaltered surface rocks and porous Basalt exhibit a high resistive overburden underlain by relatively more conductive Paleozoic sediments. A common signature of hydrothermal systems appears, and resistivities increase beneath a highly conductive clay cap in deeper parts. An intriguing feature resolved in the smoothest inversion model is a second deep conductor of 30 Ωm resistivities at a depth of 3 km, extending close to the surface. It can be related to the hot, solidified volcanic intrusions, resemblingthe heat source in a geothermal system. This study applied the two other inversion approaches for further hypothesis tests. Although the tear zone inversion re-establish the deep conductor (with 38 Ωm resistivities at 3 km depth), it is absent in the sharp boundary inversion result. This study concludes that the second deep conductor has a limited structure resolution.

Stretching AEM near-surface resolution limits related to low- and very high resistivity contrasts

Data from AEM surveys carried out in Norway, to support ground investigations for infrastructure projects, were used in this study. In large infrastructure projects, knowledge of sediment thickness is vital, along with information about sediment type as possible occurrence of highly sensitive clay. The acquisition systems, calibration and data processing are continuously improved to increase the sensitivity of the AEM systems. In an area with conductive shales over resistive bedrock, the recently introduced system response method was tested. It is applied in the inversion of SkyTEM data and makes it possible to utilize the very earliest time gates, providing information about the shallower layers. The models showed to give more pronounced structures in the near-surface, reflecting true structures observed in resistivity borehole measurements. The same outcome was observed when conducting synthetic modelling. In another setting AEM measurements were carried out along a planned road project to provide information about the extent of very conductive, possible alum shale. A volume estimate of excavated masses was sought, as alum shale poses an environmental and health risk due to the decomposition to sulfuric acid by weathering and high uranium content giving radon gas. Preliminary AEM models had a tendency to overestimate the thickness of the very resistive overburden. Experimenting with and optimizing the inversion settings resulted in models better fitting a priori information from the survey area. Limited low moment data were available due to a noisy environment. This affected the reliability of the models, illustrated by modelling and resulting real data models.

Analysis of Electromagnetic Depth Sounding Responses over a Layered Earth: Investigating Oil & Gas Seeps in the Petroleum Provinces

The present work embodies the results of theoretical and practical investigations of electromagnetic depth sounding using central frequency sounding (CFS) system over a layered earth. Failure of conventional electrical resistivity sounding in the study of geological conditions under resistive overburden calls for variable frequency sounding techniques. Electromagnetic depth sounding which involves the measurement of variation in conductivity with depth is used for solving various geoengineering, hydrogeological and shallow cases of oil & gas seeps associated with stratified earth. The CFS, which is one of the depth sounding techniques involving the measurement of vertical component of magnetic field induced at the centre of a circular or square loop, is considered in the present study for obtaining theoretical responses over a layered earth and its interpretation with shallow oil and gas seeps. Because of some limitations of contour integration and numerical integration approaches, used earlier, a more rapid digital linear filter technique is adopted for evaluation of the integral involved in the CFS theoretical expressions. Theoretical expressions for frequency-domain soundings written for layered earth models are suitably transformed for evaluation through digital linear filter. Dimensionless normalized vertical magnetic field is computed for different frequencies and loop radii for layered earth models with different layer conductivities and thicknesses. The responses computed for these cases are analysed in terms of resolution characteristics and detectability effects. In frequency-domain sounding, amplitude response curves of layer-sequences show the effect of layer conductivity, layer thickness and loop radius. Separation between individual curves on the sets for amplitude responses normally gives sufficient indications for subsurface conductivity variations of the layered earth cases. The author explores the CFS applicability and feasibility in investigating shallow oil & gas seeps in oil & gas provinces, in particular on the flanks of the rifted grabens and basin margin areas, where sediment-basement contact areas are interpreted.

3D inversion of ZTEM data for uranium exploration

. (5) SUMMARY We present a Gauss-Newton-based 3D inversion method for airborne ZTEM (Z-axis Tipper Electromagnetic) data to define resistivity structure relating to uranium deposits in the Athabasca Basin of Saskatchewan, Canada. The geophysical targets in this region can be represented by conductive plunging dykes in a resistive basement beneath a thick, more resistive overburden. We demonstrate using synthetic examples the effectiveness of the inversion method for detecting and delineating the target dykes and discuss how the inversion results are affected by various factors. It is shown that the dykes can be well imaged to depths more than 2 km even for the data from 200-m receiver height, provided the flight line is oriented perpendicular to the strike, and that the inversion results are relatively robust to the choice of the starting model. It is also shown that topographic effects are not serious for detecting the dykes at depth, because topographic effects are more significant at higher frequencies, while the sensitivity to the dykes increases with decreasing frequencies. One important finding is that if the flight line is oblique to the strike, the dependence of the starting model increases and the overall resolution decreases, compared to the 2D case, due to 3D effects.

CSRMT measurements in the frequency range of 1–250kHz to map a normal fault in the Volvi basin, Greece

Abstract In order to gain a better understanding of the geometry of surface faults, five Controlled Source/Radio Magnetotelluric (CSRMT) profiles were measured across the Volvi basin, 45 km northeast of the city of Thessaloniki in Greece. The data were collected in two frequency ranges: a) 1–12.5 kHz using a remotely controlled double horizontal magnetic dipole transmitter (CSAMT measurements), and b) 15–250 kHz using the signal from distant radio transmitters (RMT measurements). The transition from the RMT band to the CSAMT band was smooth and continuous allowing us to combine both datasets for plane-wave modeling. The surface geology shows a predominantly 2D structure, and therefore we planned the survey into profiles perpendicular to the geological strike. We have used a 2D interpretation tool to model the data in TE, TM, TE + TM and determinant modes. Using a 4% error floor on the impedance, 2D resistivity models from inversion of the determinant data provide lower RMS data fits (4.2 and 1.2 for resistivity and phase, respectively) compared to the combined TE + TM data (4.4, 2.8, overall resistivity and phase, respectively). 2D inversion of the measured tensor data shows a sharp change in the depth to the top of resistive gneiss–schist basement that is overlain by a less resistive overburden at southern basin flanks. The change in depth to the bedrock is clearly seen in all 2D models along the measured profiles suggesting the existence of normal faults with strike directions of NE–SW to E–W. The 2D electrical resistivity models suggest that the bedrock deepens towards south-west. The resistivity models are also compared with the existing borehole information in the area and show a reasonable correlation. For example the sharp change of depth to the bedrock towards the center of the basin as seen in the resistivity models are also confirmed by the borehole data.

Quantifying tomb geometries in resistivity images using watershed algorithms

Quantifying the geometries (defined here as width, height and depth of burial) of archeological structures within resistivity models produced as a result of the regularization constraints used in most inversion algorithms is difficult, especially when structures are closely spaced. Here we apply the watershed by simulated immersion method of boundary detection to smooth 2D resistivity images generated for synthetic and field data over 3D targets. The synthetic studies include a single cavity model, a model for two widely spaced cavities (spacing ≫ unit electrode spacing) and a model for two closely spaced cavities (spacing < unit electrode spacing). We also examine a single-cavity model where a relatively resistive overburden, common at archaeological sites in Egypt, is included. In the case of the single cavity models, the maximum error for any geometries are 18% for the model without the resistive overburden and 10% for the model where the overburden is included, whereas it increases to 24% for the widely spaced model and 40% for the closely spaced model. Despite, the higher errors in the closely spaced cavity model, application of the algorithm confirms the presence of two features, which is not ascertainable from the smooth resistivity images. Boundaries detected with the watershed algorithm are subsequently used to define a disconnect in the regularization, resulting in a markedly improved estimate of the resistivity structure (particularly for the closely spaced cavity model) in a second inversion step using the model obtained from the smoothness constraint inversion as the starting model. This revised resistivity model also results in a lower root-mean-square (rms) misfit between measured and theoretical data, and between synthetic and inverted models. We demonstrate how the method can be applied on images from the archaeological site at Qurnet Murai, Luxor City Egypt.

Vertical source, vertical receiver, electromagnetic technique for offshore hydrocarbon exploration

over a rock of uniform conductivity. Another model, with the same overburden resistivity but with a resistive layer gives rise to a faster temporal decay, with a maximum contrast occurring at typically at t=2-10s depending on the depth of the resistivity layer (HCs). The method has been verified on the relatively challenging Luva field, where our observed data have been found to carry a signature of the gas column there. This gas column has a relatively low resistivity contrast and is located at a depth of 1700 meters below the sea floor.

EMOD2D—a program in C++ for finite difference modelling of magnetotelluric TM mode responses over 2D earth

A program, EMOD2D, in C++, is developed to compute the apparent resistivities of two-dimensional geological structures for magnetotelluric (MT) H-polarization case, using finite difference technique. Five C++ classes with member functions are designed to compute apparent resistivitiy values. The program utilizes object oriented programming features such as multiple inheritance, encapsulation to allow the user to easily deploy and modify the given classes. This program has been used to study the responses of various ore deposit models. This modelling study is relevant to: (i) search for mineral deposits underlying conductive or resistive overburden, (ii) understand the response pattern for the ore body with different depths and conductivities, and (iii) study the response pattern for the ore body when it is in contact and without contact with the overburden. The program is compiled on Borland C++.

Appearance of Sign Reversal in Geophysical Transient Electromagnetics With a SQUID Due to Stacking

In geophysical transient electromagnetics (TEM) measurements with HTS SQUID magnetometers, a so-called 'frequency dependence' of the stacked time transients on the repetition frequency of the transmitter, and the appearance of so-called 'sign reversals', the crossover of the stacked time transients to negative values, have been observed frequently. Recently, we have shown that both of these effects can be attributed to the summing of remnant responses from earlier transmitted pulses of the repetitive transmitter waveform. Although the step function inductive response for many TEM targets decays monotonically and is positive at all times, instances of sign reversal do occur. We postulate that this sign reversal is due to the typical bipolar waveform of the TEM transmitter and the stacking procedure. In this contribution, we systematically extend our analysis to binary and ternary power-law expressions for the step function response, modeling measured responses for typical ground structures. The conditions are determined under which sign reversals appear. It is shown that the effect occurs mainly in the case where a shallow slope response is followed by a rapidly decaying response at late times. Such a signal is typically measured on a resistive overburden over a conducting medium. As an example, data are presented from a location where a sign reversal was measured with a SQUID whereas none was found in the coil data. A deconvolution procedure for determining the single pulse response from measured SQUID data is proposed.

High-frequency electromagnetic impedance method for subsurface imaging

This paper presents a high-frequency electromagnetic (EM) impedance method that extends the utility of conventional controlled-source audio-frequency magnetotelluric (CSAMT) method to the frequency range from 100 kHz to 100 MHz. In this frequency range diffusion and wave propagation must be considered together. In principle, both the electrical conductivity and the permittivity of the shallow subsurface can be imaged using impedance data gathered on the surface of the earth. The impedance approach has a distinct advantage in that coupling with the source is unnecessary, provided that the source can be positioned far enough away to yield plane waves at the receiver positions. At high frequencies the EM impedance is a function of the angle of incidence or the horizontal wavenumber, so the electrical properties cannot be readily extracted without eliminating the effect of horizontal wavenumber on the impedance. For this purpose, this paper considers two independent methods for accurately determining the horizontal wavenumber, which in turn is used to correct the impedance data. The apparent electrical properties derived from the corrected impedance data correlate poorly to the real structure, especially for the model with a resistive overburden. However, the impedance data along with the incidence angles thus estimated can be successfully inverted to yield an accurate subsurface layering through the simulated annealing inversion scheme.

A joint application of radiomagnetotellurics and transient electromagnetics to the investigation of a waste deposit in Cologne (Germany)

A joint radiomagnetotelluric (RMT) and transient electromagnetic (TEM) survey was carried out on a profile over an old waste deposit in Cologne to map lateral and vertical boundaries as well as inhomogeneities within the deposit itself. For the RMT method, three pairs of frequencies between 16.4 and 198 kHz, from radio transmitters located in roughly perpendicular directions, were used. The observed apparent resistivity ( ϱ a) and phase (φ) data clearly show the lateral boundary of the deposit to be characterized by strongly decreasing ϱ a values for all frequencies. The penetration depth for the lowest frequency is about 25 m outside and only 12 m inside the deposit. The RMT data were interpreted by one- and two-dimensional inversion techniques. On the waste site there is a resistive overburden over the highly conductive waste. Inhomogeneities within the deposit could also be resolved. However, the bottom of the deposit could not be detected by the RMT method because of the low penetration depths observed over the highly conductive waste site. TEM measurements on the same profile, however, allowed to achieve exploration depths between 10 and 100 m. Interpretation using one-dimensional conductivity models indicates the lower boundary of the waste site (20 m) and completes the two-dimensional conductivity depth section of the waste deposit.

Tensor CSAMT Study of the Fault Zone between Waikite and Te Kopia Geothermal Fields.

Closely spaced tensor CSAMT soundings have been conducted along three traverses perpendicular to the Paeroa Fault, which links the Waikite and Te, Kopia Geothermal Fields in the Taupo Volcanic Zone (TVZ) of New Zealand. Source current at frequencies of 4 Hz to 4 kHz was injected into a triangular set of three dipoles, located at least 6 km from the sounding sites. DC resistivity measurements (using tensor bipole-quadripole and Schlumberger arrays) were obtained at the same locations for comparison with the CSAMT data. As a technique appraisal, the survey demonstrated some of the advantages of CSAMT soundings, such as: good penetration through resistive overburden, a reliable signal and a high data acquisition rate. The benefits of harmonic analysis processing and the use of three current directions for least-squares tensor analysis were also demonstrated. Disadvantages included sensitivity to electric field distortions (static effects) and cultural noise. A large localised galvanic distortion, caused by an earthed water pipe, was observed in both the CSAMT and bipole-quadripole (DC) data sets. Interpretation of the CSAMT soundings suggests that low resistivity occurs below about 600 m depth between the two fields, correlating with a layer of intense hydrothermal alteration of fractured rhyolites and tuffs found in a nearby deep borehole. Similar structures have been found to extend well beyond the recognised shallow boundaries of several other TVZ geothermal fields, and may indicate widespread lateral movement of hydrothermally altering fluids at these deeper levels.

Advances in three‐dimensional magnetotelluric modeling using integral equations

Recent progress in integral equation modeling of three‐dimensional magnetotelluric responses includes the ability to simulate 3-D structures which outcrop (excluding topography), which transect layer interfaces, and which extend indefinitely in one or more directions. The most important factor in achieving this capability is an accurate treatment of the electric surface charge. In particular, a previous integro‐difference formulation for evaluating charges has been abandoned in favor of true surface integrations over the source cells with potential differencing across the field cells in the 3-D body. The new procedure constitutes a good approximation to Galerkin’s method while preserving internal consistency in terms of pulse basis functions. To verify outcropping structures, juxtaposed conductive and resistive prisms at the surface are simulated and compared to 2-D results. An elongate version of the 3-D model shows good agreement with both transverse electric and transverse magnetic modes of the 2-D response at both high and low frequencies. Apparent finite strike length effects for a short version of the 3-D body are physically reasonable. To verify structures cut by layer interfaces, an elongate tabular conductor at the base of a resistive overburden layer, which is in immediate contact with a tabular resistor at the top of a conductive basal half‐space is considered. All 3-D field components are in good agreement with 2-D transverse electric and transverse magnetic calculations from the surface down through or beside the body into the conductive basement below. The proper discontinuity of the normal component of the electric field across inhomogeneity boundaries is simulated well by the algorithm, but not exactly because of the pulse nature of the basis functions. Finally, outcropping and infinitely extended structures are checked by comparison of an offset basin model with very low frequency, thin‐layer calculation. In general, best results are obtained at cell body centers or face centers, but are not reliable over cell corners. From the offset basin study, we show also that 2-D transverse magnetic mode modeling of corresponding 3-D responses cannot be carried out indiscriminately over arbitrary structures without propagating errors to substantial depths in the 2-D model cross‐section.

A technique for the interpretation of Wenner pseudosections from basement areas of Nigeria

A simple and rapid procedure has been devised for an approximate interpretation of Wenner resistivity pseudosection profile data collected using a Microprocessor- controlled Resistivity Traversing System. The method is based on the estimation of the total longitudinal conductances of the weathered mantle from vertical sets of pseudosection data. Field results, made as part of hydrogeological investigations in basement areas of Nigeria, show that the method is very useful for determining the overburden resistivity and the depth to bedrock. The interpretation provides an initial model which can then be refined using finite difference modelling techniques.

On the interpretation of vlf resistivity measurements

VLF resistivity measurements contain at each measuring point the apparent resistivity and the phase angle between the magnetic and electric field. When interpreting the pointto-point conductivity distribution by a two-layer model, one of the three model parameters has to be assumed: overburden thickness, overburden resistivity or bedrock resistivity. The mathematical algorithm for this procedure is given. When applied to two-dimensional conductive structures, the algorithm results in several typical distortions. The inversion of perpendicular-to-strike resistivities and phases, the overburden thickness increases characteristically above thin vertical conductors, but true conductivities cannot be estimated. Above thick conductors, also conductivity information can be obtained, but the depth to the conductor is interpreted wrongly. Inversion of parallel-to-strike data estimates the conductivities of vertical structures well, but the horizontal boundaries are replaced by trade-off surface towards depth, making boundary determination more uncertain. An improved 1D algorithm with simultaneous inversion of parallel and perpendicular data to obtain the parameters of a homogeneous overburden and anisotropic bedrock is indicated.

EFFECT OF INCLINED ANISOTROPIC SUBSTRATUM ON MAGNETOTELLURIC RESPONSE*

ABSTRACTIn tectonically active regions electrical conductivity anisotropies are the dominating features. The importance of conductivity anisotropy in the interpretation of magnetotelluric data is well known. In the present study numerical results presented which show the effect of a substratum with inclined anisotropy on the magnetotelluric response. The pronounced change on the magnetotelluric response is found for the models in which the substratum underlies (i) conductive and (ii) resistive overburden.

Regions of Anomalously Large Telluric Interference in IP: The Cobar Syndrome

Regional differences are observed in the severity of telluric noise interference in induced polarization surveys. In some areas, of which Cobar (New South Wales) is an extreme example, surveys must often be postponed or confined to particular times of day. Using simple resistivity and magnetotelluric models, and typical magnetic micropulsation spectra, we show how this arises. The situation is worst when a very resistive basement is overlain by a thick conductive cover. In that case the IP signal is small on account of the conductive cover, while telluric voltages are large because of the resistive basement. For typical IP frequencies, and for the values of overburden resistivity found at Cobar, noise to signal ratio is shown to be greatest for overburden thicknesses of 200?300 m. This compares with the 80?150 m thicknesses actually observed. Noise to signal ratio is relatively insensitive to dipole length for a given separation of dipole centres.

Geophysical investigations in sweden for the characterization of a site for radioactive waste disposal — An overview

The geophysical measurements carried out for the investigations of prospective sites for radioactive waste disposal in Sweden are described. The investigations comprise detailed surface measurements with magnetic, electromagnetic, electric and seismic techniques. Measurements are made with a line spacing of 40 m and a station spacing of 20 m in an area of 4–6 km 2. At each site 7–10 deep boreholes are drilled. A number of geophysical logs are run in the boreholes, including several electric logs, providing data on temperature, salinity, and gamma ray. These geophysical data are evaluated together with the results from geological and hydrological investigations to make a three-dimensional geological and tectonical model of the site. This model defines the geometry used in the numerical modelling of the groundwater flow. In the case of thin, resistive overburden the surface investigations proved to be effective in the detection of dipping fracture zones. A line spacing as low as 40 m is needed to get adequate mapping of the fracture zones. The methods used cannot be considered efficient for mapping sub-horizontal fracture zones. The borehole logs provide data for the identification of fracture zones as well as different lithological units and give qualitative information of the hydraulic conditions. The temperature and salinity logs have provided the best hydraulic information, whereas, after testing, the induction log (borehole slingram), the differential resistance log and the VLF-log were found to be not effective for our investigations. The borehole techniques used, normally give information only about a relatively small volume of rocks around the boreholes. To obtain a detailed description of the bedrock between the boreholes, development of cross-hole geophysical, high-resolution techniques of considerable range, is needed.