Apparent Resistivity Profiles Research Articles

Application of Helicopter-Borne and Ground–Airborne Electromagnetic Detection to Tunnel Engineering Investigation

The Cedaya-S340 Holgutu Highway Construction Project is located in the Mongolian Autonomous Prefecture of Bayingolin in the Xinjiang Uygur Autonomous Region. It is an important traffic channel that connects Luntai County and Hejing County at the southern foot of Tianshan Mountains. As the major component of the highway project, the Huola Mountain Tunnel has a sharp topographic relief, and, therefore, commonly used land geophysical detection instruments cannot work on it. Therefore, we conducted a qualitative survey on the ground-to-air and airborne electromagnetic detection methods used at the Huola Mountain Tunnel site to provide basic data for the design of highway tunnels. The geophysical survey summarized the ground–airborne frequency-domain electromagnetic method (GAFEM) and the helicopter-borne time-domain electromagnetic method (HTEM) developed by Jilin University, and measured 15 measuring lines. Apparent resistivity imaging was performed for each section, and the results were consistent. This study comprehensively analyzed the apparent resistivity profile and geological mapping data. Then, the study inferred the major stratigraphic boundaries, fault fracture zones, rock fragmentation, weakness, karst development, and water content in accordance with background value, low-resistivity anomaly shape, low-resistivity anomaly value, and gradient value in the apparent resistivity profile. Finally, the study identified the scope of two main low-resistivity anomalies, located at the tunnel entrance and exit, respectively, which are basically consistent with the known fault location. The results of this study show that on the basis of the apparent resistivity maps of GAFEM and HTEM, the overall distribution law is basically consistent with site landform, hydrogeology, tectonic geology, and aerial image data. The results provide guidance for the construction of the Huola Mountain Tunnel and ensure the construction safety and progress of the tunnel.

Motion noise remove for secondary-field semi-airborne transient electromagnetic data

Semi-airborne Transient Electromagnetics (SATEM) is a low-cost and productive method for quick geophysical surveys. The system includes a grounded line source on the ground and a receiver coil hanging in the air by a drone. Motion noise usually has the biggest impact on the acquired data because the receiver coil, which is suspended several meters below the drone by a soft rope, would maintain continuing random motion due to the movement of the drone, wind, etc. It also is a common problem in airborne electromagnetic (AEM) surveys, which typically use fixed-wing aircraft or helicopters. However, due to the secondary-field acquisition of SATEM, there are data deviations between the start and end of each motion period caused by continuous coil motion and discontinuous data acquisition. And this also brings difficulty in baseline elimination by using most de-noising methods, which require time or signal continuity on the baseline and are widely applied in full-time acquisition data (full-wave data). We proposed a simple but useful single-period polynomial-fitting method to remove motion noise from secondary-field data. And to prevent the obtained polynomial from being distorted from baseline by valid signals, a haar-based wavelet was employed to automatically recognize the start-point of pulses in the original signal so that the pulse can be eliminated before the polynomial-fitting process. Through a field SATEM survey data test, we found that the polynomial-fitting with 5 parameters (4th order) is good enough to describe the motion characteristics. We compared the results with those of the most commonly used wavelet methods at different bases and levels. The comparisons of de-noising results and apparent resistivity profiles of the original signal, data after wavelet de-nosing, and data after the polynomial-fitting de-noising show that this method is superior and much more reliable than wavelet de-noising of secondary-field SATEM data.

Features of apparent resistivity anomaly of controlled-source audio-frequency magnetotellurics and prospects of oil shale in the Tongchuan area of the southern Ordos Basin, China

Four controlled-source audio-frequency magnetotelluric (CSAMT) profiles were conducted in early 2011 to investigate the distribution rules of oil shale in the Tongchuan area of the southern Ordos Basin. After terrain correction, we high-pass filtered the resulting images to prepare the 2D CSAMT apparent resistivity profiles for further analysis. Specifically, we have correlated the high-pass-filtered apparent resistivity anomalies to the distribution of oil shale seen in three available wells. The results reveal a close relationship between the oil shale distribution and the anomalously high-apparent-resistivity belt-like anomalies in the Tongchuan area. Our analysis indicates two prospective areas with different depths for oil shale within the study area.

Application of high density electricity method in detection of concealed water conveyance tunnel

We used the high density electricity method to detect underground water conveyance tunnel in a project, then we compared the difference of apparent resistivity profiles between Schlumberger device and Winner device. Lastly, we analyzed the difference between the combined inversion method and the Levenberg-Marquardt method in Res2dinv software, and inferred the trend and depth of underground water tunnel through the results of electrical detection. Several conclusions are drawn as follows. Firstly, the resistivity anomaly of underground tubular object is more prominent in Schlumberger device than in Winner device. Secondly, the Levenberg-Marquardt inversion method which contained in Res2dinv software can easily conceal the horizontal resistivity variation characteristics of small-scale tubular anomalous body and the horizontal resolution of the original apparent resistivity profile is higher than that of the inversion result. Thirdly, the combined inversion method can improve the horizontal resolution of resistivity method for detecting tubular anomalous body with larger burial depth and smaller scale.

Application of AMT in the Investigation of New Loushanguan Tunnel of Chongqing Guizhou Railway

New Loushanguan tunnel of Chongqing Guizhou railway is located in the karst and fault zone development section with strong water yield and complex engineering geological conditions. Through audio-frequency magnetotelluric(AMT) exploration, through data processing and two-dimensional inversion, the apparent resistivity profile of the tunnel is obtained. Combined with engineering geological data, the resistivity characteristics are analyzed, and the macroscopic lithology characteristics, karst development degree and fault fracture zone of the tunnel are explored the information of location and influence area provides geophysical basis for tunnel design and plays a guiding role in comprehensive geological investigation of deep tunnel.

Global apparent resistivity definition for a high-performance TEM excitation source

AbstractThe high-performance transient electromagnetic method (TEM) excitation source is a new type of source that has been proposed for urban underground space exploration. This source is composed of two trapezoid plates. To ensure that the radiation field was focused in a certain direction, the two trapezoid plate-shaped antennas were arranged into a horn shape. This new source is characterised by high power, directional excitation and high resolution. The corresponding multi-component global apparent resistivity definition method is established for a high-performance transient electromagnetic excitation source. This method is studied using the inverse function theorem. Then, the monotonic relationship between components of the electromagnetic field and resistivity is analysed. For the fields that satisfy the monotonic relationship with half-space resistivity, the apparent resistivity can be calculated correctly to ignore the time period and location in the space. This means that this definition method can eliminate the limitation of early and late times, and the near and far zones. The apparent resistivity calculation results of the theoretical layered model reveal that global apparent resistivity curves show a regular change, which smoothly and comprehensively reflects the change of electrical information in the model. The experimental results of the 3D model show that the five-layer low-resistivity anomaly contained in the urban underground space designed in this paper exhibits an obvious response in the global apparent resistivity profile. It is concluded that a high-performance TEM excitation source possesses a high resolution, clearly reflecting all of the anomalies of a complex urban underground space model.

Three-dimensional DC Resistivity Modeling using Galerkin Finite Element Method Composed by Tetrahedral Elements

Successful interpretation of DC resistivity data depends on the availability of a proper forward modeling scheme. In this study, a three-dimensional DC resistivity forward modeling scheme was developed using the finite element method. The finite element equations were obtained using a weakened form of the weighted-residual method called the Galerkin method. Discretization of the modeling domain was carried out by dividing it into smaller three-dimensional blocks and subdividing each block into five tetrahedral elements. A linear interpolation function was employed and elemental linear equations were set up, followed by formation of global matrix systems of equation and incorporation of proper boundary conditions. The conjugate gradient method was applied to solve the global system of equations, which in this study was proven to be more efficient than a direct solver, contributing to a 67% time reduction. Using a Wenner array configuration, comparison with theoretical calculation of the electric potential for a homogeneous model yielded a relative error of 3.66%. To confirm the applicability of this forward modeling scheme, apparent resistivity profiles for several basic three-dimensional subsurface resistivity models were compared with the analytical profiles, yielding an acceptable level of fitting.

Multi-depth electrical resistivity survey for mapping soil units within two 3 ha plots

Spatial information on soils generally results from local observations that are destructive and time consuming especially in the case of heterogeneous soils. Geophysical technics can be a great help for soil mapping since they are non-destructive and fast. Large areas can be surveyed with a high density of measurements. Electrical resistivity is particularly interesting for soil study because it covers a wide range of values (several decades) and depends on many characteristics of the soil.The main objective of this paper was to study soil spatial variability using an original approach to electrical data processing. Electrical data from a 3-depth survey, usually treated as three apparent resistivity maps, were considered as many electrical soundings each with three apparent resistivity values. The study of the vertical succession of these values led to define nine reference geophysical taxa. This taxonomy relies on a parameter (α) which discriminates soil layers and is defined as the interval (in Ωm) beyond which two successive apparent resistivity values on a sounding are considered different. Geophysical taxa mapping highlighted their spatial coherence, which was related to pedological characteristics such as the presence of a clay layer or the depth of the soil profile. The comparison between the spatial distribution of geophysical taxa and a pre-existing soil map showed that the delineations of taxa clusters closely matched soil unit boundaries but were much less smooth. This method allowed the assignment to each soil type of a specific apparent electrical resistivity profile which was consistent with soil profile description. The method is straightly applicable to data from other surveys, and opens the way to the development of semi-automatic soil mapping from electrical resistivity data.

The analysis on IP signals in TEM response based on SVD

During transient electromagnetic method (TEM) exploration of a copper mine, we detected the late-channel TEM signal reversal phenomenon (a voltage change from positive to negative) caused by the influence of the induced polarization (IP) effect, which affects the depth and precision of the TEM detection. The conventional inversion method is inefficient because it is difficult to process the data. In this paper, the Cole-Cole model is adopted to analyze the effect of Dc resistivity, chargeability, time constant, and frequency exponent on the TEM response in an homogeneous half space model. Singular Value Decomposition (SVD) is used to invert the measured TEM data, and the Dc resistivity, chargeability, time constant and frequency exponent were extracted from the measured TEM data in the mine area. The extracted parameters are used for interpreting the detection result as a supplement. This reveals why the TEM data acquired in the area has a low resolution. It was found that the DC resistivity and time constant do not significantly change the results, however, the chargeability and frequency exponent have a significant effect. Because of these influences, the SVD method is more accurate than the conventional method in the apparent resistivity profile. The area of the copper mine is confined accurately based on the SVD inverted data. The conclusion has been verified by drill and is identical to the practical geological situation.

Effect of the soil moisture variation on apparent resistivity profiles and a method of correction

In order to get an apparent resistivity map of quite a large area which required a considerable amount of time (months) and because of the occasional availability of equipment and bad weather conditions causing postponing of certain campaigns, the area was divided into adjacent sub-areas which were mapped in a considerably separated measuring days. Because of the rainfall changes from one campaign to another, the final apparent resistivity map obtained by superposing all the different measuring day’s maps is obviously misleading. In the first stage, this disturbing effect was highlighted by carrying out profiles in the beginning of each measuring day on a specific line of the grid (the first line) just before mapping the corresponding sub-area. This was achieved first for the shallowest part of the grid, and then for the deeper parts. In the second stage, a removal of this effect was performed by applying a method of correction which consisted of bringing a specific measuring day apparent resistivity values close to reference day values. Once a given measuring day apparent resistivity was corrected, the remaining random errors were estimated. Keywords: Moisture, temporal resistivity, rainfall, day reference, line reference.

Saltwater intrusion modelling in Jorf coastal aquifer, South‐eastern Tunisia: geochemical, geoelectrical and geostatistical application

AbstractMarine intrusion is the most serious problem facing the coastal Jorf shallow aquifer, located in south‐eastern Tunisia on the Mediterranean Sea. Jorf Aquifer is intensively exploited to supply the growing needs of agriculture and domestic sectors. This work proposes a multidisciplinary investigation, involving hydro‐geochemical, geoelectrical survey and geostatistical techniques for modelling the saltwater intrusion. For this purpose, 36 water samples were conducted and analysed. Electric conductivity, pH, total dissolved solids and major ions were measured and analysed. Pie and Durov Diagrams, Q‐mode hierarchical cluster and geostatistical analysis were considered to identify the main groundwater mineralization processes. Results revealed that the Na‐Cl‐Ca‐SO4 is the dominant water type suggesting that dissolution of halite and gypsum was the main mineralization source of groundwater in the central and southern part of study area. However, saltwater intrusion was shown to control groundwater quality essentially in coastal areas. Variographic analyses were used to select the variographic model that best fits the spatial development of apparent resistivity. Kriged apparent resistivity profiles showed an abnormal decrease of resistivity values in the coastal zone, implying highly saline water because of seawater intrusion. Apparent resistivity values also decrease considerably in the faulted areas, suggesting a contribution of faults to seawater intrusion. Finally, saltwater mixing ratio was computed for each sample, and a refined seawater intrusion map was developed. Copyright © 2012 John Wiley & Sons, Ltd.

Estimation of groundwater levels with vertical electrical sounding in the semiarid area of South Keerqin sandy aquifer, China

Abstract To develop a simple, accurate, and non-destructive method for estimating the groundwater level (GWL) in an unconfined sandy aquifer, field measurements of soil electrical resistivity were conducted at the Daqinggou Ecological Station (DES) in 2005 and the Experimental Base of the Institute of Wind-Sand Land Improvement and Utilization (IWLIU) in 2009. The resistivity data were acquired through a series of vertical electrical soundings (VES) using a Wenner array. For comparison with the VES method, the GWLs were also manually monitored in wells. The results showed that the thirty VES profiles decreased or first increased and then decreased with increasing electrode spacing (i.e., becoming more conductive with depth). The depth of the GWL was obtained by calculating the turning points, as inferred from inflections in the apparent resistivity profiles. The GWL variations between the VES method and manual measurement ranged from 0.22 to 1.03 m at the DES, with a mean value of 0.52 m, and from 0.03 to 0.82 m at the IWLIU, with a mean value of 0.10 m. The significant differences between the GWLs obtained by the VES method and manual measurement at the DES were due to the higher GWLs with capillary action; there were no significant differences in the GWLs obtained at the IWLIU. The linear regression coefficient of determination was 0.97 for the IWLIU GWL values, indicating a good agreement between the VES method and manual measurements. Therefore, we conclude that the VES method is a sound measuring tool for estimating GWLs in unconfined sandy aquifers when the GWL is sufficiently deep (e.g., GWL > 3.98 m).

Evaluation of gas hydrate deposits in an active seep area using marine controlled source electromagnetics: Results from Opouawe Bank, Hikurangi Margin, New Zealand

Several known gas seep sites along the Hikurangi Margin off the east coast of New Zealand were surveyed by marine controlled source electromagnetic (CSEM) experiments. A bottom-towed electric dipole–dipole system was used to reveal the occurrence of gas hydrate and methane related to the seeps. The experiments were part of the international multidisciplinary research program “New Vents” carried out on German R/V Sonne in 2007 (cruise SO191) to study key parameters controlling the release and transformation of methane from marine cold vents and shallow gas hydrate deposits. Two CSEM lines have been surveyed over known seep sites on Opouawe Bank in the Wairarapa region off the SE corner of the North Island. The data have been inverted to sub-seafloor apparent resistivity profiles and one-dimensional layered models. Clearly anomalous resistivities are coincident with the location of two gas seep sites, North Tower and South Tower on Opouawe Bank. A layer of concentrated gas hydrate within the uppermost 100 m below the seafloor is likely to cause the anomalous resistivities, but free gas and thick carbonate crusts may also play a role. Seismic data show evidence of fault related venting which may also indicate the distribution of gas hydrates and/or authigenic carbonate. Geochemical profiles indicate an increase of methane flux and the formation of gas hydrate in the shallow sediment section around the seep sites. Takahe is another seep site in the area where active venting, higher heat flow, shallow gas hydrate recovered from cores, and seismic fault planes, but only moderately elevated resistivities have been observed. The reasons could be a) the gas hydrate concentration is too low, even though methane venting is evident, b) strong temporal or spatial variation of the seep activity, and c) the thermal anomaly indicates rather temperature driven fluid expulsion that hampers the formation of gas hydrate beneath the vent.

Resistivity Level Runs to Detect Water-filled Mine Voids between Drill Holes

Resistivity level runs are collected by lowering a current source down one well and measuring the resulting voltage at the same depth in another well. Mine voids between the wells that contain acid water appear as conductive anomalies on the resulting apparent resistivity profiles. Resistivity level runs can be collected rapidly and without lowering expensive equipment down holes of unknown stability. The data can be interpreted on-site, and are relatively insensitive to positioning errors. The method is well suited to sites where several drill holes have failed to intersect a known mine void. We demonstrated the feasibility of resistivity level run profiling at an abandoned mine complex in central Pennsylvania, where resistivity level runs were successfully used to locate haulage ways containing mine water.

Crustal variations and terrane boundaries in southern Mexico as imaged by magnetotelluric transfer functions

To investigate the complex crustal structure of southern Mexico magnetotelluric (MT) data were acquired at 75 stations along two north–south profiles, 200–250km apart, crossing several terrane boundaries whose interpretation has been controversial in the literature. Resulting sounding curves can be grouped, according to their shapes, into 10 different sets. Each of them may represent a crustal unit with a characteristic depth dependent electrical conductivity distribution. Often, the spatial extent of such a crustal unit matches well with a geological terrane. However, we also observed transition zones or sharp contrasts in crustal electrical properties which do not correspond to any defined terrane boundary. Transition zones are associated with low angle crustal structures (regional thrust faults or mylonitic zones) and/or with areas of plutonism, which are adding heterogeneities to the crust. In other cases our data suggest that terrane boundaries proposed on the basis of surface geology are probably shallow structures, which may not continue at depth. The comparison of apparent resistivity profiles with the available geologic information indicates that the MT method is capable of clearly defining zones affected by recent and active volcanism and tectonics as well as the part of the crust with Paleozoic and Precambrian metamorphic rock assemblages.

Fault Detection by 2D Resistivity Inversion on a Topographic Area

Direct current (DC) resistivity data acquired on rough terrain can be interpreted by using an appropriate two-dimensional (2-D) finite element inversion with the topography-incorporated technique developed by Tong and Yang (1990). This technique is computation ally efficient and provides a geological interpretation that is an improvement on the standard 2-D inversion. The main feature of this scheme is that sounding data incorporated the topography into the inversion without any previous topographic correction of field data. Thus errors inherited by the inversion data from an imperfect topographic correction can be avoided, especially in an area that has complex subsurface structures. In addition, the penetration depth of a Schlumberger four symmetric collinear electrode (4-electrode survey) array is often limited by obstacles at ground-level; especially in mountainous areas. This drawback may be overcome by using a Schlumberger three asymmetric collinear electrode (3-electrode survey) array. In order to demonstrate the advantage of using the 3-electrode survey, model studies were performed, and the results show that the DC resistivity responses obtained from the 4-electrode and 3-electrode surveys have similar features for a given spread length. However, under the same topographic constraints, the 3-electrode configuration has a more extension of a spread length, and therefore, a greater penetration depth. To seek verification of our strategy, DC resistivity sounding was applied to the Kanchiao fault area of northern Taiwan. The results stressed fault detection in this area. A Schlumberger 3-electrode survey was carried out in the field, and the data collected were inverted to the geoelectric models by using Tong and Yang's scheme (1990). In addition, synthetic apparent resistivity pseudosections were also computed from the final inversion models. Comparison of the synthetic with the observed apparent resistivity profiles is sufficient to reveal the Kanchiao fault position. The implications of the results obtained by combining the special electrode layout of the DC resistivity soundings with the previously mentioned finite element inversion technique are highlighted. The techniques used here does provide users with a guide of what to do in a topographic area. The techniques are superior to conventional techniques as applied to a complex subsurface structure.

Landslide surveys in Tertiary soft rock areas using HEM

This paper describes case histories for landslide surveys in Tertiary soft rock areas in Japan using a Helicopter-borne Electromagnetic (HEM) system operating at five frequencies between 220 Hz and 137,500 Hz.The usual method for conducting landslide surveys consists of airphoto interpretation, field investigation, and test borings. The ground dc resistivity method is seldom used for landslide surveys, because of the intermixture of topographic effects, the lack of uniformity of data and the difficulty of obtaining closely spaced survey lines along the slopes. On the other hand, HEM data are densely sampled, are very uniform over a survey area and are not as likely to suffer from topographic effects since the HEM “footprint” is relatively small compared with terrain changes. These factors make HEM measurements ideal for detecting the small resistivity changes caused by the landslides.HEM results show that the landslide locations coincide well with anomalously low resistivity zones in HEM apparent resistivity profiles and maps. This correlation occurs because the sliding surfaces of Tertiary type landslides are always accompanied by montmorillonite clay, which lowers the resistivity of the rocks. Furthermore, landslide locations very often correspond to an upheaval or raised pattern of low-resistivity zones in imaged AEM-resistivity sections. Shallow low-resistivity zones such as sliding surfaces also lower the apparent resistivity values at lower frequencies.The HEM method is effective in detecting and delineating landslides associated with cohesive soil, collapsed soil, or weathered zones in areas of Tertiary soft rocks.

Downward continuation of (Vs)L profile in 1P (time‐domain) with linear excitation using point electrodes. A short notet1

AbstractIt is seen that the apparent chargeability (Ma)L anomaly over a 2D graphite body splits into two distinct (Vs)L anomalies which closely follow the apparent resistivity profile. This suggests that the electric field amplitude is distorted due to a superficial inhomogeneity creating a (Vs)L anomaly, which bears no relation to the polarized body. The target depth obtained by continuation of such a profile is therefore, not acceptable.

Gradient Array Profiles over Conductive Veins

Under certain circumstances thin target bodies in geoelectric profiling exhibit a form of equivalence that is more commonly associated with thin horizontal beds in electrical sounding operations. In particular, gradient array responses measured over inclined conductive veins with sufficiently large depth of burial (in relation to the vein thickness) and sufficiently large conductivity (with respect to the surrounding medium) are essentially equivalent to those over conducting ribbons with the same conductivity-thickness product. This equivalence behaviour is exploited in order to investigate the influence of the body parameters on the geoelectric profiles measured with a gradient electrode arrangement over thin conductive veins. The effect of dip, depth of burial, depth extent and conductivity-thickness product are considered individually. It is demonstrated that the form of the apparent resistivity and apparent chargeability profiles over conductive veins display a simple inverse relationship. Further, it is found that the conductivity-thickness product has little influence on the form of the response profiles ? it only affects the anomaly amplitudes. A similar effect is noted with the dip angle of veins having an extended depth extent. Here the form of the profiles is increasingly antisymmetric and largely independent the dip. This behaviour has significance to the interpretation problem. In this context conductive veins are conveniently classified into two basic types depending on their relative depth extent. The greatest problem occurs with veins of extended depth extent. For these bodies it is concluded that the interpretation procedure can only define the location of the vein apex. All other vein parameters influence only the amplitude of the profiles and not their shape, so that the interpretation problem is fundamentally underdetermined in the absence of further information.

Use of Electrical Resistivity Soundings to Estimate the Depth of Underground Lignite Mine Workings Near Wilton, North Dakota

Active subsidence is occurring above lignite mine workings near Wilton, North Dakota. The subsidence poses a threat to State Highway 36, and impacted areas are in the process of being reclaimed by the North Dakota Public Service Commission. This reclamation work includes an extensive drilling program and provides a unique opportunity to evaluate the use of vertical electrical resistivity soundings (VES) to identify underground workings. Four locations having good subsurface information at the site were chosen for VES work. Apparent resistivity profiles using a symmetrical Schlumberger array were modeled with the automatic interpretation method described by Zohdy (1989). The results indicate unmined and unsaturated lignite of the Bullion Creek Formation has a resistivity similar to the surrounding clays. When modeled as a homogeneous layer, unflooded areas which have been mined have model resistivities approximately two to five times greater. Resolution of the depth to the workings appears to be within several meters, provided the mined horizon lies at a depth of less than 30 m. The predominant orientation of the workings appears to influence significantly the apparent resistivity profile and the model. Although not tested in this study, azimuthal soundings about a common midpoint may provide additional information on the orientation of mine workings in areas where drilling data or mine maps are not available.