Magnetometric Resistivity Method Research Articles

Validation of a new magnetometric survey for mapping 3D subsurface leakage paths

Techniques for more reliable detection of 3D subsurface flow paths are highly important for most water-related geotechnical projects. In this case study, a magnetometric resistivity method with a new approach and state-of-the-art technology (“Willowstick survey”) was applied to the testbed dam (YD dam) site, and its applicability was validated by geotechnical investigation techniques including borehole drilling and sampling, Lugeon test, flow direction and velocity test, and seismic tomography. In addition to the magnetometric survey, a 3D electrical resistivity survey was performed independently and the results were compared and discussed. The electrical resistivity survey was effective in detecting groundwater levels, but it was limited in mapping leakage paths. On the other hand, the Willowstick magnetometric survey effectively detected geologic weaknesses (e.g., fault fracture) and potential leakage paths of the dam site foundation rocks. The results of this research are expected to be effective for water infrastructures where leakage is an important issue.

Magnetometric resistivity: a new approach and its application to the detection of preferential flow paths in mine waste rock dumps

The injection of a low frequency electrical current in the ground between two electrodes A and B generates a magnetic field that can be measured at the ground surface with sensitive magnetic sensors. The map of the magnetic field, measured at the frequency of the injected current, can be used to determine the paths of the current through the ground. When the current is channelled along preferential conductive paths, the MagnetoMetric Resistivity (MMR) method can be used to detect these paths. Conductive current paths can be associated with preferential flow paths of groundwater when the two electrodes A and B are in the direction of the flow and when the flow path is highly electrically conductive with respect to the background. We first review the background equations for the magnetic field in MMR. Then, we provide the kernel of the problem using Biot and Savart law to connect the components of the observed magnetic field to the current density distribution. We also develop a simple approach to invert the magnetic field in terms of electrical current paths. To illustrate how the method works, we develop five synthetic models to test the sensitivity of the method to the properties of the conductive targets channelling the electrical current. The targets are characterized by different shapes, sizes, depths, and conductivity contrasts with the background. Then, we proceed with a case study for which the MMR method is used to identify and map preferential groundwater flow paths bypassing a mine waste rock dump drainage collection trench into the tailings pond. In this case, the conductivity of the flow paths is much stronger than the background conductivity due to the high mineralization of the ground water along these paths. The method underlines the 3-D architecture of these flow paths.

The magnetometric resistivity method in a stratified medium having resistivities varying exponentially with depth

Using a known solution for the electric potential and Ampère’s law, the azimuthal component of the magnetic field is deduced in a horizontally layered medium with a current point source placed anywhere, considering that the resistivity in each layer varies exponentially with depth. This theoretical result contributes to model the magnetometric resistivity method, which had been applied onshore, e.g., for mineral exploration, offshore to investigate the permafrost layer at bottom sea, hydrothermal flux, and natural resources. We have numerically tested the obtained formulation against previous results found in the literature that use distinct electrode and sensor dispositions, with models having three and four layers. Introducing the exponential variation, it verified the sensitivity to physical and geometric parameters comparing the exponential and homogeneous models.

Time-dependent magnetometric resistivity anomalies of groundwater contamination: Synthetic results from computational hydro-geophysical modeling

We present a forward-modeling investigation of time-dependent ground magnetometric resistivity (MMR) anomalies associated with transient leachate transport in groundwater systems. Numerical geo-electrical models are constructed based on the hydrological simulation results of leachate plumes from a highly conceptualized landfill system and the resultant MMR responses are computed using a modified finite difference software MMR2DFD. Three transmitter configurations (i.e., single source, MMR-TE, and MMR-TM modes) and two hydrological models (i.e., uniform and faulted porous media) are considered. Our forward modeling results for the uniform porous medium indicates that the magnetic field components perpendicular to the dominant current flow contain the most information of the underground targets and the MMR-TE mode is an appropriate configuration for detecting contaminant plumes. The modeling experiments for the faulted porous medium also confirm that the MMR method is capable of mapping and monitoring the extent of contaminant plumes in groundwater systems.

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A Magnetometric Resistivity (MMR) method is one of the controlled source methods, which can be used to survey an electrical resistivity structure of the uppermost oceanic crust. The MMR method is composed of source and receivers. The source consists of two electrodes, one near the sea surface and the other near the seafloor, which supplies an artificial vertical bipole electrical current. The receivers are Ocean Bottom Magnetometers (OBMs), which measure the magnetic field variations generated by the artificial electrical current. The electrical resistivity of the oceanic crust is directly proportional to the electrical current, inversely to the amplitude of the magnetic field variation, and to the square of source-receiver separation. The electrical resistivity of the oceanic crust mainly depends on the porosity of the oceanic crust and the fluid temperature within the oceanic crust. In this paper, six MMR surveys for oceanic resistivity structure are reviewed. One-dimensional resistivity structures estimated from three surveys revealed the resistivity and the thickness of sedimentary layers. The other three surveys have discovered the differences of the oceanic crust resistivity structures between on spreading axis and off spreading axis. In only two of them, one-dimensional inversion was applied to estimate the resistivity structures. Accurate estimations of the three-dimensional resistivity structures (e.g. a hydrothermal circulation system) can not be made by the one-dimensional inversion. Therefore, the development of three-dimensional inversion for the MMR data is prerequisite for precise investigation of the oceanic crust.

Geo-Electrical Responses Associated with Hydrothermal Fluid Circulation in Oceanic Crust: Feasibility of Magnetometric and Electrical Resistivity Methods in Mapping Off-Axis Convection Cells

Recent developments in theory and instrumentation have led to increasing interest in the use of geo-electrical techniques to map seafloor structure and to explore mineral deposits. Electrical experiments conducted at sea are difficult and costly to perform, reinforcing the need for theoretical design studies before any seagoing programs get underway.I present in this paper the first theoretical investigation of geoelectrical responses associated with hydrothermal fluid circulation in a mid-ocean ridge flank environment. A 2D conceptual electrical model is constructed based on hydrothermal modelling results, and its responses to two major ‘galvanic’ techniques (magnetometric resistivity (MMR) and electrical resistivity methods) are calculated using a finite difference computer package. Forward modelling results reveal that the marine MMR method is capable of detecting off-axis hydrothermal convection cells with equivalent or even greater resolution than traditional seafloor heat flow surveys. However, the electrical resistivity method is not applicable because this system suffers a very severe ‘shorting effect’ of the overlying seawater layer, which almost totally masks the contribution from the underlying oceanic crust.

1-D resistivity structures of the oceanic crust around the hydrothermal circulation system in the central Mariana Trough using Magnetometric Resistivity method

Abstract We present preliminary results of an electromagnetic survey over a region of hydrothermal circulation in the spreading axis of the central Mariana Trough. The Magnetometric Resistivity (MMR) method is used to determine the electrical resistivity structure of the oceanic crust around the hydrothermal system. 1-D electrical resistivity structure is calculated from the relation between the amplitude of magnetic field and source-receiver separation. The amplitudes of magnetic field variation of ocean bottom magnetometers (OBMs) placed on axis are larger than those of OBMs about 700 m away from the spreading axis, for source-receiver separations larger than 400 m. We estimated two resistivity structure models: one for the axial OBMs, and another for the off-axis OBMs. A region of lower resistivity between depths of 100–300 m is identified in the on-axis model. The hydrothermal source of the Alice Springs Field probably exists beneath the spreading axis, and the size of this source is smaller than 700 m. Lower resistivities at a depth of 50 m are identified in both the on-axis and the off-axis models, suggesting that the porosity in the uppermost oceanic crust is largest at this depth.

A downhole magnetometric resistivity technique for electrical sounding beneath a conductive surface layer

The magnetometric resistivity (MMR) method has not been used systematically for vertical electrical sounding because surficial measurements of the magnetic field caused by static current flow from a point source of current at the surface are independent of the form of the variation of earth resistivity with depth. However, data obtained from an adaptation of the MMR method in which the ratio of the magnetic fields below and above a known conductive surface layer is measured as a function of the horizontal range from the current source can be inverted to obtain the unknown resistivity of deeper material. The practical limitation is that the integrated conductivity of the surface layer must not be much smaller than the integrated conductivity of the unknown zone in order to observe diagnostic ratios significantly different from unity. The expressions derived for the magnetic‐field ratio and for the sensitivity of the ratio to changes in the resistivity of a plane‐layered unknown zone are closed‐form inverse Hankel transforms. The transforms are evaluated analytically for a model of constant, uniform resistivity. One possible geotechnical application of the method is the detection of offshore, resistive permafrost beneath the shallow Beaufort Sea. A theoretical design study in which ratios of the magnetic fields at the bottom and top of the sea are synthesized and the sensitivity of those ratios to changes in sea‐floor resistivity is determined as a function of range and depth, respectively, reveals that the resistivity of a permafrost zone 50 m thick buried 20 m below the sea floor can be identified. The maximum range needed is of the order of 200 m, which appears to be smaller than the maximum spacing required for a Schlumberger sounding to yield similar information.

The Down-hole Magnetometric Resistivity (DHMMR) Method

Summary The down-hole magnetometric resistivity method (DHMMR) is a pseudo-DC grounded dipole geophysical survey technique which allows absolute direction to a conductor from a borehole to be established. Anomalies produced by elongated block conductors are similar in shape and can be modelled in a similar way to gravity anomalies produced by block-shaped bodies having a density contrast. The anomalies are therefore simpler in shape than TEM anomalies and are easier to recognize in areas of geological complexity. The DHMMR technique is faster and more directional than down-hole TEM but has poorer resolution of conductor size and shape, and hence its ideal role is as a complement to down-hole TEM surveys. Case histories from two volcanogenic base metal prospects, near Charters Towers, Queensland, demonstrate the directivity of the method in the search for off-hole massive sulphides.

Nonlinear integral equations for electromagnetic inverse problems

The scaling properties of Maxwell’s equations allow the existence of simple yet general nonlinear integral equations for electrical conductivity. These equations were developed in an attempt to reduce the generality of linearization to the exclusive scope of electromagnetic problems. The reduction is achieved when the principle of similitude for quasi‐static fields is imposed on linearized forms of the field equations. The combination leads to exact integral relations which represent a unifying framework for the general electromagnetic inverse problem. The equations are of the same form in both time and frequency domains and hold for all observations that scale as electric and magnetic fields do; direct current resistivity and magnetometric resistivity methods are considered as special cases. The kernel functions of the integral equations are closely related, through a normalization factor, to the Frechét kernels of the conventional equations obtained by linearization. Accordingly, the sensitivity functions play the role of weighting functions for electrical conductivity despite the nonlinear dependence of the model and the data. In terms of the integral equations, the inverse problem consists of extracting information about a distribution of conductivity from a given set of its spatial averages. The form of the new equations leads to the consideration of their numerical solution through an approximate knowledge of their kernel functions. The integral equation for magnetotelluric soundings illustrates this approach in a simple fashion.

Downhole magnetometric resistivity response of a half‐plane conductor

The magnetometric resistivity (MMR) methods measure the magnetic field of the galvanic current flow introduced by current electrodes in the ground. The downhole MMR version overcomes some of the limitations imposed by a conductive overburden on the surface MMR methods. This paper is concerned with downhole MMR responses of buried conductors. The model considered is an infinitely conductive half‐plane conductor in a uniform whole‐space host, as an ideal representation of a plate‐like conductive orebody buried deep underground. Exact analytical downhole MMR responses are derived for arbitrary locations of current electrode sources and magnetic field sensors. Type curves are shown for various source‐sensor‐target orientations to illustrate some characteristics of the responses.

Polymetallic sulfide exploration on the deep sea floor: The feasibility of the MINI‐MOSES experiment

The recent discoveries of polymetallic sulfide deposits on the deep sea floor have created an interest in geophysical techniques for mapping them. A magnetometric resistivity method (MINI‐MOSES) has been developed for detecting the distribution of sub‐sea‐floor electrical resistivity. A vertical, long‐wire bipole feeds electric current into the ocean. Some of the current enters the sea floor, and its magnetic field is measured at various distances by a low‐frequency induction coil magnetometer. The method is sensitive to conductive and resistive zones at depth, and information about three‐dimensional resistivity structures can be obtained. The small load resistance of an electric transmitter grounded in the ocean allows high current to be driven by a battery‐powered instrument. The induction coil receiver is more sensitive and less noisy than conventional flux‐gate magnetometers over the selected frequency band. This receiver is therefore well‐adapted to the almost noise‐free deep sea‐floor environment. Both the transmitter and the receiver are completely self‐contained, recording averaged transmitted and received signals in random access memory. The technique was tested near a sulfide deposit at 47° 57′N, 129°06′W on the axis of the Juan de Fuca Ridge. The deep submersible ALVIN positioned and moved the instruments about on the sea floor. Just enough data were obtained to demonstrate the viability of the technique for this kind of geologic mapping and mineral exploration. The electrical resistivity of 14 Ω ⋅ m obtained for a uniform half‐space model is consistent with Deep Sea Drilling Project (DSDP) results for the seawater‐saturated basalt.

Three‐dimensional terrain corrections for mise‐à‐la‐masse and magnetometric resistivity surveys

Three‐dimensional modeling of topographic effects in mise‐à‐la‐masse and magnetometric resistivity surveys is accomplished using the surface integral equation method. The technique provides a means for (1) analyzing these effects on earth models of homogeneous conductivity; and (2) removing terrain effects from field data. A new method combining current source images with surface charge is developed to treat the electric field boundary conditions at the air‐earth interface. The method uses an image of each subsurface current source positioned above the surface, so as to induce a surface charge distribution which approximately cancels the charge distribution induced by the subsurface current source. The resulting residual surface charge distribution varies spatially more gradually than either of the original charge distributions, and hence may be represented accurately on a coarsely segmented model surface with simple basis functions. The topographic surface is modeled by a finite number of facets, each with constant slope and surface charge density. Charge values are obtained with an iterative solution technique. Surface electric fields are calculated from the surface charge distribution, current sources, and images. The magnetic field is found by evaluating a surface integral involving surface slopes and electric fields. The numerical solution is verified by comparisons with dipole‐dipole resistivity results from a two‐dimensional finite‐element model of a valley, and with analytic solutions for the magnetic fields over a dipping interface. Methods for terrain correcting mise‐à‐la‐masse and magnetometric resistivity data are demonstrated with examples using actual field measurements. The results of this study show that (1) rugged topography can significantly distort measurements in mise‐à‐la‐masse and magnetometric resistivity surveys; and (2) the described modeling technique provides an effective means of calculating terrain corrections for both the mise‐à‐la‐masse and magnetometric resistivity methods over complex three‐dimensional topography.

Three-Dimensional Terrain Corrections for mise-á-la-masse and Magnetometric Resistivity Surveys

Three dimensional modeling of topographic effects in mise-a-la-masse and magnetometric resistivity surveys is accomplished using the surface integral equation method. The technique provides a means for (1) analyzing these effects on earth models of homogeneous conductivity; and (2) removing terrain effects from field data. A new method combining current source images with surface charge is developed to treat the electric field boundary conditions at the air-earth interface. The method uses an image of each subsurface current source positioned above the surface, so as to induce a surface charge distribution which approximately cancels the charge distribution induced by the subsurface current source. The resulting residual surface charge distribution varies spatially more gradually than either of the original charge distributions, and hence may be represented accurately on a coarsely segmented model surface with simple basis functions. The topographic surface is modeled by a finite number of facets, each with constant slope and surface charge density. Charge values are obtained with an iterative solution technique. Surface electric fields are calculated from the surface charge distribution, current sources, and images. The magnetic field is found 'by evaluating a surface integral involving surface slopes and electric fields. The numerical solution is verified by comparisons with dipole dipole resistivity results from a two-dimensional finite-element model of a valley, and with analytic solutions for the magnetic fields over a dipping interface. Methods for terrain correcting mise-a-la-masse and magnetometric resistivity data are demonstrated with examples using actual field measurements. The results of this study show that (1) rugged topography can significantly distort measurements in mise-a-la-masse and magnetometric resistivity surveys; and (2) the described modeling technique provides an effective means of calculating terrain corrections for both the mise-á-la-masse and magnetometric resistivity methods over complex three-dimensional topography.

A BOREHOLE MAGNETOMETRIC RESISTIVITY EXPERIMENT*

AbstractA borehole magnetometric resistivity (MMR) experiment is described in which an attempt is made to determine the extent and orientation of zones of fissuring within an otherwise massive sequence of carboniferous limestone. The region under investigation lies beneath a landfill site and the main parameter of interest is the direction of flow of ground water, which will be influenced by the orientation of the faults or fissures within the rock. The MMR method possesses some extremely advantageous features for application to hydro‐geological problems of this kind. The method is sensitive to electrical current channelling within an otherwise relatively resistive medium, and the detection of the magnetic field within the borehole does not depend upon electrical contact of the receiver with the ground. Consequently, the method can be used in dry or plastic‐cased boreholes. A direction of maximum electrical current flow is deduced from the MMR data which coincides with the predominant direction of jointing within the region.We are most grateful to the Water Research Centre, Medmenham, Berkshire, for supporting this research and particularly to Dr M. Fleet and Mr K. Beesley for their help and advice.

On measuring the electrical conductivity of the oceanic crust by a modified magnetometric resistivity method

Magnetotelluric sounding reveals a conductive zone beneath the Pacific Ocean at depths in excess of 60 km but does not resolve the resistivity of the lithosphere above this zone. Further resolution can be obtained by controlled source electrical methods. The simplest of these are the galvanic techniques. Dipole‐dipole resistivity sounding is not suitable because dipole separations of thousands of kilometers would be required to obtain values of the resistivity. A viable alternative is to measure on the ocean floor the magnetic field of a vertical bipolar source extending from the sea surface to the seafloor. Magnetometer transmitter separations of only a few kilometers are sufficient to determine the resistivity of a half space beneath the ocean. Sounding curves similar to those of the resistivity method may be constructed to resolve the resistivity of a layered lithosphere. The curves constructed are valid at alternating frequencies which are small compared with a skin frequency not in the ocean but in the lithosphere. The depth of penetration is of the order of half the transmitter‐receiver separation. Magnetic field amplitudes are in the range of picotesla for reasonable lithospheric resistivities and separations up to 10 times the length of the bipole. Modern instrumentation, modified for the ocean floor, can detect such signals at a range of 20 km at a frequency of about 0.02 Hz averaged over several hours.

A FIELD TEST OF THE MAGNETOMETRIC RESISTIVITY (MMR) METHOD

The electrical prospecting method, known as the Magnetometric Resistivity (MMR) method, is based on the measurement of the low level (about 100 milligamma), low‐frequency (1–5 Hz) magnetic fields associated with noninductive current flow in the ground. The horizontal component of the magnetic field is measured along profiles which are at right angles to a baseline joining two widely separated current electrodes. The field test was conducted on a plateau in the western cordillera, where the topography is characterized by steep hills, bold ridges, gullies and narrow canyons. A steep faulted contact between basement rocks of differing resistivity is exposed on one flank of the plateau, beneath over 500 m of Tertiary volcanics and sediments. The object of the test was to determine if the basement contact could be mapped by the MMR method, working entirely on top of the plateau. The plan position of the contact could be inferred approximately from measurements at the outcrop. The object was achieved with a minimum of data processing. Using a theoretical model which resembles a thick, outcropping vertical dike of infinite vertical extent, the contrast in resistivity across the contact is estimated. A further model, that of an exponential “alpha” center, is also fitted to the data in an effort to pin‐point an anomalous region which may have unusually high conductivity.

The Magnetometric Resistivity Method and its Application to the Mapping of a Fault

An electrical prospecting method, known as the Magnetometric Resistivity (MMR) method, is described both in theory and in practice. It is based on the measurement of the low level (<5 gamma), low-frequency (1–5 Hz) magnetic fields associated with noninductive current flow in the earth. The field techniques are similar to those employed in the gradient array resistivity method, except the potential electrodes are replaced by a highly sensitive component magnetometer. The horizontal component of the magnetic field, which is at right angles to the line joining the current electrodes, is usually measured. The MMR anomaly is the difference between the measured values and the 'normal' values, which are the values of the component expected at the surface of a uniform earth.It is shown that an earth composed of uniform, horizontal layers gives no MMR anomaly. But the MMR anomalies due to a current dipole, a sphere and a fault are calculated. The anomaly due to a sphere is compared with anomalies for the sphere obtained by other geophysical methods. The principal profile of the anomaly is similar to that of the vertical field anomaly of a vertically magnetised sphere.It is argued that the MMR method may be superior to the gradient-array resistivity method in mapping inhomogeneities that are embedded at depth in a conductive host or are located beneath conductive overburden. The latter is demonstrated using an analogue model.The field apparatus and the field techniques of the method are described. The data taken during a field test of the method over the Gloucester Fault, at Leitrim, Ontario, are presented and are interpreted in terms of a conductivity contrast across the fault. The contrast determined agrees quantitatively with estimates obtained using other electrical methods.