Magnetic measurements in electrical prospecting by resistivity methods

The electrical resistivity and induced polarization (IP) methods are widely used in geological mapping, prospecting and exploration of mineral deposits, engineering geology, hydrogeology, archaeology, and geotechnical and environmental applications. Historically, these methods have formed the basis of the electrical prospecting technique. In these methods, a DC or low-frequency AC electrical current is introduced into the earth through a grounded transmitter line. The measured quantity is the electric field. However, if the earth’s resistivity or chargeability changes horizontally, this change gives rise to an anomalous magnetic field, which is studied by the magnetometric resistivity (MMR) and magnetic induced polarization (MIP) methods, respectively. Along with advantages, some shortcomings are inherent in the MMR and MIP techniques. Apparently, the main drawback of these methods is that the magnetic fields of both the transmitter line wire and ground electrodes on the surface are several orders of magnitude greater than the anomalous magnetic field response. This introduces a significant “noise” to magnetic-resistivity data. We investigate the potential of using a circular electric dipole (CED) in magnetometric resistivity techniques. It has been found that the application of a CED, instead of a conventional transmitter line, dramatically enhances the signal-to-noise ratio.     

Biaxial anisotropy in geoelectric prospecting

In applied studies on electromagnetic well-logging and electric prospecting, a medium is supposed to be either isotropic or with conventional anisotropy of conductivity. However, in the meantime, there is a clear hint that a medium may exhibit even biaxial anisotropy; i.e., the resistances along all three directions X, and Y, Z are different. Based on the analytical solution by the method of separation of variables, the paper considers an algorithm for the calculation of the electromagnetic field in a layered medium with biaxial conductivity anisotropy involving an arbitrary direction of horizontal conductivities in each layer. In this case, the theoretical solution and the algorithm display substantial peculiarities, and the numerical implementation involves many complexities. These problems were examined and solved mainly due to the constant comparison with the calculations carried by the finite element method and their analysis. Programs for the calculation of electromagnetic fields in the frequency and time domains were developed, and results of geoelectric interest were obtained. For example, the excitement of a horizontally-layered section by a magnetic dipole produces a vertical electric component of the field.     

Generalized Tikhonov's algorithm for accurate calculation of one-dimensional transient responses directly in time domain

In the last two decades, forward modelling for the time domain (transient) electromagnetic method has concentrated almost entirely on multi-dimensional models and algorithms. At the same time, the interpretation of real field data is still mainly one dimensional. This is caused by the lack of an efficient multi-dimensional acquisition procedure supported by sufficiently fast and reliable inversion software, on the one hand, and by the great efficiency of one-dimensional field set up and interpretation of the data on the other hand. The latter is particularly true for the short offset transient electromagnetic method, which is much less sensitive to multi-dimensional effects, compared to long offset methods. The most commonly used one-dimensional forward modelling algorithms are based on the spectral method, which requires calculating rapidly oscillating Fourier–Bessel (Hankel) integrals. Due to the very fast decay of short offset responses, the integrals become computationally unstable at late times of the transient process. Although this problem has been successfully solved for practically feasible measurement times of conventional short offset systems using transverse electric and mixed transverse electric and transverse magnetic fields, it turned out crucial for novel methods based on the use of unimodal transverse magnetic fields. These methods are much more sensitive to geoelectric parameters of the Earth in general and those of resistive targets, in particular, but they generate responses, which drop at late times significantly faster than those of conventional methods. Such behaviour of transverse magnetic fields represents severe computational problem for the spectral method, but is successfully solved by direct time domain algorithms. This article describes a generalization of the well-known Tikhonov's solution to a boundary value problem directly in time domain, which is applied to an arbitrary one-dimensional earth model excited by an arbitrary source. Contrary to existing spectral algorithms, the described method allows accurate calculations of both transverse electric and transverse magnetic transient responses at arbitrarily late times. On the other hand, it is more time efficient than finite-difference/finite element direct time domain algorithms and provides analytical late-stage asymptotic solutions.     

Formation of the macroanisotropic geoelectric parameters of a thin-layered geologic medium and the resolution of electrical prospecting

The accuracy (detail, resolution) of controlled-source electrical prospecting is a combination of several disparate elements. They include the model framework used in the interpretation. A real geologic medium has a complex structure. Sedimentary rocks have a layered structure with fractal properties: The layers split into smaller ones. Large-scale geoelectrical studies (for example, electrical prospecting) require a proper geoelectrical model. By necessity, the 1D geoelectrical model in electrical prospecting is horizontally layered, with thick (hundreds of meters) homogeneous layers, whereas a fine structure is neglected. To study some aspects of this problem, we performed a set of numerical experiments. They were aimed at studying the TEM response from a formation consisting of many thin layers with random geoelectric parameters, mainly resistivity.     

Transient response of a homogeneous half space with due regard for displacement currents

The solution for the half-space model is represented directly in the time domain as computationally stable convolution integrals. The influence of the geoelectric parameters of the earth and transmitter current waveform are then investigated for both infinitesimal and finite-dimensional transmitter loops. Simple empirical formulae are derived to account for the finite duration of the transmitter current turn off time.The whole transient process is divided into three essentially different stages: the propagation stage, the intermediate stage and the diffusion stage. The first is characterized by extremely complicated signal behavior. Apparently, interpretation of the field data using any kind of model fitting inversion algorithm is impossible in this stage. The diffusion stage virtually coincides with that used in the quasi-static case and is, therefore, unsuitable for detecting the dielectric properties of the earth. The intermediate stage is, thus, the only possible time range in which the dielectric properties can be detected using the dynamic characteristics of the signal.The duration of each stage is evaluated depending on the geoelectric parameters of the earth for different transmitter current waveforms.     

Transient electromagnetic surveys with unimodal transverse magnetic field: ideas and results

The theory behind transient electromagnetic surveys can be well described in terms of transverse magnetic and transverse electric modes. Soundings using transverse magnetic and transverse electric modes require different source configurations. In this study, we consider an alternating transverse magnetic field excitation by a circular electric dipole. The circular electric dipole transmitter is a horizontal analogue of the vertical electric dipole. Offshore surveys using circular electric dipole might represent an alternative to the conventional marine controlled‐source electromagnetic method at shallow sea and/or for exploring relatively small targets. Field acquisition is carried out by recording either electric or magnetic responses. Electric responses bear information on the 1D structure of a layered earth and successfully resolve high‐resistivity targets in marine surveys. Land‐based circular electric dipole soundings are affected by induced polarisation. On the contrary, magnetic responses are absent on the surface of a 1D earth, and as a result, they are very sensitive to any and even very small 3D conductivity perturbations. In addition, they are sensitive to induced polarisation or some other polarisation effects in the subsurface. At present, circular electric dipole transmitters and magnetic receivers are successfully used in on‐land mineral and petroleum exploration.     

TEM-TDEM soundings with the use of vertical loops

In the course of scientific collaboration, we were involved in discussion on the capacity of a vertical loop configuration to resolve thin high-resistivity layers, which is quite an interesting and largely debated point. We report a small forward modeling study including an algorithm based on an analytical solution by separation of variables and a respective program for computing the time-domain TEM field of a horizontal magnetic dipole. We infer that the subsurface vertical loop system shows no critical advantage in resolving thin insulating inclusions.     

Geosystem as an object of landscape study

GeoJournal - Systems' ideas are organically inherent in the integrational potential of geography. Analysis of evolution of understanding of a “geosystem” witnesses for ecologization...     

Excitation of a half-space by a radial current sheet source

Some advantages and problems of the new geoelectrical prospecting method, i.e., vertical electric current soundings (VECS) are discussed. This method is based on using a new source, namely a circular electric dipole (CED). The source is installed by one of the transmitter poles grounded in the central point and the other pole uniformly grounded around with a radius determined by the depth of investigation desired. It can be defined as a noninductive source. The previous research was based on the diffusion approach. In this paper the author uses the solution with due regards for displacement currents in the frequency and time domain. A major disadvantage of the CED scheme is the need to provide a symmetrical grounding of the outer ring electrode. A possible way to avoid this requirement is to adopt an ungrounded CED array.