Self-potential Anomalies Research Articles

Quantitative analysis of self-potential anomalies in archaeological sites of Israel: an overview

Self-potential (SP) method is one of the most non-expensive and unsophisticated geophysical methods. However, its application limits absence of reliable interpreting methodology, first for the complex geological–environmental conditions. The typical disturbances appearing in the SP method and ways for their removing (elimination) are discussed. Some brief review of the available interpretation methods indicates their insufficient effectivity, especially for the complex environments. For the magnetic method of geophysical prospecting, special quantitative procedures applicable under non-trivial environments (oblique polarization, rugged relief and unknown level of the normal field) have been recently developed. Performed analysis allowed to revealing some essential common peculiarities between the magnetic and SP fields. These common aspects make it possible to apply the advanced procedures developed in magnetic prospecting to SP method. Besides the reliable determination of the depth of anomalous target, these methodologies enable to calculate the polarization effect and corrections for the non-horizontal SP observations. For classification of SP anomalies is supposed to employ a new parameter—‘self-potential moment’. These procedures (improved modifications of characteristic point and tangent techniques) have been successfully tested on SP models and employed in the real situations in various archaeological sites in Israel. The present paper generalizes the earlier performed studies. The obtained results indicate the practical importance of the developed methodology.

Global Optimization for Delineation of Self-potential Anomaly of a 2D Inclined Plate

A fast and efficient technique for explanation of self-potential anomalies is of immense importance for exploration, engineering, and environmental problems. Estimation of model parameters of ore bodies in the subsurface is the primary concern in mineral exploration. In most cases, self-potential data are delineated considering various simple or idealized structures for the interpretation of lateral and vertical variations of subsurface ore bodies. In this context, we developed an inversion algorithm to determine the different parameters associated with a 2D inclined plate-type structure, which does not require any a priori information. The developed algorithm can interpret appropriately every parameter with minimum uncertainty. The position of causative source body (x0), its half-width (w) and its depth (z) were the parameters interpreted using the developed algorithm. It was found that these parameters were well resolved within the estimated uncertainty, although solutions for w showed wide variability. The technique was verified with synthetic data without noise and with different degrees of Gaussian noise. The technique was also confirmed with three field datasets for mineral exploration, and the interpreted parameters were in fair agreement with those reported in earlier works.

PGE Prospecting Features by Results of SP and Geoelectrochemistry Methods

Platinum-group elements (PGE) prospecting can be carried out with use of geochemical or geophysical methods. Geochemical methods can study objects located at the surface only, and geophysical methods are not always effective because of too low concentrations of PGE in the host rocks do not change the ores physical properties. Geoelectrochemical methods are suitable for deep-lying ores prospecting. Problem of movable forms creation of passive PGE can be fixed takin into account natural electrical processes. It is shown by laboratory experiment and tested during field works. Laboratory data confirmed the influence of electric current on the transition of PGE dozens presents in the dissolved form. The complex geophysical and geoelectrochemical surveys was carried out on the territory of Aganozerskiy (South Karelia), Svetloborskiy and Kachkanar (Middle Urals) massifs containing PGE mineralization. Preconditions for the creation of self-potential (SP) anomalies of redox nature are allocated within these massifs and characteristics of self-potential anomalies are investigated. SP sources are prerequisite for the mobile PGE forms halos creation if they are close to platinum mineralization zones. Moving up in the jet halos by natural ionic flotation mechanism and capillary rise in a circle of meteoric water evaporation and seepage can form zones of elevated concentrations of PGE secondary fixed forms near the surface, which can be detected by different geoelectrochemical methods. It can be concluded that geoelectrochemical methods are not always effective, but in the presence of sources of SP in a geological environment. The interpretation of the geoelectrochemistry anomaly for noble metals should take into account the effect of activators of the mobile forms formation.

Groundwater Flow Monitoring by Fusion Probability Tomography of Self-Potential Data

Groundwater flow could produce an observable self-potential (SP) signal; it is, therefore, useful to use SP signal to investigate the condition of water flow, which is meaningful to the processes to which the water flow is critical, such as landslide warning, contaminant transport, and volcanic eruption. Most of the SP data inversion methods are based on prior in situ electrical resistivity information, which is difficult to be accurately obtained in the continuous monitoring of the vadose zone. We suggest a fusion scheme of integrating the electric charge occurrence probability (ECOP) tomography and the continuous complex wavelet transform (CCWT) method to process and interpret SP data when it lacks prior electrical structure information on the background media. A model of fracture flow with precipitation recharge is used to testify our proposed method. The results indicate that the combination scheme can locate the source of SP anomalies with improved accuracy. The proposed fusion probability tomography can locate the fracture flow with an explicit region of probability anomaly and suppress the irrelevant noises.

Underground Coal fire Mapping Using Analysis of Self-potential (SP) Data Collected from Akashkinaree Colliery, Jharia Coalfield, India

Abstract Sub-surface coal mine fires burn valuable resources and give rise to great threat in the local environs and people’s health. Jharia coal field is one of utmost fire affected coal fields in India. This paper deals with the study of sub-surface coal fire using self-potential (SP) method, a cost-effective and non-invasive passive geophysical technique. SP method in coal fire mapping works based on redox potential generated by oxidation of coal and also on Thomson potential due to temperature gradient. Initially, SP anomaly map generated using electrode drift corrected field data. The SP anomalies in the present study area vary from ~95 to ~ -40mV. SP anomaly map has been continued to a height of 10m from the ground to remove the possible shallow surface noises. The upward continued anomaly map has been enhanced using first vertical derivative. Further, Euler’s depth solutions have been estimated to understand possible geometry and source depth of sub-surface coal fires. The local geological map and field photograph corroborate well with the results for the characterization of coal seam fires.

Robust interpretation of single and multiple self-potential anomalies via flower pollination algorithm

Self-potential (SP) method is suitable for identifying positions of fractures, ore bodies, leakage, corrosion of metal, etc. The identification of SP anomalies can be implemented through inversion process, which not only serves to determine the best model parameters but also to estimate their uncertainties. The model parameters and their uncertainty can be determined from the posterior distribution model (PDM) of the SP anomaly inversion. To find the required PDM, in this paper, generalized likelihood uncertainty estimation (GLUE) thresholding is proposed as a joint approach together with flower pollination algorithm (FPA). This overall method is then tested on several synthetics and fields (i.e., metallic drum, Weiss, Sawoo, and LUSI anomalies) SP data containing single and multiples of SP sources. The results demonstrate that the proposed algorithm is robust for solving quantitative interpretations of SP data. Moreover, this method does not require prior assumptions over the shape of the anomaly source.

Numerical modeling of biogeobattery system from microbial degradation of underground organic contaminant

Redox fields observed near contaminated sites are related to electrochemical and electrobiological reactions. The term biogeobattery is used to describe the redox processes in underground organic contaminated sites which are biogeochemical systems, where bacteria, conductive minerals, oxygen, and organic contaminants create numerous anode–cathode pairs by transferring electrons from reducing areas under the water table or other anaerobic conditions to oxidizing areas above the water table. A numerical biogeobattery model established based on the geobattery and inert electrode model is the theoretical basis for numerical modeling. A coupled method of tetrahedral finite elements and hexahedral infinite elements is proposed for the algorithm basis of numerical modeling. Then a small-scale biogeochemical system and a field-scale landfill system are modeled using the numerical biogeobattery model and the finite–infinite element coupling method. The results suggest that the accuracy of the coupled method is superior to the finite element method. The amplitude change in the embedded redox field has a great effect on self-potential anomalies, while the gradient change in the embedded redox field has no significant impact on self-potential anomalies. The resolution of the surface self-potential decreases with the depth of biogeochemical systems. The biogeobattery model based on the inert electrode model is an effective numerical model in biogeochemical systems.

Detection of the Possible Buried Archeological Targets Using the Geophysical Methods of Ground Penetrating Radar (GPR) and Self Potential (SP), Kom Ombo Temple, Aswan Governorate, Egypt

Kom Ombo temple is one of temples which were belted over high plateau close to the River Nile, near to Aswan in Egypt in the Greek-Roman period. The expected archaeological remains in the selected area are the hidden tunnels of the mummified crocodiles. The aim of the present work is to detect any of these tunnels by the application of the (GPR) and (SP) methods. The interpretation of the 10 GPR profiles revealed some locations of possible hidden tunnels. These locations show different contrasts and high amplitudes of the reflected signals, compared to the enclosing soil; also the scattering of the signals is higher than the bed layer in these locations, which may reveal the possible buried mummified crocodile tunnels in the study area. The depths of the possible targets range from 2.0 m to 2.5 m. The SP electric map shows that the study area possesses a range of about 135 mV of the potential differences between the measured stations. The positive response of the SP data is mainly concentrated at the central part of the study area. The relatively weak, negative SP anomalies may be related to moisture in the soil. The positive SP anomalies on the SP electric map display possible significant correlation between them and the inferred tunnel locations from the GPR data. The calculated depths from the SP profiles show significant agreement with that estimated from GPR data depths, which indicate that the SP electric method can be used as a successful tool in detecting buried archaeological remains in support of GPR.

Delineation of gold mineralization near Lawa village, North Singhbhum Mobile Belt, India, using electrical resistivity imaging, self-potential and very low frequency methods

North Singhbhum Mobile Belt (NSMB), India, is almost 200 km long elongated zone. The region has stretched out shear zones that are in E-W direction with a complex history of lithology, structure, metamorphism and tectonic settings, which closely mirrors other gold producing orogenic metamorphic mobile belts. The study area, Lawa village, is located in Nimdih Block of Seraikella-Kharsawan district, Jharkhand, India. Detailed Electrical resistivity tomography (ERT), Self-Potential (SP) and Very Low Frequency Electromagnetic (VLF-EM) surveys were carried out to investigate appropriate conducting zones associated with CuU mineralization for gold exploration. Apparent resistivity pseudo sections obtained and compared with the current density pseudo sections of VLF data achieved from KH filtering technique and SP anomaly curves to mark out considerable lateral extension of prospective source. Conventional filtering methods cannot help in eliminating non-linear and non-stationary noises that contaminated the VLF-EM data, obtained from the field. To regain signals with significant geologic information, we have used empirical mode decomposition (EMD) technique. Results obtained show an excellent correlation between ERT and VLF profiles marked by low resistivity, high current density zones. This correlation indicates that the conducting features have shallow depth and are steeply dipping with a strike along E–W direction. However, SP anomaly curves were not clear enough to harmonize with the above result. This study boosts the scope of further geo-electrical and EM investigations in this region to explore appropriate target locations for gold exploration, which may be later, utilized on a significant scale for commercial purposes.

Estimating Model Parameters from Self-Potential Anomaly of 2D Inclined Sheet Using Whale Optimization Algorithm: Applications to Mineral Exploration and Tracing Shear Zones

The use of spontaneous potential (SP) anomalies is well known in the geophysical literatures because of its effectiveness and significance in solving many complex problems in mineral exploration. The inverse problem of self-potential data interpretation is generally ill-posed and nonlinear. Methods based on derivative analysis usually fail to reach the optimal solution (global minimum) and trapped in a local minimum. A new simple heuristic solution to SP anomalies due to 2D inclined sheet of infinite horizontal length is investigated in this study to solve these problems. This method is based on utilizing whale optimization algorithm (WOA) as an effective heuristic solution to the inverse problem of self-potential field due to a 2D inclined sheet. In this context, the WOA was applied first to synthetic example, where the effect of the random noise was examined and the method revealed good results using proper MATLAB code. The technique was then applied on several real field profiles from different localities aiming to determine the parameters of mineralized zones or the associated shear zones. The inversion parameters revealed that WOA detected accurately the unknown parameters and showed a good validation when compared with the published inversion methods.

Estimating the location of a causative body from a self-potential anomaly using 2D and 3D normalized full gradient and Euler deconvolution

Changes in the size and depth of sources greatly affect self-potential (SP) anomalies. Therefore, it is important to determine the location of the source accurately. In the present study, applications of the normalized full gradient (NFG) method and Euler deconvolution (EUD) were described to determine the location of the sphere-like SP body as complementary approaches to other optimization algorithms. The NFG and EUD methods were tested on synthetic, noise-free, and noisy anomalies caused by sphere-like models in two-dimensional (2D) and three-dimensional (3D) cases. Subsequently, the methods were applied to real field data. The importance of the present study lies in the fact that it is the first 3D application of these methods to the SP anomaly caused by the sphere-like model in the literature. In order to determine the optimum harmonic number in the NFG method, a new criterion was used instead of the usual trial-and-error method, providing more reliable selection possibilities. In a similar way, average values were used to determine the window size accurately in the EUD method. The test results of the synthetic and real field models were satisfactory. They showed that both methods are applicable to determine the location of sphere-like structures, such as ore deposits, in self-potential surveys.

PEMODELAN KONTINU 2-D BERBASIS EFEK ELEKTOKINETIK DENGAN FINITE ELEMENT PENDEKATAN GALERKIN

Method refers to a measurement of naturally electric potential on the surface. Self Potential is one of thegeophysical methods that can be used to identify fluid flow in the subsurface. Finite element method hasbeen used to fluid flow velocity modeling, current source, and potential distribution modeling based onelectrokinetic effect in 2-D profile. The sign of potensial respon (negative or positive) is an important factorfor the interpretation of self-potential anomalies. Finite element method is one of the numerical methods forsolve the boundary value problems and the boundary conditions in the form of differential equations in SelfPotentialmodeling

Monitoring redox sensitive conditions at the groundwater interface using electrical resistivity and self-potential

Assessing redox conditions in soil and groundwater is challenging because redox reactions are oxygen sensitive, hence, destructive sampling methods may provide contact with air and influence the redox state. Furthermore, commonly used redox potential sensors provide only point measurements and are prone to error. This paper assesses whether combining electrical resistivity (ER) and self-potential (SP) measurements can allow the mapping of zones affected by anaerobic degradation. We use ER imaging because anaerobic degradation can release iron and manganese ions, which decreases pore water resistivity, and produces gas, which increases resistivity. Also, electrochemical differences between anaerobic and aerobic zones may create an electron flow, forming a self-potential anomaly. In this laboratory study, with four sand tanks with constant water table heights, time-lapse ER and SP mapped changes in electrical/electron flow properties due to organic contaminant (propylene glycol) degradation. Sampled pore water mapped degradation and water chemistry. When iron and manganese oxides were available, degradation reduced resistivity, because of cation release in pore water. When iron and manganese oxides were unavailable, resistivity increased, plausibly from methane production, which reduced water saturation. To bypass the reactions producing methane and release of metallic cations, a metal pipe was installed in the sand tanks between anaerobic and aerobic zones. The degradation creates an electron surplus at the anaerobic degradation site. The metal pipe allowed electron flow from the anaerobic degradation site to the oxygen-rich near surface. The electrical current sent through the metal pipe formed an SP anomaly observable on the surface of the sand tank. Time-lapse ER demonstrates potential for mapping degradation zones under anaerobic conditions. When an electrical conductor bridges the anaerobic zone with the near surface, the electron flow causes an SP anomaly on the surface. However, electrochemical differences between anaerobic and aerobic zones alone produced no SP signal. Despite their limitations, ER and SP are promising tools for monitoring redox sensitive conditions in unsaturated sandy soils but should not be used in isolation.

Monitoring time evolution of self-potential anomaly sources by a new global optimization approach. Application to organic contaminant transport

Complex interactions among organic contaminant, soil and water change the electrical properties of the subsurface often causing strong self-potential (SP) anomalies, whose monitoring is proposed as a useful tool to determine the temporal evolution of the contaminant plumes. In the present study, we focus on the problem of organic contaminant transport related to olive oil mill wastes (OOMWs), which represent an important environmental problem in Mediterranean countries. The diffusion of the contaminants into the subsurface is studied by using a global optimization procedure on SP data measured, at different times, in a well-studied contaminated pilot area located next to the Keritis river in western Crete island (Greece). Despite the complex hydrogeological conditions related to the proximity to both the Keritis river and a small seasonal stream, the analysis of five SP datasets acquired along the same profile shows that the proposed hybrid Genetic-Price algorithm is able to reproduce the main features of the SP signals, thus identifying probable multiple SP anomaly sources and their changes over time. In particular, preferential horizontal pathways of OOMW and their migration in the vadose zone during the summer season were identified, providing some useful insights for future planning of remediation actions.

On studying flow through a fracture using self-potential anomaly: application to shallow aquifer recharge at Vilarelho da Raia, northern Portugal

We demonstrate a strategy of investigating recharge potential of a shallow aquifer in a hard rock terrain through flow in a fractured zone using self-potential (SP) anomaly and other ancillary (published) data related to the area of investigation. Our study area is Vilarelho da Raia, northern Portugal which possesses a shallow aquifer within a granitic terrain. We explain the phenomenon responsible for generating SP anomaly due to groundwater flow through a fractured conduit associating a fault. We propose delineating parameters associated with such structure from SP profile across it using a thin 2D conducting sheet model buried below subsurface. We propose a data based robust interpretation using derivative analysis of SP anomaly. We demonstrate that for sufficiently smooth SP anomaly across a 2D buried sheet model the first and the second order vertical derivatives of SP anomaly are the only necessary tools in delineating model parameters using characteristic features of derivatives curves and a set of closed form formulas. We propose using Savitzky–Golay derivative filter to get robust estimate of the first and second order horizontal derivatives. The first and second order vertical derivatives are obtained using Hilbert transform and Laplaces harmonic equation respectively. We applied the proposed technique on SP anomaly profile across a fault at Vilarelho da Raia in estimating parameters of a sheet model which represents the fault plane. We used the estimated parameters and the ancillary data taken from published literature to delineate hydrogeological parameters related to recharging of an aquifer within a granitic terrain.

A particle swarm optimization method for interpreting self-potential anomalies

Abstract This paper describes the use of the particle swarm optimization (PSO) method for interpreting observed self-potential anomalies measured along a profile. First, the technique applies the second moving average to the observed self-potential data in order to eradicate the possible influence of the regional anomaly (up to the third-order polynomial effect) via the filter of consecutive window lengths (s-values) and to calculate the residual anomaly. Following that, the PSO method is applied to the residual response to infer the source parameters: amplitude coefficient (K), depth (z), polarization angle (θ) and the shape factor (q) of the underlying buried target. The technique has been applied to three different theoretical and two field examples from the USA and Turkey. Comparisons have shown that the source parameters retrieved from the technique described here are in good agreement with the available geologic and geophysical information.

Self-potential signals associated with localized leaks in embankment dams and dikes

The self-potential method can be used to detect and monitor anomalous seepages in dams and embankments. In such a case, an electrical field of electrokinetic nature (i.e., associated with pore water flow) can be measured using a set of non-polarizable electrodes typically located at the ground surface or in some wells. This field can be in turn related to the pattern of groundwater flow. We built an experimental dam to investigate to which extent the self-potential method can help characterizing seepages in dams. We first use the finite element method to simulate the ground water flow in a heterogeneous porous and permeable material by solving the groundwater flow equation. The resulting groundwater flow solution is then used to compute the electrical potential distribution by solving the corresponding elliptic partial differential equation. In a preliminary experiment, we could not measure any self-potential anomaly associated with the infiltration of water in the dam. Our numerical simulations showed that the magnitudes of the self-potential anomalies were controlled by (1) the nature of the flow regime (viscous laminar versus inertial laminar flow regimes) and (2) the presence of insulating Polyvinyl Chloride (PVC) tubes located at the end of the preferential flow channels in the structure of the dam. Thanks to these numerical simulations, we added sand at the entrance of the infiltration area in order to reduce the effects of the PVC tubes and to restrain the flow regime to the viscous laminar flow regime. New experiments allowed for detecting a self-potential anomaly with an amplitude of around −9 mV consistent with that obtained through numerical modelling with a finite element simulator. This comparison was used to test the accuracy of the modelling approach and define the strengths and weaknesses of the self-potential method to determine preferential seepages in earth dam structures.

Determination of source parameters of simple-shaped geologic subsurface structures from self-potential anomalies using enhanced local wavenumber method

Simple geometry model structures can be useful in quantitative evaluation of self-potential data. In this paper, we solve local wavenumber equation to estimate the horizontal position, the depth and the type of the causative source geometry by using a linear least-squares approximation. The advantages of the algorithm in determining the horizontal position and depth measure are its independency to shape factor of the sources and also its simple computations. The algorithm is built in Matlab environment. The validity of the algorithm is illustrated on variable noise-free and random noise included synthetic data from two-dimensional (2-D) models where the achieved parametric quantities coincide well with the actual ones. The algorithm is also utilized to real self-potential data from Ergani Copper district, Turkey. The results from the actual data application are in good agreement with the published literature for the study area. The source code of the algorithm is available from the authors on request.

Quantitative interpretation of multiple self-potential anomaly sources by a global optimization approach

Multiple self-potential (SP) anomalies are analyzed by using a Genetic-Price Algorithm (GPA), which has been recently introduced for the inversion of SP data. The proposed approach is tested on multiple synthetic anomalies, which are modeled by horizontal cylinders. First, a forward modeling is used to analyze the resolution of such anomalies by varying all model parameters. Then, GPA is applied to invert synthetic multiple SP anomalies. The numerical analyses show that the proposed approach is able to fully characterize the anomaly sources by providing the correct values of the model parameters as well as the number of sources, even if Gaussian random noise is added to the synthetic data. Furthermore, to show the computational efficiency of GPA, the results of a comparative analysis with the Very Fast Simulated Annealing algorithm are given. The validity of the GPA approach is confirmed by its application to three examples of self-potential field data from mineral exploration and groundwater investigations, which are presented and discussed in relation to other inversion approaches. Finally, the quantitative interpretation of multiple anomalies along a SP profile crossing the Mt. Somma-Vesuvius volcano caldera (southern Italy) is provided.

Electrical Survey Results in a Sub-Seabed Co2 Release Experiment (QICS)

A controlled sub-seabed CO2 release experiment was carried out during May–October 2012 in Ardmucknish Bay in Scotland, UK. A total of 4.2 tons of CO2 gas was released for 37 days from May to June at a maximum flow rate of 80 L/min (210 kg/d) at a depth of 11 m beneath 12 m deep sea floor. During and after the CO2 release experiment, electrical resistivity and self-potential (SP) surveys were conducted to monitor CO2 migration under the seabed. Three electrical survey lines with a length of 45 m were completed on the seabed. The electrical resistivity surveys were conducted September 23, 2012 and May 27, 2016. The resistivity survey data were acquired with a pole-pole electrode configuration. Two-imensional analysis was performed in the three lines. The determined resistivity structures showed that positive anomalies of the resistivity structure of 2012 compared with the structure of 2016 distributed 10-20 m south-west of the CO2 released point. In this area CO2 bubbles was seen escaping from the seabed by divers and video cameras. CO2 migration under the seabed was estimated in this area by the seismic survey. The SP survey results measured during the CO2 release experiment showed that positive SP anomaly of a maximum of 0.05 V was observed when CO2 flow rate increased from 60L/min to 80 L/min. The positive anomalies distributed south-west of CO2 released point. This area is almost same as the positive electrical resistivity anomaly area in 2012. These results show that the CO2 migration under the seabed caused positive electrical resistivity anomalies and also positive SP anomalies.