Self-potential Anomalies Research Articles

Estimation of the electrical potential distribution along metallic casing from surface self-potential profile

Corroding casings of wells generate negative self-potential (SP) anomalies, increasing from about −10 to −500mV in the vicinity of the well to 0mV at large distances. As reported in previous laboratory experiment, SP can be used to retrieve the distribution of electrical potential along the casing, which is somehow a proxy for the corrosion state of the casing. These studies used 3D (whole space) or surface 2D (whole surface) measurements of SP distribution; here we reported a field example, for which only a 1D surface SP profile is available. In order to retrieve the most probable associated potential distribution (defined by a spline) along the 11.1-m long metallic casing, we develop a direct model based on geometrical and geoelectrical properties of the medium, which was then used in a (non-deterministic) optimization procedure by simulated annealing, including some physical constrains. Tests carried out on a synthetic case allowed the initial source to be correctly retrieved, provided that the number of nodes used for the spline defining the potential distribution along the casing is large enough. The inversion of real field data provided a dipolar anomaly, with minimal negative amplitude of around −600mV at 5m, and maximal positive amplitude of about 1100mV at 9m (close to the level of the water table), this shape being in agreement with the results of previous laboratory studies.

A geological interpretation of geophysical self-potential anomalies in the Radzimowice ore district, Sudetes, south-western Poland

The analysis of the geophysical surveys results carried out in the Radzimowice area reveals that the anomalous zones of significant SP (self-potential) anomalies of natural SP fields do not substantially find a reflection in existing geological papers. Confrontation of existing geological elaborations with the results of SP analysis shows that the selfpotential studies expose significant elements of geological setting – i.e. run of faults, lithological boundaries, occurrence of ore mineralization – which can be helpful in clarifying the geological setting of the area. In the study, archived self-potential data were used and additionally new SP measurements were executed to justify their treatment as a valuable source of information.

Research on Self-potential Base Line Drift Processing in Wangji Oilfield

<span style="font-family: "Times New Roman","serif"; font-size: 10pt; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA;" lang="EN-US">The positive amplitude anomalies and base line drifts of self-potential (SP) curves make it difficult to recognize the amplitude anomalies and impossible to carry on the standardized calibration. In this way, it becomes extremely hard to read the amplitude anomalies of self-potential curves, increasing the difficulty in lithology recognition and stratigraphic correlation. What’s more, it is impossible to apply the computer technology to do the batch processing. Aimed at the practical problems, taking advantages of the collected digital logging data, positive self-potential anomaly recognition and transformation models have been established, converting the positive anomalies in the sandstone intervals into negative ones, solving the problem of two types of amplitude anomalies in the same scale range. Self-potential base line drift processing moves the biased base line to the null line step by step, adopting the method of piecewise fitting differential migration. With satisfying results, the conformation, amplitude and jugged degree are accordant with the primary curve, forming the technique of self-potential curve processing, laying foundation for logging translation and geological research.</span>

Laboratory modelling of self-potential anomalies due to spherical bodies

The relationship between the self-potential (SP) produced by a polarised sphere and its depth was studied in a laboratory experiment. This was carried out by using a sphere made up of two hemispheres of different materials: one copper and the other zinc. Self-potentials were measured by placing the sphere at a given depth in a rectangular glass tank filled with water. The surface of the water was covered by a sheet with 684 brass electrodes. A sensitive, high impedance digital voltmeter was used to measure potentials from each electrode to a ‘base’. We have measured the SP response of the metallic body and our work shows that SP signals of several millivolts are generated due to the sphere placed within water. The gridded SP data show a clear anomaly over the sphere at shallow depths, and as the depth of the sphere increases, the measured SP signal due to the sphere decreases. An analytical formula is given to determine the maximum depth of the sphere at which the presence of the anomaly can be detected. Responses from other geometries are examined as well.

Integrated Geophysical Mapping of Near-surface Structures and Utilities in Ile-Ife, Nigeria

Magnetic, self-potential (SP) and Ground-penetrating radar (GPR) methods were employed to investigate the near-surface conditions beneath a road in Obafemi Awolowo University Campus, Ile-Ife with the aim of mapping previously suspected dyke, fractures, buried utilities in the area. Fifty stations were established for the magnetic and SP methods along five and four traverses respectively while GPR data was acquired along three traverses, all parallel to the general strike of the rocks in the area (NW-SE). The magnetic and SP readings obtained were interpreted qualitatively and quantitatively using least-squares to identify the location/trends and determine source parameters of features of interest. Also, GPR sections were analyzed using basic radar processing tools. The magnetic field readings ranged from -310nT to 220nT while the SP readings with range -126mV to 55mV. The anomaly source was delineated on low closures on the iso-magnetic map while the inflexion points on the SP profiles lie above the target source. Results of quantitative interpretation of magnetic and SP anomalies revealed a spherical, sub-vertical source with depth to top values of 1.6m to 3.85m. The radargrams revealed the positions and depths of utilities such as pipes and cables, shallow-dipping fractures, a road culvert and partially weathered bedrock layers overlying fresh basement in the area. The penetration depth of the GPR was 14 m and the depth to top of the basement ranged from 4 m to 6 m. The techniques employed mapped the subsurface structural/cultural features in the study area and determined their positions and depth extents. crust than the other deeper layers. As a result increased understanding of the dynamics of this part of the Earth is paramount (1). In this paper, the primary aim was to map the shallow portions of the earth within a defined area in Ile-Ife; up to a depth of about 15 m, and confirm the presence of previously suspected fractures, fault, dyke and man-made utilities. Three geophysical methods namely Magnetic, Self-potential (SP) and Ground Penetrating Radar (GPR) were employed in the present study. The magnetic method involves the measurement of variations in total magnetic field of the earth, caused by local differences in the magnetization of the subsurface rocks and soils. It has a wide range of applications and has been used in locating buried metalliferous man-made objects such as cables and pipelines, location of boundaries between rocks, faults, and dykes (2). The SP method is a passive geophysical technique that responds to naturally-occurring potential differences generated mainly by electrochemical, electrokinetic, and thermoelectric sources in the subsurface and has many applications ranging from mineral exploration, hydrogeological, environmental, geotechnical and engineering investigations. There is recorded success in the use of the SP method in shallow subsurface mapping of simple geometrical bodies (3) and sheet-like bodies (4). The GPR technique (which is a novel technique in Nigeria) detects electrical discontinuities due to features such as bedding planes, fault planes etc. in the shallow subsurface (usually less than 50m) by the generation, transmission, propagation, reflection and reception of discrete pulses of high-frequency electromagnetic energy with high-resolution imagery (5). GPR is effective for subsurface fracture and fracture zone imaging (6), soil and rock stratigraphy mapping (7) and underground utility mapping (8). Consequently, this integrated study was done to verify the presence of the geologic structures and cultural utilities in the area, and also to demonstrate the efficacy of the GPR technique in shallow subsurface mapping in a basement complex terrain of southwestern Nigeria.

Tracing of paleo-shear zones by self-potential data inversion: case studies from the KTB, Rittsteig, and Grossensees graphite-bearing fault planes

AbstractThis paper describes a new method for tracing paleo-shear zones of the continental crust by self-potential (SP) data inversion. The method falls within the deterministic inversion framework, and it is exclusively applicable for the interpretation of the SP anomalies measured along a profile over sheet-type structures such as conductive thin films of interconnected graphite precipitations formed on shear planes. The inverse method fits a residual SP anomaly by a single thin sheet and recovers the characteristic parameters (depth to the toph, extension in deptha, amplitude coefficientk, and amount and direction of dipθ) of the sheet. This method minimizes an objective functional in the space of the logarithmed and non-logarithmed model parameters (log(h), log(a), log(k), andθ) successively by the steepest descent (SD) and Gauss-Newton (GN) techniques in order to essentially maintain the stability and convergence of this inverse method. Prior to applying the method to real data, its accuracy, convergence, and stability are successfully verified on numerical examples with and without noise. The method is then applied to SP profiles from the German Continental Deep Drilling Program (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschla - KTB), Rittsteig, and Grossensees sites in Germany for tracing paleo-shear planes coated with graphitic deposits. The comparisons of geologic sections constructed in this paper (based on the proposed deterministic approach) against the existing published interpretations (obtained based on trial-and-error modeling) for the SP data of the KTB and Rittsteig sites have revealed that the deterministic approach suggests some new details that are of some geological significance. The findings of the proposed inverse scheme are supported by available drilling and other geophysical data. Furthermore, the real SP data of the Grossensees site have been interpreted (apparently for the first time ever) by the deterministic inverse scheme from which interpretive geologic cross sections are suggested. The computational efficiency, analysis of the numerical examples investigated, and comparisons of the real data inverted here have demonstrated that the developed deterministic approach is advantageous to the existing interpretation methods, and it is suitable for meaningful interpretation of SP data acquired elsewhere over graphitic occurrences on fault planes.

Integrated geophysical studies to elicit the subsurface structures associated with Uranium mineralization around South Purulia Shear Zone, India: A review

South Purulia Shear Zone in India is an important zone that hosts uranium mineralization. Since detailed geophysical studies have not been carried out in this region, an integrated geophysical study using self-potential, resistivity, very low frequency electromagnetic and radiometric methods was performed to identify the subsurface structures that could host the hydrothermally altered uranium deposits in the area. The study reveals the wide and large magnitude of negative self-potential anomaly across the South Purulia Shear Zone. The peak negative self-potential anomalies are correlated with the low gravity and low resistivity anomalies measured along various profiles. The low self-potential, gravity and low resistivity anomaly zones are also correlated with conducting zones inferred from very low frequency electromagnetic measurements. Interpretation of self-potential data reveals multiple thick sheet-type vertical and/or inclined structures which might be associated with uranium mineralization. Schlumberger resistivity sounding data reveals an increasing trend of apparent resistivity with current electrode separations/depth. Apparent conductance measured simultaneously with resistivity measurement reveals an increase in current flow of current as depth increases. This exhibits the presence of thin conducting layers at these depths, which is not reflected in resistivity sounding data due to suppression problem. Also this conducting layer is consistent in various soundings and is connected from Raghunathpur to the South Purulia Shear Zone. Correlation of very low frequency and self-potential data shows that the structures are comparable and a radiometric profile also advocate that the conducting structure is associated with radioactive minerals. These structures are likely to be mineralized zones as hydro-uranium anomaly has also been reported from various locations in the area. Moreover, drilling results at a specific mine near the study area also confirms the presence of uranium mineralization. The hydrothermal activity associated with uranium mineralization seems to be still active in the area. Such combined geophysical studies are essential to understand this highly complex shear zone for the economic exploitation of its natural resources.

Self-potential monitoring of a thermal pulse advecting through a preferential flow path

Summary There is a need to develop new non-intrusive geophysical methods to detect preferential flow paths in heterogeneous porous media. A laboratory experiment is performed to non-invasively localize a preferential flow pathway in a sandbox using a heat pulse monitored by time-lapse self-potential measurements. Our goal is to investigate the amplitude of the intrinsic thermoelectric self-potential anomalies and the ability of this method to track preferential flow paths. A negative self-potential anomaly (−10 to −15 mV with respect to the background signals) is observed at the surface of the tank after hot water is injected in the upstream reservoir during steady state flow between the upstream and downstream reservoirs of the sandbox. Repeating the same experiment with the same volume of water injected upstream, but at the same temperature as the background pore water, produces a negligible self-potential anomaly. The negative self-potential anomaly is possibly associated with an intrinsic thermoelectric effect, with the temperature dependence of the streaming potential coupling coefficient, or with an apparent thermoelectric effect associated with the temperature dependence of the electrodes themselves. We model the experiment in 3D using a finite element code. Our results show that time-lapse self-potential signals can be used to track the position of traveling heat flow pulses in saturated porous materials, and therefore to find preferential flow pathways, especially in a very permeable environment and in real time. The numerical model and the data allows quantifying the intrinsic thermoelectric coupling coefficient, which is on the order of −0.3 to −1.8 mV per degree Celsius. The temperature dependence of the streaming potential during the experiment is negligible with respect to the intrinsic thermoelectric coupling. However, the temperature dependence of the potential of the electrodes needs to be accounted for and is far from being negligible if the electrodes experience temperature changes.

Resolution of multiple sheet-type structures in self-potential measurement

The resolution of self-potential anomalies due to closely spaced multiple sheet-like bodies by the potential difference and potential gradient is studied in this paper. Self-potential anomalies due to several synthetic models were inverted through a very fast simulated annealing (VFSA) global optimization. Increase in depth to the top, polarization constant and depth extent of the body decreases resolution at a particular target separation. It has been observed that depth to the top and separation between two targets play an important role in the resolution. Vertical sheets at equal depth can be resolved in the potential difference measurement only if they are separated by at least four times their depth, while they can be resolved in the gradient method, if they are separated by twice the depth. Resolution using potential difference becomes more difficult for dipping sheets, although the potential gradient method can resolve them efficiently. Efficacy of potential gradient data in the inversion is demonstrated in the study using synthetic data as well as field measurement from South Purulia Shear Zone related with uranium investigation.

Localization of a coal seam fire using combined self-potential and resistivity data

The remediation of coal seam fires requires, as a first step, the precise localization of the burning front. Various geophysical methods can be employed for this purpose but not all of them can provide an unambiguous localization of this type of target. We propose a combined inversion of self-potential and resistivity data and the use of joint attributes (characterizing the target) to localize the burning front of a coal seam fire. This new methodology is applied to the 3D reconstruction of a shallow coal seam fire in Boulder, Colorado. The coal seam fire is located in the Gorham subbituminous coal formation at a depth comprised between 10 and 15m. The self-potential survey comprises 160 stations (5 profiles and 32 stations per profile) located at the ground surface with self-potential anomalies amounting +70 to −50mV with respect to an arbitrary reference located further away from the burning front. The resistivity survey comprises 5 resistivity profiles with 118 measurements per profile (590 measurements in total). The burning front is associated with very low value of the electrical resistivity (<15Ωm) and source current density probably of thermoelectric nature. The combined inversion and interpretation of the self-potential and resistivity data indicates clearly the position of the coal seam fire, which is corroborated by a thermal anomaly observed at depth of 30cm and indicating that the minimum depth of the thermal source is approximately 9m. This methodology can be easily extended to greater depths using resistivity derived from galvanometric or airborne electromagnetic methods and large-scale self-potential surveys.

Comparative study of geophysical and soil–gas investigations at the Hartoušov (Czech Republic) natural CO2 degassing site

Our study at this natural analog site contributes to the evaluation of methods within a hierarchical monitoring concept suited for the control of CO2 degassing. It supports the development of an effective monitoring concept for geological CO2 storage sites—carbon capture and storage as one of the pillars of the European climate change efforts. This study presents results of comprehensive investigations along a 500-m long profile within the Hartousov (Czech Republic) natural CO2 degassing site and gives structural information about the subsurface and interaction processes in relation to parameters measured. Measurements of CO2 concentrations and investigation of the subsurface using electrical resistivity tomography and self-potential methods provide information about subsurface properties. For their successful application it is necessary to take seasonal variations (e.g., soil moisture, temperature, meteorological conditions) into consideration due to their influence on these parameters. Locations of high CO2 concentration in shallow depths are related to positive self-potential anomalies, low soil moistures and high resistivity distributions, as well as high δ13C values and increased radon concentrations. CO2 ascends from deep geological sources via preferential pathways and accumulates in coarser sediments. Repetition of measurements (which includes the effects of seasonal variations) revealed similar trends and allows us to identify a clear, prominent zone of anomalous values. Coarser unconsolidated sedimentary layers are beneficial for the accumulation of CO2 gas. The distribution of such shallow geological structures needs to be considered as a significant environmental risk potential whenever sudden degassing of large gas volumes occurs.

2-D electrical resistivity imaging for investigating an active landslide along a ridgeway in Burdur region, southern Turkey

Two-dimensional (2-D) electrical resistivity imaging (ERI) technique has been used to characterize a landslide which occurred in a ridgeway between Burdur and Isparta in southern Turkey. The problems related to this road showed an increase due to road constructions and heavy rainfall. Primary attention was drawn to the determination of the thickness of the mobilized material, its possible sliding surface, and evaluation of the groundwater conditions related to the occurrence of the landslide. The eight survey profiles in the landslide area were implemented. Electrical resistivity survey has been carried out by the Wenner–Schlumberger array for data acquisition and smoothness-constrained least-squares method for data inversion. The self-potential (SP) survey data are presented as one-dimensional profile for each of the lines. ERIs pointed out a great heterogeneity of the near surface or mobilizing material and presence of impermeable material that could be associated to lateral and vertical limits of permeability. SP anomalies displayed that the positive values were located in water discharge on the surface whereas negatives in the sites of infiltration. The sliding surface is at a depth of about 10 m.

Asymmetrical structure, hydrothermal system and edifice stability: The case of Ubinas volcano, Peru, revealed by geophysical surveys

Ubinas volcano, the historically most active volcano in Peru straddles a low-relief high plateau and the flank of a steep valley. A multidisciplinary geophysical study has been performed to investigate the internal structure and the fluids flow within the edifice. We conducted 10 self-potential (SP) radial (from summit to base) profiles, 15 audio magnetotelluric (AMT) soundings on the west flank and a detailed survey of SP and soil temperature measurements on the summit caldera floor. The typical “V” shape of the SP radial profiles has been interpreted as the result of a hydrothermal zone superimposed on a hydrogeological zone in the upper parts of the edifice, and depicts a sub-circular SP positive anomaly, about 6km in diameter. The latter is centred on the summit, and is characterised by a larger extension on the western flank located on the low-relief high plateau. The AMT resistivity model shows the presence of a conductive body beneath the summit at a depth comparable to that of the bottom of the inner south crater in the present-day caldera, where intense hydrothermal manifestations occur. The lack of SP and temperature anomalies on the present caldera floor suggests a self-sealed hydrothermal system, where the inner south crater acts as a pressure release valve. Although no resistivity data exists on the eastern flank, we presume, based on the asymmetry of the basement topography, and the amplitude of SP anomalies on the east flank, which are approximately five fold that on the west flank, that gravitational flow of hydrothermal fluids may occur towards the deep valley of Ubinas. This hypothesis, supported by the presence of hot springs and faults on the eastern foot of the edifice, reinforces the idea that a large part of the southeast flank of the Ubinas volcano may be altered by hydrothermal activity and will tend to be less stable. One of the major findings that stems from this study is that the slope of the basement on which a volcano has grown plays a major role in the geometry of the hydrothermal systems.Another case of asymmetrical composite cone edifice, built on a steep topography, is observed on El Misti volcano (situated 70km west of Ubinas), which exhibits a similar SP pattern. These types of edifices have a high potential of spreading and sliding along the slope owing to the thicker accumulation of low cohesion and hydrothermally altered volcanic products.

Self-potential monitoring for hydrologic investigations in urban covered-karst terrain

In the covered karst of west-central Florida, USA, sinkholes (sandy collapse conduits) provide locally concentrated recharge to underlying aquifers. For water management, it would be beneficial to understand the rates at which collapse conduits recharge an underlying aquifer. Self-potential (SP) monitoring has promise as a noninvasive, relatively low-cost method for assessing temporal variability in flow. Previous studies suggested that negative SP anomalies over collapse conduits correspond to downward-flowing groundwater; however, before SP surveys can be reliable indicators of conduit flow, SP from ET, soil conductivity changes, redox, electrode effects, and cultural noise must be better understood. A year of continuous SP monitoring with two grids of approximately 15 Pb/PbCl/KCl nonpolarizing electrodes each was combined with high-resolution ground-penetrating radar surveys and intermittent water table monitoring over two small covered-karst conduits in Tampa, Florida, USA. Although variations in SP resulting from changes in cultural noise, soil conductivity, ET, redox, and rainfall were evident, separate and unrelated positive and negative SP anomalies episodically manifested over conduits, which suggested that conduit flow could be dynamic, not static. Three flow regimes in conduits were postulated: conduit permeability higher than in surrounding surficial sediments, conduit permeability lower than in surrounding surface sediments, conduit permeability matched surrounding surface sediments. Numerical steady-state 2D simulations in Comsol created the three postulated flow regimes and revealed that a different SP polarity could result from different rates of flow: positive SP corresponded to higher permeability conduits, negative SP corresponded to lower permeability conduits, no or minimal SP appeared when conduits and surrounding sediments had equal permeability. In these models, downward flow was not responsible for generating negative SP. To assess the hydrology of a conduit, it appears that SP should be monitored continuously. Further monitoring of field sites with hydrologic sensors is needed.

Theory and application of magnetic and self-potential methods in the detection of the Heshituoluogai coal fire, China

Coal fires are a major problem throughout the world. They threaten the environment and the health of people living nearby and result in significant economic losses. Efficient and economical control of these fires requires that the extent of the subsurface coal fire be delineated. In this paper, we first present laboratory experiments, revealing that new preferential alignment of magnetic moments, newly formed magnetite and thermoremanent magnetization are the root causes of magnetic anomalies in coal fire area. The redox potential and Thomson potential, which are the basis of the self-potential anomalies, are proposed additionally for application. Then, the geological setting and an overview of the Fifth Fire Area (FFA) of the Heshituoluogai coal fire in Xinjiang are introduced in detail. Finally, the magnetic and self-potential methods are combined to delineate the extent of the fire. Several data processing methods such as diurnal fluctuation rectification, reduction to pole and upward continuation are used to process the data to make the interpretation of results more straight forward. The locations of subsurface fire regions delineated by the magnetic and self-potential methods are consistent with the results of ground surveys, indicating that these two methods can be used effectively as a tool for the detection of coal fires.

Characteristics of Resistivity and Self-Potential Anomalies over Agbani Sandstone, Enugu State, Southeastern Nigeria

The characteristics of the Resistivity and Self-Potential (SP) anomalies over Agbani Sandstone have been carefully and painstakingly carried out. The study was aimed at investigating the possible rock types and their mineralogical potentials. Data was acquired using the high resolution versatile ABEM SAS 4000 resistivity meter, employing the profiling method. Datasets were analyzed using the Excel toolkits. Interpretation was basically qualitative. Based on the resistivity interpretation, Agbani Sandstone is laterally limited in extent while the mineralization potential is high as a result of the high negative SP anomalies. The negative SP values range is -200mV to -500mV. This is practically indicative of a sulphide orebodies – possibly pyrite (FeS2). Comparative profile plots show that the observed zones of sulphide orebodies are within the gradational contact of Agbani Sandstone with Awgu Shale. Stream sediment analysis and rock geochemical study are recommended. However, the study has shown that contact zones of sandstone deposits are possibly ore enrichment zones.

Characterization of Focused Seepage Through an Earthfill Dam Using Geoelectrical Methods

Resistivity and self-potential tomography can be used to investigate anomalous seepage inside heterogeneous earthen dams. The self-potential (SP) signals provide a unique signature to groundwater flow because the source current density responsible for the SP signals is proportional to the Darcy velocity. The distribution of the SP signals is also influenced by the distribution of the resistivity; therefore, resistivity and SP need to be used in concert to elucidate groundwater flow pathways. In this study, a survey is conducted at a small earthen dam in Colorado where anomalous seepage is observed on the downstream face at the dam toe. The data reveal SP and direct current resistivity anomalies that are used to delineate three anomalous seepage zones within the dam andto estimate the source of the localized seepage discharge. The SP data are inverted in two dimensions using the resistivity distribution to determine the distribution of the Darcy velocity responsible for the observed seepage. The inverted Darcy velocity agrees with an estimation of the Darcy velocity from the hydraulic conductivity obtained from a slug test and the observed head gradient.

Structural control of monogenetic volcanism in the Garrotxa volcanic field (Northeastern Spain) from gravity and self-potential measurements

We report new geophysical observations on the distribution of subsurface structures associated with monogenetic volcanism in the Garrotxa volcanic field (Northern Spain). As part of the Catalan Volcanic Zone, this Quaternary volcanic field is associated with the European rifts system. It contains the most recent and best preserved volcanic edifices of the Catalan Volcanic Zone with 38 monogenetic volcanoes identified in the Garrotxa Natural Park. We conducted new gravimetric and self-potential surveys to enhance our understanding of the relationship between the local geology and the spatial distribution of the monogenetic volcanoes. The main finding of this study is that the central part of the volcanic field is dominated by a broad negative Bouguer anomaly of around −0.5 mGal, within which a series of gravity minima are found with amplitudes of up to −2.3 mGal. Inverse modelling of the Bouguer data suggests that surficial low-density material dominates the volcanic field, most likely associated with effusive and explosive surface deposits. In contrast, an arcuate cluster of gravity minima to the NW of the Croscat volcano, the youngest volcano of this zone, is modelled by vertically extended low-density bodies, which we interpret as a complex ensemble of fault damage zones and the roots of young scoria cones. A ground-water infiltration zone identified by a self-potential anomaly is associated with a steep horizontal Bouguer gravity gradient and interpreted as a fault zone and/or magmatic fissure, which fed the most recent volcanic activity in the Garrotxa. Gravimetric and self-potential data are well correlated and indicate a control on the locations of scoria cones by NNE–SSW and NNW–SSE striking tectonic features, which intersect the main structural boundaries of the study area to the north and south. Our interpretation of the data is that faults facilitated magma ascent to the surface. Our findings have major implications for understanding the relationship between subsurface structures and potential future volcanic activity in the Garrotxa volcanic field.

SP Monitoring at a Sea Dike

ABSTRACTThe self‐potential (SP) method is widely used in seepage evaluation hydrological studies to monitor the integrity of infrastructure such as dams, sea dikes, and other types of flood control devices because the electric signals that are measured are directly related to seepage rate. At leaking areas along sea dikes, large SP anomalies can be generated by the rising and falling of tides. Unfortunately, SP data are often contaminated with several types of noise, such as that from drifting electrodes, telluric disturbances, and external electrical noise. Furthermore, SP signals can have high levels of spatial variability due to heterogeneity in lateral resistivity at the locations where the electrodes are installed. Because of these issues, it is very difficult to correlate the measured SP voltages with the streaming potentials associated with groundwater flows at particular points in time. To alleviate these problems, we developed a simple but effective interpretation method for SP monitoring data that involves subtracting consecutive SP voltages collected at different time points from a particular monitoring station. This subtracting procedure is able to effectively reduce spurious SP anomalies caused by electrode drift, change in resistivity, and other types of interference. Therefore, any changes observed in SP measurements over certain time frames were interpreted as resulting primarily from temporal changes in seepage flow. To demonstrate the performance of this method, we analysed SP monitoring data measured at a sea dike located on the southern coast of Korea. Our results confirmed that the SP interpretation method is able to explain changes in streaming potentials depending on the tide change over time and to detect the horizontal location of anomalous seepage zones along the sea dike.

Forward modelling and inversion of self-potential anomalies caused by 2D inclined sheets

Self-potential anomalies observed over sulfide ore bodies can be closely associated with electrochemical reactions and the ohmic potential drop within the rocks. Self-potential surveys based on laboratory measurements of electrochemical potentials allow us either to measure the amplitude of the anomalies generated by this mechanism or to determine the model parameters. In order to achieve these goals, two sheets of zinc and copper were joined together to simulate sheet-like ore bodies. Self-potential surveys were conducted over 684 electrodes with the purpose of revealing the influence of various angles of the sheet. In a laboratory experiment, four different inclinations were chosen to perform the forward modelling. The last part of this paper involves the inversion of measured data to recover the distribution of generated self-potential signals. The inversion results show a satisfactory agreement with the laboratory measured data. Finding the geometry of the buried source from the shape of the SP response is not intended as it is fixed in advance. The first aim of this paper is to show how the SP response is affected under the presence of a 2D conductive structure (sheet-like) in tank experiments. The second aim is to obtain one of the model parameters (coefficient M) using data regression.