Negative Self-potential Anomalies Research Articles

Comparative Assessment of Aquifer Vulnerability Near Major Dumpsites around Karu-Abuja and Keffi in Nasarawa State, Nigeria, Using Integrated Geophysical Methods

This study investigates aquifer vulnerability to leachate infiltration near major dumpsites in Karu-Abuja and Keffi, Nasarawa State, Nigeria, using integrated geophysical methods – Vertical Electrical Sounding (VES), 2-D Electrical Resistivity Tomography (ERT), Self-Potential (SP), and Very Low Frequency Electromagnetic (VLF-EM). The study area spans basement complex and sedimentary formations. Data were collected from nine VES points, four ERT profiles, ten SP profiles and sixteen VLF transverses using Ohmega Allied resistivity meter and a Gem VLF receiver. The measurements identified groundwater saturation zones and contamination pathways such as fractures and faults. Data interpretation employed tools such as WINRESIST, RES2DINV, GRAPHER, SURFER and KHFFILT. Results delineated five to six geoelectric layers, including, topsoil, clayey sand, weathered/fractured, and fresh bedrocks. Keffi's topsoil resistivity values (47.1-224.2 Ω.m, in depths ≥ 2.1 m) indicate thicker overburden and better aquifer protective layers compared to Karu-Abuja's resistivity values (16.5-294.0 Ω.m, in depths ≥ 0.5 m). Leachate infiltration is observed in both areas, with materials of low resistivity values ranging from (7.2 to 9.9 Ω.m, in depths ≥ 7.7 m) and (2.8 to 9.6 Ω.m, in depths ≥ 6.37 m) in Karu-Abuja and Keffi study areas respectively, interpreted as contaminated zones. Negative SP anomalies ranging from (– 339.9 to -1.1 mV) and (-135 to -1.65 mV) attributed to electro-kinetic reactions, and high positive VLF current-density ranging from (5 to 10 %, in depths ≥ 14 m), further corroborated contamination pathways. The study evaluated the Aquifer Protective Capacities (APC) of Keffi and Karu-Abuja, revealing that Keffi showed better protection compared to Karu-Abuja, with Keffi showing poor rating of 66%; along VES 1 (0.043 S), VES 4 (0.05 S), VES 5 (0.01 S) and VES 6 (0.02 S) and a moderate rating of 33.33 %, along VES 3 (0.29 S) and VES 4 (0.33 S). Only VES 2 (0.89), representing 16.6%, had a good rating. The Karu-Abuja study area showed poor APC rating of 66.6%, along VES 1 (0.0063 S) and VES 3 (0.002 S), and a weak rating of 33.3% along VES 2 (0.1 S). The findings emphasise the need for regular Environmental Impact Assessments (EIAs) and the installation of geo-synthetic clay liners at dumpsite bases to safeguard groundwater resources.

Hydrogeological structure of a seafloor hydrothermal system deduced from a pair of positive and negative self-potential anomalies observed at the Oomuro-dashi hydrothermal field in the Izu-Ogasawara Arc, south of Japan

This paper presents and interprets two new self-potential data measured over a hydrothermally active field associated with a Quaternary rhyolitic volcano, Oomuro-dashi, in the northern Izu-Ogasawara Arc, south of Japan. The measured data show a pair of positive and negative anomalies of the order of one millivolt at 5 m above the seafloor. The observation of a positive self-potential near a seafloor hydrothermal system is new, in spite that negative self-potential anomalies have been regularly reported in various studies for different locations. Determining the dominant mechanism(s) is therefore key to further understanding the subseafloor structure of seafloor hydrothermal systems. To this end, we also conducted long-term observations of subseafloor temperatures at two sites in the area of the self-potential anomaly to estimate the Darcy velocity. We found a downward fluid flow of the order of tens of metres per year at both sites. The flow in the area of the negative self-potential anomaly is stronger than in the area of the positive anomaly. Based on these observations, we propose two end-member models to explain the paired self-potential anomaly. The first model considers a horizontal geo-battery, in which part of a subhorizontal electrically conductive body is crossed by a subvertical redox front. In this model, the oxidised part of the geo-battery causes a negative self-potential anomaly, as in the previous observations, while the reduced counterpart of the geo-battery, which is normally buried, is exposed near the seafloor and causes a positive anomaly. In this case, a conductive body is expected to lie beneath both anomalies, and we could access the reduced part of the geo-battery. This model is consistent with the results of the Darcy velocity estimation if the strong hydrothermal circulation would cause the redox horizon to deepen. The second model is a combination of the thermal and streaming potentials causing both positive and negative self-potential anomalies. This model does not necessarily require a buried conductive body beneath the self-potential anomalies. These end-member models could be distinguished by resistivity imaging, which identifies the distribution of conductive bodies beneath self-potential anomalies, although they would overlap in natural systems.Graphical

Numerical Modeling of Marine Self-Potential from a Seafloor Hydrothermal Ore Deposit

The redox field generated by electrically conductive minerals is one of the main constituents of self-potentials. It can be explained by electrochemical reactions in which conductors participate. The location and outline of seafloor hydrothermal ore deposits can be detected using marine self-potential anomalies that can be approximated through a marine geobattery model. The numerical modeling of marine self-potentials could be the foundation of corresponding data inversion and interpretation and improving the application effect of the self-potential method in detecting seafloor hydrothermal ore deposits. In this study, the inert electrode model and the on-land geobattery model are introduced to build the marine geobattery model, and the finite-infinite element coupling method is derived to deal with the truncated boundary problem effectively. Also, two tests are conducted to study the effect of model parameters on ground self-potential anomalies. A seafloor sulfide deposit model is built to study the self-potential characteristics. The numerical modeling results suggest that the precision and efficiency of the coupled method are superior to that of the traditional finite element method. Self-potential anomalies are greatly affected by medium resistivity, complex terrain, and amplitudes of embedded redox fields. Gradient changes in embedded redox fields do not cause significant self-potential anomalies but lead to mutations of current sources. The self-potential anomaly from the sulfide deposit model shows that the self-potential method can be effectively used to explore seafloor hydrothermal deposits that accompany negative self-potential anomalies above the ore bodies. The coupled method is quite suitable for multi-source models such as self-potential models.

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.

Correlation of Self Potential and Ground Magnetic Survey Techniques to Investigate Fluid Seepage in Archaeological site, Sungai Batu, Lembah Bujang, Kedah, Malaysia

One of the substantial of geophysics is to investigate the subsurface condition of the earth (groundwater) using appropriate geophysical techniques. In this research the correlation of self potential (SP) and ground magnetic methods was used to investigate fluid seepage in Archaeological site, Sungai Batu, Lembah Bujang, Kedah, Malaysia. Self-potential method was used to determine flow of water, and Ground magnetic method was used to find object that can influence the result of self potential measurement and the aquifer depth, the lines were spread 0m ≤ x ≤ 9m, 0m ≤ y ≤ 30m with a trace intervals of 1.5m and 0.75m per electrode spacing respectively. The result display by Self Potential signals gives a clear understand that water flow from higher value (central) towards the lower value which is mostly at the southwest part than other areas and distinct level of feasible flow at different part ranges from -30mV to +35mV,which are very related to seepage flow patterns, negative SP anomalies were related with subsurface seepage flow paths (recharge zone) and positive SP anomalies were related with areas of seepage outflow (discharge zone); and Ground Magnetic signals shows good details of the buried materials with high magnetic values which was interpreted as baked clay bricks and low magnetic values indicate groundwater seepage with depth of 5m. Therefore, the two results have correlation significant at 0.8 which show good correlation in groundwater investigation in this study, which validates the results.

Assessing the internal structure of landslide dams subject to possible piping erosion by means of microtremor chain array and self-potential surveys

An integrated geophysical approach comprising microtremor chain array and self-potential surveys was used to assess the internal structure of landslide dams subject to possible piping erosion in selected sites in Japan and Kyrgyzstan. The non-invasive geophysical approach is cost effective, environmentally friendly and portable, and hence, it has proven to be valuable for the geotechnical assessment of landslide dams where piping can trigger failure of the dam. While the microtremor chain array survey results revealed the internal structure of the landslide dam, the self-potential survey results indicated the path of anomalous seepage zones. In the surveyed sites of long-existing landslide dams, the presence of a seepage path in the dam was confirmed by a good correlation between the areas of low phase velocity and large negative self-potential anomalies. In summary, this integrated geophysical approach could be useful for the early risk assessment of landslide dams and prediction of landslide dam failure by piping.

Integrated surface geophysical approach to locate a karst conduit: a case study from Royal Spring Basin, Kentucky, USA

Groundwater flow in karst terrains is difficult to map because it can be concentrated through conduits that do not necessarily coincide with the surface features. We applied electrical resistivity (ER) and self-potential (SP) techniques at three sites to locate an inferred trunk conduit feeding a major spring in the Inner Bluegrass region of Kentucky (USA). Royal Spring is the primary water supply for the city of Georgetown; the upper part of its basin coincides with the Cane Run watershed. ER profiles (972 m total length) were measured using a dipole–dipole electrode configuration with 2- to 3-m spacing. SP measurements were taken along those ER lines and an additional test profile (230 m) using one stationary reference electrode and another roving electrode at a fixed interval. The SP technique has been used by many researchers to detect the electro kinetic potential generated by groundwater flow. The low resistivity of water in the conduit, as compared to the high background resistivity of limestone bedrock, was the ER exploration target. A negative SP anomaly corresponded to a low ER anomaly for most of the profiles, but a few are not comparable. Although SP data collected over multiple days along the test profile differed significantly, they showed similar trends. Field drift in SP data was found to be highly sensitive to temperature changes during the time of measurement. Although the overall trends of the final SP profiles for different dates were similar, the SP magnitudes varied with the amount of precipitation and the average soil temperature. The low-resistivity anomalies in the 2D inverted sections and corresponding negative SP anomalies at two sites (Berea Road and Kentucky Horse Park) encountered water-filled conduits, although mudfilled voids encountered during drilling at University of Kentucky Agricultural Research Farm sites suggest that these may be tributary conduits rather than the trunk conduit.

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 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.

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.

Self-potential investigation of moraine dam seepage

Self-potential (SP) and electrical resistivity measurements are used to investigate seepage at a remote moraine dam in the Sierra Nevada of California. The site is a small terminal moraine impounding roughly 300,000 m3 of water at ~ 3400 m a.s.l. Suspicious fine sediment in a small lake at the dam's downstream toe prompted initial concerns that anomalous seepage may be eroding matrix material from the moraine. 235 individual SP measurements covering the surface of the dam were collected in order to investigate electrokinetic current sources resulting from seepage, while resistivity soundings probed moraine stratigraphy and suggest that the till contains interstitial ice. Contoured SP data reveal a non-uniform voltage distribution over the moraine dam and two distinct negative SP anomalies. The first, located in the central area of the moraine, shows a broad negative SP zone around the crest and increasingly positive SP moving downhill towards both the upstream and downstream toes. This anomaly can be explained by shallow gravitational groundwater flow in the near subsurface combined with upward groundwater flux through evapotranspiration; numerical simulation of the combined effect matches field data well. The second SP anomaly has a tightly localized distribution and can be explained by vertically descending flow into a bedrock fault conduit. Our conceptual seepage model suggests that flow travels from Dana Lake first at the boundary of ice-filled moraine and bedrock before converging on a concentrated channel in the subvertical fault zone. Positive SP near the dam abutments results from groundwater inflow from adjacent hillslopes. Combined analyses suggest that seepage erosion is not currently affecting the moraine dam, and that the sediment observed on the bed of the downstream toe lake is likely a remnant of past outflow events.

Self‐potential signals associated with preferential groundwater flow pathways in sinkholes

Two self‐potential (SP) surveys have been conducted at a test site located in Normandy, in the northwest of France, in a chalk karst in spring and summer 2005. The spring survey showed circular negative SP signals associated with the position of sinkholes and crypto‐sinkholes, while the survey conducted during the summer showed fewer anomalies with lesser magnitudes. The negative SP anomalies observed in the spring survey were several tens of millivolts less than a reference located outside the ridge along which the sinkholes are located. In addition to the SP surveys, we also performed a DC electrical resistivity survey. The electrical resistivity tomogram shows the position of the interfaces between the chalk and the overlying clay with flint and loess covers and the position of the sinkholes. A linear relationship is observed between the SP signals and the thickness of the loess layer. In addition, large negative SP anomalies are associated with the sinkholes themselves. These SP signals are explained by solving the boundary value problem for the coupled hydroelectric problem. We fixed the boundary condition at the ground surface in terms of hydraulic flux associated with the infiltration of the meteoric water and a no‐flow condition on the vertical boundaries. A finite element numerical simulation of this coupling is performed, and a fairly good agreement is obtained between the simulation and the observed SP signals. We show for the first time how the SP signals reflect mainly the topography of the clay with flint/loess interface and help to identify the location of sinkholes. Three‐dimensional tomography algorithms are proposed to locate the source of these self‐potential signals in the ground.

Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self-potential and CO 2 surveys

This work addresses the study of fluid circulation of the Stromboli island using a dense coverage of self-potential (SP) and soil CO 2 data. A marked difference exists between the northern flank and the other flanks of the island. The northern flank exhibits (1) a typical negative SP/altitude gradient not observed on the other flanks, and (2) higher levels of CO 2. The general SP pattern suggests that the northern flank is composed of porous layers through which vadose water flows down to a basal water table, in contrast to the other flanks where impermeable layers impede the vertical flow of vadose water. In the Sciara del Fuoco and Rina Grande–Le Schicciole landslide complexes, breccias of shallow gliding planes may constitute such impermeable layers whereas elsewhere, poorly permeable, fine-grained pyroclastites or altered lava flows may be present. This general model of the flanks also explains the main CO 2 patterns: concentration of CO 2 at the surface is high on the porous north flank and lower on the other flanks where impermeable layers can block the upward CO 2 flux. The active upper part of the island is underlain by a well-defined hydrothermal system bounded by short-wavelength negative SP anomalies and high peaks of CO 2. These boundaries coincide with faults limiting ancient collapses of calderas, craters and flank landslides. The hydrothermal system is not homogeneous but composed of three main subsystems and of a fourth minor one and is not centered on the active craters. The latter are located near its border. This divergence between the location of the active craters and the extent of the hydrothermal system suggests that the internal heat sources may not be limited to sources below the active craters. If the heat source strictly corresponds to intrusions at depth around the active conduits, the geometry of the hydrothermal subsystems must be strongly controlled by heterogeneities within the edifice such as craters, caldera walls or gliding planes of flank collapse, as suggested by the correspondence between SP–CO 2 anomalies and structural limits. The inner zone of the hydrothermal subsystems is characterized by positive SP anomalies, indicating upward movements of fluids, and by very low values of CO 2 emanation. This pattern suggests that the hydrothermal zone becomes self-sealed at depth, thus creating a barrier to the CO 2 flux. In this hypothesis, the observed hydrothermal system is a shallow one and it involves mostly convection of infiltrated meteoric water above the sealed zone. Finally, on the base of CO 2 degassing measurements, we present evidence for the presence of two regional faults, oriented N41° and N64°, and decoupled from the volcanic structures.

Hydrothermal circulation beneath Mount Pelée inferred by self potential surveying. Structural and tectonic implications

Self-potential (SP) surveys were made on Mount Pelée volcano (Martinique Island, French West Indies) in 1991 and 1992 in order to recognize its hydrothermal system, the associated groundwater channeling and the main superficial structures of the massif. Almost 70 km of profiles were carried out with an average sample spacing of 50 m. Measurements essentially reveal negative SP anomalies, down to −1700 mV, with high gradients (−1.83 mV/m) due to the infiltration of meteoric water into the massif. Rims of summit calderas Morne Macouba and Etang-Sec present sharp negative SP anomalies on the western, northern, and eastern flanks. Negative SP anomalies indicate no upward water flow beneath Mount Pelée summit. On the southwestern volcano flank, a 3.5×6 km horseshoe-shaped structure corresponding to a southwest flank collapse event, older than 25,000 years BP, is clearly identified by the SP mapping. High gradients border the inner southern rim from Morne Calebasse to St Pierre town and the Caribbean Sea. Along the northern rim of the horseshoe-shaped structure the negative SP anomalies give place to a positive SP anomaly, up to 200 mV, of SW–NE trend. This zone covers the area of two active hot springs (Sources Chaudes and Puits Chaud: 40–65°C). Marine magnetic surveys and bathymetry show that the horseshoe-shaped structure spreads into the Caribbean Sea up to about 10 km from the coast. Buried structural discontinuities are evidenced inside the flank collapse structure. The upper one deviates the groundwater flow coming from the summit toward the south flank where the flow finds an indentation to expand again downwards. This discontinuity is either an old hypothetical caldera rim partly destroyed by the collapse of the south–southwestern flank and covered by recent pyroclastic deposits, or more probably the trace of a bulge landslide. A circulation model of the hydrothermal waters is proposed. Rainfall (5–6 m/year) is partly drained inside the summital calderas and the flank collapse zone through pyroclastic flows down to an impermeable basement. There the groundwater constitutes perched aquifers at the contact of the bulge landslide, or of the hypothetical old caldera rim. Along the inner northern border of the flank collapse structure the phreatic water is reheated. Warm groundwater flows along the northern avalanche structure rim and discharges near the coast in ground and marine outcrops, of medium temperature. Finally, the main part of the meteoric water is channeled along the old caldera rim, or along the bulge landslide towards the south flank of Mount Pelée, where some gaps in the rim exist. There the groundwater finds again a subhorizontal gravitational circulation along Mount Pelée slopes into the Caribbean Sea.

Results of Electrical Prospecting and Well-boring at the Oshuku Hot Springs in Iwate Prefecture

The hot water veins have been detected as negative self-potential anomalies, having the maximum gradient of 2.8 mV/m. (Fig.3), and the potential profiles of the anomaly zone were equal independently from both the topography and the geology, at the Oshuku hot springs in Iwate Prefecture. (Fig. 4) Accordingly the negative anomalies there are surely caused by the chemical reactions due to the intrusion of hot mineral water. In addition, a structural line, which may be hardly detected by surface geological exploration, has been found out by the electrical method.The bore-site, decided after the electrical prospecting, was supposed to be in the outside of the outcrop of hot water vein, and the bore was expected to penetrate the another vein, different from that of the existing hot springs, at the depth of 71.5 m. below the ground surface.These suppositions have been proved by the actual practice of well-boring. Namely the temperatures of the bore-hole was never beyond 30°C down to the depth of 70 m., in spite of the short distance of only 12 m. from the nearest hot spring, and they raised suddenly just from the depth of 71.7 m. up to 56.5°C at the depth of 79.3 m. (Fig.6) And the pumping test proved that the well yielded the hot water of 53.8°C and the pumping gave no influence to the pre-existing hot water sources.