Self-potential Values Research Articles

Implications of Self-Potential Distribution for Groundwater Flow System in a Nonvolcanic Mountain Slope

Self-potential (SP) measurements were conducted at Mt. Tsukuba, Japan, which is a nonvolcanic mountain, to infer groundwater flow system in the mountain. Survey routes were set around the northern slope, and the reliability of observed SP anomaly was checked by using SP values along parallel survey routes; the error was almost within 10 mV. The FFT analysis of the spatial SP distribution allows us a separation of raw data into two components with shorter and longer wavelength. In the shorter (altitudinal) wavelength than ∼200 meters, several positive SP peaks of more than 100 mV in magnitude are present, which indicate shallow perched water discharges along the slope. In the regional SP pattern of longer wavelength, there are two major perturbations from the general trend reflecting the topographic effect. By comparing the SP and hydrological data, the perturbation around the foothill is interpreted to be caused by heterogeneous infiltration at the ground surface. The perturbation around the summit is also interpreted to be caused by heterogeneous infiltration process, based on a simplified numerical modeling of SP. As a result, the SP pattern is well explained by groundwater flow and infiltration processes. Thus, SP data is thought to be very useful for understanding of groundwater flow system on a mountain scale.

Structural control of collapse events inferred by self-potential mapping on the Piton de la Fournaise volcano (La Réunion Island)

Field surveys were performed on the terminal cone of Piton de la Fournaise in 2006 and 2008 to precisely map the self potential (SP) signal and determine the zonation of the hydrothermal activity both on the flanks of the cone and in the summit area, including inside the Bory and Dolomieu craters. SP maps inside the craters have been performed 8 months before the 5–7 April 2007 caldera collapse. Zonations appear both at the scale of the cone and of the summit and allow new interpretation of the electrical signal distribution on the terminal cone of Piton de la Fournaise. Superimposed to the SP maxima linked to the rift-zones, several areas of SP maxima associated with collapse structures have been detected: (1) in the summit area, the Bory and Dolomieu craters show the strongest SP values with amplitudes exceeding 2 V with respect to the base of the cone, and with a sharp lateral variation to the East, corresponding to the inner boundary of the Dolomieu caldera, collapsed on 5–7 April 2007, and (2) in the paleo pit craters surrounding the summit which show amplitudes similar to the Dolomieu–Bory craters. The analysis of the variations of the signal with time evidences a modification of the fluid flow pattern with a higher associated SP signature to the east in 2008. We interpret the amplification of fluid flow to the east in 2008 as a consequence of the eastward motion of the eastern flank of the volcano during the April 2007 eruption. The acquisition of SP data during two periods separated by the April 2007 eruption turns out to be a good opportunity to correlate the SP signal to the Piton de la Fournaise structure and to its evolution in term of hydrothermal and eruptive activity.

Correlation of Self Potential (SP) and Resitivity Survey Methods for Evaluation of Groundwater Potentials: Case Study of Imo State, Nigeria

A total of seventeen vertical electric surroundings were carried out in Mbaitolu Local Government Area (L.G.A) of Imo State Nigeria, during which apparent resistivity and SP values were simultaneously measured. The field curves obtained reveals a multilayered earth with series of crest and trough for all the sounding stations. VES 6 showed only positive maxima and minima. The distribution of SP values for electrode spacing AB/2 = 100m divides the area into two zones of positive and negative potentials respectively. The anomaly high areas are those of favourable aquifer which form discharge zones with strong lateral flow and high aquifer thickness and transmissivity. Anomaly low areas correspond to areas of infiltration. The depth to aquifer determined from SP survey ranged from 40m to 115m and correlated fairly well with those obtained from resistivity result in 16 out of the 17 stations. The distribution of equipotential lines shown by the contour map divides the area into two zones by a zero potential line suggesting a possible ground water divided in terms of water quality or geologic setting or both. The cross-correlation coefficient computed statistically for each pair of depth points gave correlation significance at the 0.01 level, which validates the results.

Detection and evaluation of horizontal fractures in earth dams using the self-potential method

The self-potential method can be applied to evaluate the degree of water seepage into earth or rockfill dams. Spatial distribution of measured self-potential data can indicate possible anomalous water flow. Phenomena, such as piping, can be modelled as cylindrical bodies. Internal erosion can develop structures, which can be represented by a sphere or point source. Differential settlement in the dam structure occasionally creates horizontal fractures, which require equivalent geometrical body formula. An equation, which allows the calculation of a self-potential profile over a horizontal contact, was developed. This equation can also be applied to the inspection of water flow into horizontal drainage filters. When inverse interpretation returns bodies of relatively small amplitudes, then their probability can be tested statistically. A test, based on cross-correlation between a modelled curve and field data, can be used to evaluate their existence at a given probability level. An acceptance criterion is computed, using the concept of likelihood coefficient. The equation was applied to two case histories. The first is an example of water flow evaluation into a horizontal drainage filter. Several anomalous bodies were interpreted from residual self-potential data, i.e. from the difference between the theoretical response of the filter and the measured self-potential values, then, most of the interpreted bodies were statistically tested. The second case deals with detection and evaluation of a horizontal fissure. The interpreted parameters of the detected body corresponded well to a horizontal fracture found when the water level in the reservoir was lowered.

Combined application of surface geoelectrical methods for groundwater flow modeling: A case history

Electrical methods [electrical profiling and sounding, induced polarization (IP), and self-potential (SP)], combined with geochemical and hydrochemical surveys, were applied to characterize shallow sediments at a site contaminated by oil-related brines. The site is located within the Romashkinskoye oil field (Tatarstan Republic, Russian Federation). Results of geochemical and hydrochemical surveys, as well as those of electrical profiling, revealed no individual haloes of contaminant in the soil and groundwater, which suggests that the entire site is contaminated. Vertical electrical IP soundings (VES-IP) allowed us to obtain the stratigraphic characteristics of the site and to distinguish a shallow aquifer among clayey aquitards. Interpretation of SP data on the basis of forward numerical modeling helped us to develop a groundwater-flow model of the site. To reduce ambiguities, we kept fixed the values of layer resistivity derived from the inverted VES-IP data. Boundary conditions on water head and water flow were based on drilling and sampling data. We varied only the values of hydraulic conductivity to fit the calculated SP values to the measured SP values. We found a strong misfit between the measured and calculated SP values for the one-aquifer model. By inferring a second, deeper-seated aquifer, we computed a two-aquifer model that produced a satisfactory SP fit.

Fluid circulation in the active emerged Asal rift (east Africa, Djibouti) inferred from self-potential and Telluric–Telluric prospecting

In 1996 Self-Potential (SP) and Telluric–Telluric (TT) surveys were performed in the Asal rift, in the Republic of Djibouti, in order to study water circulation through the rift, from Ghoubbet Strait to Asal lake located 11 km Northwest, and at the altitude of 155 m below sea level. SP surveys reveal two main positive anomalies, several kilometres wide, up to 150 mV in amplitude, parallel to the rift and separated by a relatively large (1 km) negative anomaly. The northern anomaly is well centred above the active rift. The central part of the anomaly is cut by a sharp and narrow negative anomaly (200 mV in amplitude, 150 m width) due to a major fault running from Ghoubbet Strait to Asal lake. The two positive anomalies are interpreted by ground seawater flowing toward Asal lake and warmed up during their advance by the strong geothermal flux existing beneath the rift. The negative anomalies are most probably due to large and rapid fluid transfer along faults and high permeable geologic beds bending toward Asal lake. The rapid flow would prevent any large heating of the ground water. The largest negative anomaly, which isolates the two positive ones, gives new valuable information on the tectonics. The extension of the northern positive anomaly to the Southwest is stopped by a fault located about 1 km to the North of a normal fault considered until now, as a major rift structure. The 16 Telluric–Telluric (TT) surveys point out three behaviours of the telluric field. Near Ghoubbet Strait and inland the telluric field is well polarised in a N90–100°E direction and the amplitude expresses a medium to a low resistivity beneath the recording sites. This pattern is the signature of the coast effect and the regional polarisation of the field. Along normal faults bordering the rift the SE–NW polarisation seems to be controlled by the rift tectonics. Inside the rift the telluric field becomes weaker, indicating a decay of the resistivity. In the central part of the rift the telluric field amplitude becomes 1/3 of the generally recorded amplitude and the polarisation cannot be estimated. It is corresponding to very low resistive areas in agreement with high positive SP values.

Internal structure of the Merapi summit from self-potential measurements

Recent activity at Merapi volcano occurred mainly inside the avalanche caldera related to a huge, prehistoric Mt. St-Helens-like event ( Berthommier, 1990). Since 1786, the activity has been mainly related to the growth and collapse of domes, all contained within the large crater named Pasarbubar. Some events (1822–1823, 1872 and 1930–1931) were more powerful than the average event and formed wide craters. The relative importance of the different craters and their vent locations are not well known, because of thick deposits, which partially hide these structures. We applied the self-potential (SP) method in 1990–1991 in order to improve our knowledge of these structures, in the area between the crater Pasabubar and the present dome. SP measurements allow us to partially identify the geometry of the different craters and thickness variations of the deposits inside the craters. The SP profile show two main negative anomalies, interpreted to represent the crater Pasarbubar, and the 1822 crater. The 1872 crater is only partially recognizable on the profile. From the SP anomalies, it is possible to estimate the vadose-zone thickness, but not the full thickness of the crater fill material. North from the summit, one traverses a positive anomaly. The northern limit of this anomaly, as observed on the profile, is interpreted as the limit of the 1930–1931 crater. This crater is now entirely filled by a dome complex, involving the present and older domes. The high, regular SP values occupying the entire summit region are interpreted to reflect the presence of a convective hydrothermal cell. The risk of an important slide from the wall of these different craters should be considered, as was the case of the prehistoric Mt.-Saint-Helens-type event.

Antipersistent dynamics in short time scale variability of self-potential signals

Time scale properties of self-potential signals are investigated through the analysis of the second order structure function (variogram), a powerful tool to investigate the spatial and temporal variability of observational data. In this work we analyse two sequences of self-potential values measured by means of a geophysical monitoring array located in a seismically active area of Southern Italy. The range of scales investigated goes from a few minutes to several days. It is shown that signal fluctuations are characterised by two time scale ranges in which self-potential variability appears to follow slightly different dynamical behaviours. Results point to the presence of fractal, non stationary features expressing a long term correlation with scaling coefficients which are the clue of stabilising mechanisms. In the scale ranges in which the series show scale invariant behaviour, self-potentials evolve like fractional Brownian motions with anticorrelated increments typical of processes regulated by negative feedback mechanisms (antipersistence). On scales below about 6 h the strength of such an antipersistence appears to be slightly greater than that observed on larger time scales where the fluctuations are less efficiently stabilised.