DC Resistivity Soundings Research Articles

Monitoring subterraneous water regime at the new Ain Shams university campus in Al-Obour city (northeast of Cairo–Egypt) using both azimuthal very low frequency–electromagnetic and DC–resistivity sounding techniques

A joint azimuthal very low frequency–electromagnetic (VLF–EM) and DC–resistivity sounding survey was conducted at the new Ain Shams university campus in Al-Obour city, northwest of Cairo, Egypt. The main objective of the survey was to highlight the applicability and reliability of such non-invasive surface techniques in mapping and monitoring both the vertical and lateral electrical conductivity structures of waterlogged areas, by subterraneous water accumulations, at the campus site. Consequently, a total of 743 azimuthal VLF–EM and 4 DC–resistivity soundings were carried out in June, 2011, 2012 and 2013. The data were interpreted extensively and consistently in terms of two-dimensional (2D) transformed EM equivalent current-density and stitched inverted electrical resistivity models, without using any geological a-priori information. They could be used effectively to image the local anomalous lower electrical resistivity (higher EM equivalent current-density) structures and their near-surface spreading with time, due to the excessive accumulations of subterraneous water at the campus site. The study demonstrated that a regional azimuthal VLF–EM and DC–resistivity sounding survey could help design an optimal dewatering program for the whole city, at greatly reduced execution time.

Use of salinity and resistivity measurements to study the coastal aquifer salinization in a semi-arid region: a case study in northeast Nile Delta, Egypt

To understand the impact of sea level rise (SLR) and aquifer intensive use would cause to the groundwater level and saltwater intrusion, an integrated relationship between salinity and electrical resistivity of a Quaternary aquifer is established in the northern part of East Nile Delta (El Sharkia area), Egypt. Historical data of groundwater salinity are mapped and compared with the current water samples to better understand the salinity spatial variability. The salinity maps show that salinization has increased sustainably. Additionally, the transition/dynamic zone related to SLR and/or excessive pumping can be addressed. In particular, the surface DC resistivity soundings were carried out to demonstrate the vertical and horizontal salinity distributions in the area. In the course of this study, the 1D model generation using a hybrid genetic algorithm (GA) was applied and tested using borehole information. The constructed geoeletrical cross-sections emphasize and delineate the extension of saltwater intrusion. Cleary, it is found that due to excessive pumping from shallow wells over the last decades, the subsurface resistivity and TDS vertical distributions can change rapidly within a short distance. Additionally, the results show that despite the dominance of brackish and saltwater at the northern part of the area, perched low conductive lenses are observed reflecting a low level of groundwater salinization. To obtain a link between water salinity and aquifer resistivity, an empirical relationship was derived to predict the salinity variations at different depths. In comparison with the measured total dissolved solids (TDS), the predicted salinity map appears realistic. These results demonstrate the important role of the integration between resistivity and salinity measurements for mapping the groundwater salinization with depth, and call for further research to plan and manage the area’s water resources.

An efficient 1D O CCAM’S inversion algorithm using analytically computed first- and second-order derivatives for dc resistivity soundings

An efficient algorithm has been developed for 1D resistivity inversion problem using both first- and second-order derivatives, which are computed analytically. The second-order derivative matrix, which is not used in the OCCAM's inversion, has been incorporated into the algorithm employing analytical expressions. Computation of complicated second-order derivatives in each iteration is circumvented by a new algorithm. These modifications result in stable convergence of the OCCAM's inversion and in general, better misfit can be achieved specially for smoothing parameter, μ < 1 . The modified inversion algorithm, coded in MATLAB was tested using two synthetic Schlumberger resistivity sounding examples. Its application has been illustrated with field data from south India.

Mountain permafrost distribution in Dovrefjell and Jotunheimen, southern Norway, based on BTS and DC resistivity tomography data

The influence of climate and topography on the distribution of permafrost within the Dovrefjell and Jotunheimen areas, southern Norway, is analysed. A dataset of 972 BTS (bottom temperature of winter snow) measurements was analysed in relation to altitude, potential direct incoming radiation, aspect, snow depth, curvature and slope. To confirm and characterise permafrost-transition zones indicated from the BTS measurements, miniature temperature data-loggers, borehole temperatures, one-dimensional DC resistivity soundings and two-dimensional DC resistivity tomography were used. In addition, small-scale variance analyses upon the BTS values were performed using spatial-statistical methods. Results confirm that BTS values are highly correlated with altitude. Based on analysis of BTS data, the lower limit of possible permafrost is 1490 m a.s.l. on Dovrefjell and 1460 m a.s.l. in Jotunheimen. The relation between altitude and BTS suggests that the climate conditions on Dovrefjell and in Jotunheimen are similar with respect to permafrost distribution. Potential direct incoming radiation (PR) has a minor influence on BTS. A significant correlation between both surface moisture and surface type conditions in summer and BTS measurements were found. The results from Jotunheimen suggest that 20-45% of the variance in BTS results that are not explained by altitude are explained by small-scale spatial variance within a 20-30 m range. The results from the BTS measurements and the two-dimensional DC resistivity tomography were highly consistent.

Investigation of mountain permafrost in the Kozia Dolinka valley, Tatra Mountains, Poland

The Kozia Dolinka valley lies at an altitude above 1900 m a.s.l. on the northern slope of the main ridge of the High Tatra Mountains. Mountain permafrost occurrences were studied with the use of BTS, infrared imaging, water and ground temperature measurements and DC resistivity soundings. The data suggest the existence of isolated patches of permafrost. The lowest observed bottom temperature of winter snow values was in the order of-10C. DC soundings revealed the existence of a high resistivity layer of limited extent. Permafrost seasonal monitoring was conducted with resistivity soundings. Measurements were carried out in spring-autumn 1999, when a distinct change in permafrost thickness was observed.

Evaluation of small-loop transient electromagnetic soundings to locate the Sherwood Sandstone aquifer and confining formations at well sites in the Vale of York, England

Shallow-depth transient electromagnetic (TEM) soundings have been performed at six borehole locations in an intensively farmed area in northern England to evaluate their usefulness in mapping geological formations under a thick cover of glacial drift deposits. The regionally important Triassic Sherwood Sandstone (SS) Group aquifer is directly overlain by Triassic Mercia Mudstone in the eastern two-thirds of the study area and by drift deposits in the west. Owing to the difficulty of deploying large loops and the overriding need to minimize lateral effects on the depth probes, square transmitter loops of 20, 40 and 50-m side-lengths were deployed in the central-loop configuration with the Geonics EM47 and PROTEM47/57 field equipment. Using a two-stage data interpretation technique, it is found that the effective depth of mapping ranged from about 8 to 150 m at most sounding locations. Comparison of inversion models with borehole data shows that the SS and some overlying sedimentary rocks may be discerned from the TEM soundings; there is a consistent pattern of resistivity distribution within each geological formation at all the borehole sites enabling a realistic identification of the key stratal units. However, a 7–11-m-thick upper layer is found in all the constructed models, which does not correlate with any known formation boundaries, but appears to be justified by comparison with sample dc resistivity soundings at two locations; it would also appear that the earliest time windows (<0.016 ms) are somewhat distorted by the band-limitation operation of the TEM instrumentation. This pilot study demonstrates that the TEM method is a potent tool for stratigraphic mapping in the region, but the upper 5–8 m remains largely inaccessible to the method using state-of-the-art equipment and conventional data processing techniques. It may therefore be necessary to combine TEM and short spread-length ( AB/2≤25 m) dc resistivity depth soundings to accurately map the near-surface in this glacial-covered terrain.

Mountain permafrost distribution based on BTS measurements and DC resistivity soundings in the Daisetsu Mountains, Hokkaido, Japan

In order to map mountain permafrost distribution and to discuss geomorphological indicators for permafrost on the Daisetsu Mountains, BTS measurements and DC resistivity soundings were conducted. Low BTS values were obtained on north-facing slopes at elevations above 2050 m and sporadically below 1900 m. DC resistivity soundings indicate that the permafrost thickness is from 6 to 26 m at elevations above 2050 m, and less than several metres below 1900 m. A diagnostic BTS value of −°C for permafrost occurrence was examined by means of DC resistivity soundings. The diagnostic BTS value disagreed with the results from the DC resistivity soundings in areas with sporadic permafrost. Both BTS measurements and DC resistivity soundings indicate the permafrost is continuous beneath north-facing and wind-blown ground at elevations above 2100 m and sporadic between 1750 and 1900 m. This distribution is controlled by local factors such as slope direction and snow accumulation. At elevations above 2100 m, blockstreams on north-facing slopes experience creep phenomena, reflecting the presence of permafrost. Copyright © 2000 John Wiley & Sons, Ltd. Pour cartographier la distribution du pergélisol et pour discuter des indicateurs de pergélisol dans les montagnes Daisetsu, des mesures BTS et des sondages de résistivité DC ont été réalisés. De faibles valeurs de BTS ont été obtenues sur les pentes exposées au nord à des altitudes au-dessus de 2050 m ainsi que sporadiquement sous 1900 m. Les sondages de résistivité indiquent que l'épaisseur du pergélisol varie de 6 à 26 m aux altitudes supérieures à 2050 m et est inférieure à quelques mètres en dessous de 1900 m. Une valeur diagnostique de BTS de −2°C pour la présence de pergélisol a été confrontée aux résultats des mesures de résistivité. La valeur diagnostique BTS ne correspond pas aux mesures de résistivité dans les régions de pergélisol sporadique. A la fois les mesures BTS et de résistivité indiquent l'existence d'un pergélisol continu sous les pentes exposées au nord et sous les surfaces dégagées de la neige par le vent aux altitudes supérieures à 2100 m et sporadiquement entre 1750 et 1900 m. Cette distribution est contrôlée par des facteurs locaux comme l'orientation des pentes et l'accumulation de neige. Aux altitudes supérieures à 2100 m, des coulées de blocs s'étirant sur des pentes exposées au nord subissent des phénomènes de creep attestant la présence d'un pergélisol. Copyright © 2000 John Wiley & Sons, Ltd.

Monitoring Winter Freezing in a Silt Soil in Southern Manitoba, Canada Using Surface DC Resistivity Soundings

The sensitivity of the electrical conductivity of soil to the temperatures just beneath 0 °C allows the application of electrical geophysical methods for monitoring soil freezing. Surface DC‐resistivity soundings were performed over Winter 1996–97 to examine the electrical response of winter freezing of a silt soil in Winnipeg, Manitoba, Canada (49 °48′30″ N, 97 °07′22″ W). Temperature measurements were made over a 2.7 m deep section during this time interval providing a semi‐continuous soil‐temperature profile. Additional geophysical measurements including seismic refraction, terrain conductivity, and time‐domain electromagnetics were completed to characterize the lateral and vertical variations in the soil properties. During the recordings the surface temperature in Winnipeg decreased from 5.6 °C in October to in January. The 0 °C isotherm reached a depth of 15 cm in mid‐November and a depth of 75 cm in late‐January. The freezing produced a two‐order of magnitude increase in the electrical resistivity of the soil which could be detected by Wenner DC‐resistivity soundings with electrode spacings of less than 3 m. Geophysical inversion of the resistivity soundings provides an image of the time‐varying electrical‐resistivity structure of the soil. When the frozen layer is modelled as a single layer of uniform resistivity, it increases in thickness from around 20 cm in December to 45 cm in February. These values represent estimates of the minimum thickness of the true layer. The resistivity of the frozen layer increases from in December when the average temperature of the frozen layer is to in February when the average temperature of the layer is This increase is probably due to increasing ice content in the layer. The results suggest the DC‐resistivity method provides a useful method for monitoring temporal and spatial changes in the frozen soil layer.

Electrical resistivity structure of the Canterbury Plains, New Zealand

Magnetotelluric (MT) soundings from nine locations across the Canterbury Plains, South Island, New Zealand, are used to deduce the electrical resistivity structure of the Plains down to c. 3 km depth. The Quaternary gravels covering the Plains represent a resistive layer which is inferred to be c. 600 m thick and are underlain by more conductive Tertiary sediments. Over most of the Plains, a rise in resistivity at greater depth cannot be unambiguously identified as greywacke basement, but correlation of the electrical structure with earlier seismic and gravity work suggests that the gravels and Tertiary sediments thin towards the western edge of the Plains where greywacke is very close to the surface, dc resistivity soundings are used as a check on the correction of static‐shift in the MT data and are shown to be compatible with the MT results. Joint interpretation of the dc and MT data allows hydrological implications to be drawn on the porosity and pore fluid resistivity of the layers.

Use of a Square‐Array Direct‐Current Resistivity Method to Detect Fractures in Crystalline Bedrock in New Hampshire

AbstractAzimuthal square‐array direct‐current (dc) resistivity soundings were used to detect fractures in bedrock in the Mirror Lake watershed in Grafton County, New Hampshire. Soundings were conducted at a site where crystalline bedrock underlies approximately 7 m (meters) of glacial drift. Measured apparent resistivities changed with the orientation of the array. Graphical interpretation of the square‐array data indicates that a dominant fracture set and (or) foliation in the bedrock is oriented at 030° (degrees). Interpretation of crossed square‐array data indicates an orientation of 027° and an anisotropy factor of 1.31. Assuming that anisotropy is due to fractures, the secondary porosity is estimated to range from 0.01 to 0.10.Interpretations of azimuthal square‐array data are supported by other geophysical data, including azimuthal seismic‐refraction surveys and azimuthal Schlumberger dc‐resistivity soundings at the Camp Osceola well field. Dominant fracture trends indicated by these geophysical methods are 022° (seismic‐refraction) and 037° (dc‐resistivity). Fracture mapping of bedrock outcrops at a site within 250 m indicates that the maximum fracture‐strike frequency is oriented at 030°.The square‐array dc‐resistivity sounding method is more sensitive to a given rock anisotropy than the more commonly used Schlumberger and Wenner arrays. An additional advantage of the square‐array method is that it requires about 65 percent less surface area than an equivalent survey using a Schlumberger or Wenner array.

Study of the Santa Catarina aquifer system (Mexico Basin) using magnetotelluric soundings

A tensor magnetotelluric test survey was carried out in the region of Santa Catarina, located in the Chalco sub-basin of the Mexico Basin. The objective was to define the stratification at depth with an emphasis on the geometry of the main aquifer of that region which is partially known from DC resistivity soundings and drilling. High-quality magnetotelluric soundings could be recorded in the immediate vicinity of large urban zones because the sub-surface is very conductive. Interpretation shows that the solid bedrock is located at a depth of at least 800 m to the south and 1300 m to the north; it could, however, be much deeper. Using complementary DC resistivity sounding and well-logging data, three main layers have been defined overlying the bedrock. These layers are, from surface to bottom, an unsaturated zone of sand, volcanic ash and clay about 10 m thick, followed by a very conductive (1.5 ohm·m) 200 m thick layer of sand and ash with intercalated clay, saturated with highly mineralized water, and finally a zone with resistivity increasing gradually to 60 ohm·m. The investigated deep aquifer constitutes most of this third layer. It consists of a sequence of sand, gravel, pyroclastites and mainly fractured basalts. MT resistivity soundings and magnetic transfer functions also indicate that a shallow resistive structure is dipping, from the northwest, into the lacustrine deposits of the basin. This geologic feature is likely to be highly permeable fractured basaltic flows, which provide a channel by which water contaminated by the Santa Catarina landfill may leak into the basin.

AC resistivity sounding

DC resistivity sounding is a galvanic method, and the type of information obtained from application of th at method is determined by the behaviour of galvanic currents in the ground. This behaviour imposes inherent limitations on the scope for determination of earth parameters from the measurements. Among these limitations the equivalences of high-resistivity layers and low-resistivity layers are weil known. The high-resistivity equivalence, where neither the thickness nor the resistivity of a highly resistive layer embedded in better conducting surroundings may be determined but only the product of the two, is annoying in the context of many practical applications. In many cases highly resistive layers of dry sand and gravel are underlain by either wet and better conducting layers of sand and gravel or by low-resistivity c1ays. In these cases it becomes impossible to determine the amount of dry sand and gravel from DC resistivity soundings alone when prospecting for raw materiaIs, and in hydrogeological applications the depth to the water table is of ten undetermined. Furthermore, DC resistivity measurements do not all ow a determination of the anisotropy of the ground. Besides the galvanic method of DC resistivity sounding, there are a number of electromagnetic or inductive methods such as SLINGRAM and AMT (audiomagnetotelluric) with or without controlled source. The information gained from these methods is determined by the behaviour of induced currents in the ground and differs from the gal van ie information. Measurements with the inductive methods are strongly influenced by the presence of good conductors, while poor conductors are more or Iess invisible. The depth to a good conductor is usually accurately determined from inductive methods.

Electromagnetic Mapping of Fresh‐Water Lenses on Small Oceanic Islands

ABSTRACTMany small oceanic islands have fresh‐water lenses that can be used for domestic water supplies or dry season augmentation for rainfall catchment systems. However, these small lenses are often irregular in extent and thickness. Some islands have several small, isolated lenses. By making a few reasonable assumptions, the electromagnetic (EM) profiling method can be used to estimate lens thickness. If it can be assumed that the water table is close to sea level, and that the unsaturated and fresh‐water saturated geologic units have low bulk conductivities, the interface depth can be obtained from a three‐layer solution. The first layer is the unsaturated zone. Its thickness is assumed to be the land surface elevation above sea level. The third layer is the salt‐water saturated zone; it is assumed to be infinitely thick. The layer conductivities are either estimated or obtained from DC resistivity soundings. In resistive environments only the third‐layer conductivity has a significant effect on the calculated interface depth. The only remaining unknown is the second‐layer thickness, which represents the fresh‐water saturated zone. The second‐layer thickness is sensitive to the value used for the third‐layer conductivity. Calibration of the EM‐derived interface depth with other geophysical or water‐quality data is recommended, unless relative lens thicknesses are sufficient to meet the survey objectives. Application of this method to field studies of a sandy barrier island and a carbonate key, both in Florida, has provided useful data which correlate well with other geophysical and water‐quality data.

Evaluation of Transient Electromagnetic Soundings for Deep Detection of Conductive Fluids

ABSTRACTTransient electromagnetic sounding methods (TEM) have several advantages over DC resistivity soundings. The principal advantage is that sounding depths several times the receiver‐transmitter spacing can be achieved, resulting in less cumbersome field work and more flexible sounding site selection. The TEM method was tested using a commercially available instrument (Geonics EM‐37) in the Gulf Springs region of west‐central Florida to locate the saltwater interface in the Floridan Aquifer. The Floridan is a thick (200 m), unconfined, karstic, carbonate aquifer within the study area. High annual rainfall and internal drainage create a deep and dynamic interface.Eighty transient soundings were completed in 18 field days. Seventy‐one soundings used a square 80 m × 80 m transmitter loop, and the other nine used a 160 m × 160 m coil. The interface was easily detected from a minimum depth of 60 m to its maximum depth of about 140 meters. Also, a high‐resistivity, low‐porosity zone was detected at depths ranging from 180–250 meters. The TEM data compare well with available chloride concentration data. The TEM interface corresponds to chloride values in excess of 200–400 mg/1. The sounding curves obtained suggest that with presently available instruments the TEM method is not well‐suited for shallow soundings (less than 40–50 meters).

Electrical DC resistivity soundings with long profiles on rock glaciers and moraines in the alps of Switzerland : Fisch, W; Fisch, W; Haeberli, W Z gletscherkunde Glazialgeol, V13, N1–2, 1977, P239–260

Electrical DC resistivity soundings with long profiles on rock glaciers and moraines in the alps of Switzerland : Fisch, W; Fisch, W; Haeberli, W Z gletscherkunde Glazialgeol, V13, N1–2, 1977, P239–260

INTERPRETATION OF MAGNETOTELLURIC RESISTIVITY AND PHASE SOUNDINGS OVER HORIZONTAL LAYERS

The present paper deals with a new inverse method for quantitatively interpreting magnetotelluric apparent resistivity and phase‐lag sounding curves over horizontally stratified earth sections. The recurrent character of the general formula relating the wave impedance of an (n−1)‐layered medium to that of an n‐layered medium suggests the use of the method of reduction to a lower boundary plane, as originally termed by Koefoed in the case of dc resistivity soundings. The layering parameters are so directly derived by a simple iterative procedure. The method is applicable for any number of layers but only when both apparent resistivity and phase‐lag sounding curves are jointly available. Moreover no sophisticated algorithm is required: a simple desk electronic calculator together with a sheet of two‐layer apparent resistivity and phase‐lag master curves are sufficient to reproduce earth sections which, in the range of equivalence, are all consistent with field data.