Induced Polarization Soundings Research Articles

Geophysical exploration strategy for Cu-Ni-Co deposits in China: A review

Nickel (Ni) and Cobalt (Co) deposits are important mineral resources, and China is the largest consumer of these resources. Magmatic, hydrothermal, sedimentary, and meta-sedimentary ore-bearing rocks are the most significant Co-Ni deposit types in China, accounting for 98% of the Co production. Thus, exploring the Chinese Co-Ni resources and reserves is beneficial for promoting its economic development. We have analyzed the metallogenetic geologic model and the physical characteristics of the rocks and ores of magmatic Co-Ni sulfide and hydrothermal Cu-Co deposits and the sedimentary and meta-sedimentary rocks that host the Cu-Co ore deposits to develop an effective geophysical exploration technology system. For magmatic deposits, the small ore-forming intrusion is the main detection target; in general, gravity, magnetic, and seismic exploration and artificial-source electromagnetic methods are widely used to determine the precise location of the ore-forming intrusion. For hydrothermal deposits, ore-controlling structures and favorable positions for mineralization (e.g., alteration belts) are the main exploration targets, and the combination of hyperspectral remote sensing, gravity, magnetic-excitation, chemical prospecting, and artificial-source electromagnetic detection techniques can help locating such ore body types. For sedimentary and meta-sedimentary ore-bearing deposits, a combination of soil debris geochemical exploration measurement, induced polarization (IP) scanning, gravity method, IP sounding, and the transient electromagnetic method has been used to scan and locate the ore-controlling and ore-bearing layers. Finally, upon selecting the Xiarihamu magmatic Co-Ni deposit in the East Kunlun orogenic belt as our study area, we have conducted controlled-source electromagnetic exploration and obtained reliable results. Our study can serve as a reference for determining the most effective geophysical technique for the exploration of Cu-Ni-Co deposits in China.

Application of Induced Polarization Method in Mineral Resource Exploration

The search for hidden minerals with a large, buried depth depends on the guidance of geological and metallogenic theory, but it is difficult to master the geological characteristics of deep deposits. In this paper, taking a survey area as an example, the stimulated polarization geophysical method is used to obtain the information of deep minerals indirectly. Through the analysis and study of IP (induced polarization) anomaly and its characteristics in a tin polymetallic mining area, we delineate the IP anomaly distribution zone of metal ore, identify the distribution of polarized abnormal bodies by IP sounding, and determine the distribution of metallogenic fracture zones in the working area. Through the least square inversion of resistivity and polarizability, the spatial correspondence between the abnormal area of resistivity and polarizability and the location of ore bodies are obtained. The strategy of deep-mineral-induced polarization analysis adopted by us can provide a reliable geophysical basis for rapid deep geological prospecting.

Exploration of Erdaohe Silver Polymetallic Deposit in Inner Mongolia based on Induced Polarization Method

The exploration of Erdaohe Cu-Ag-Pb-Zn deposit in Zhalantun City, Inner Mongolia has entered the stage of blind exploration. It is a difficult problem at this stage whether common geophysical methods can find anomalies caused by 300∼1000 m or deeper Pb-Zn deposits in this area. Whether the problem is solved or not has a strong practical significance for the current and future prospecting work in this area. According to the theory of induced polarization sounding in time domain, it is considered in this paper that when the parameters such as high power, suitable device, and time delay are well coupled with the deep blind ore, it is possible to find anomalies caused by deep lead-zinc ore larger than 300 m. In order to prove the feasibility of the deep application of this method, the high-power induced polarization sounding experiment with different devices and different power supply cycles was carried out in Erdaohe copper-silver lead-zinc mine. The test results show that by adopting the measurement method of the encrypted equipotential device and selecting the appropriate polar distance, when the power supply period is 32 s, the time delay is 200 s and the sampling width is 200 s, a good deep lead-zinc mine anomaly can be found. The equipotential symmetrical quadrupole sounding MN/AB = 1/10 and the depth correction coefficient is 0.41 for inversion can accurately determine the anomaly location, which is worthy of popularization and application in this area. The ore bodies in this area are controlled by structures and distributed in lenticular layers along the fault footwall interlayer fracture zone. The ore bodies may change with the occurrence of the fracture surface. The characteristics of low resistivity, low slow, and high polarization of IP and single peak anomaly of IP sounding are important signs of lead-zinc prospecting in this area.

An integrated ore prospecting model for the Nyasirori gold deposit in Tanzania

ABSTRACT The Nyasirori gold deposit, located in the middle-western end of the Musoma-Mara Archean greenstone belt in Tanzania, is a tectonic altered rock type gold deposit controlled by shear tectonic zone. This work conducted high-precision ground magnetic measurements to delineate fault structures and favorable prospecting targets, utilized induced polarization (IP) intermediate gradient to roughly determine the distribution and extension of the tectonic altered zone and gold ore (mineralized) bodies, and further carried out IP sounding and magnetotelluric sounding to locate the tectonic altered zone and gold ore (mineralized) bodies. The anomalous gradient belt of the combination of positive and negative micromagnetic measurements reflects the detail of shallow surface tectonic alteration zone and gold mineralization body. Micromagnetic profile anomalies indicate the spatial location and occurrence of concealed tectonic alteration zone and gold (mineralized) ore bodies. Soil geochemical measurements indicate that the ore-forming element Au correlates well with As and Sb, and As and Sb anomalies have a good indication to gold orebodies. Based on the multi-source geological-geophysical-geochemical information of the Nyasirori gold deposit, this work established an integrated prospecting model and proposed a set of geophysical and geochemical methods for optimizing prospecting targets.

The effect of compaction on complex electrical resistivity of shaly sands

ABSTRACTPore space properties of sedimentary rocks are of fundamental importance with regard to reservoir evaluation and fluid flow modelling or special geotechnical applications such as stability considerations for reservoirs, dams, or embankments. Processes such as compaction due to pressure drawdown in weakly consolidated reservoirs, effects caused by stimulation of reservoirs with decreasing productivity, or the compaction of building ground can be effectively studied by verifying and monitoring changes in porosity. The close connections between electrical resistivity or conductivity and pore space geometry as well as a high sensitivity to even slight changes in pore space characterising parameters are well‐established.Therefore, the results of complex electrical conductivity measurements in the frequency range from 0.05 Hz to 1 kHz on a set of heterogeneous shaly sand samples during increasing compaction are presented in this study. The major objective of these investigations was to quantify the porosity reduction in shaly sands during the step‐wise compaction of the samples in a specially designed measurement cell. Overall, ten unconsolidated shaly sand samples with varying grain size distribution were analysed. In addition, selected shaly sandstone data are presented to enhance the observations made for the sand samples with regard to the potential influence of cementation as expressed by the cementation exponent.With increasing compaction, the measured complex conductivity data of the fully water‐saturated samples show two co‐occurring effects. On the one hand, the real part decreases due to the dominating effect of Archie’s law. On the other hand, the imaginary part increases due to the increasing contribution of interface conductivity. This effect is due to an increase in the internal surface‐area‐to‐porosity ratio. Cementation exponent and the considered porosity range seem to be controlling the magnitude of this effect. These observations may be explained by using a simple complex conductivity model that relates conductivity components to porosity, specific surface area, and cementation exponent. An interpretation algorithm is proposed that allows determining relative porosity variations based on a baseline and a single repeat measurement without prior knowledge of further rock characteristics.To demonstrate the applicability of the algorithm on field measurements, data from spectral induced polarization soundings obtained at a test site for compaction techniques were interpreted. It could be shown that these observations bear further potential for enhancing the prediction of porosity, changes of compaction, and, hence, changes in hydraulic permeability.

Three-dimensional tomography using high-power induced polarization with the similar central gradient array

Induced polarization (IP) 3D tomography with the similar central gradient array combines IP sounding and IP profiling to retrieve 3D resistivity and polarization data rapidly. The method is characterized by high spatial resolution and large probing depth. We discuss data acquisition and 3D IP imaging procedures using the central gradient array with variable electrode distances. A 3D geoelectric model was constructed and then numerically modeled. The data modeling results suggest that this method can capture the features of real geoelectric models. The method was applied to a polymetallic mine in Gansu Province. The results suggest that IP 3D tomography captures the distribution of resistivity and polarization of subsurface media, delineating the extension of abrupt interfaces, and identifies mineralization.

Geoelectric Soundings for Delineation of Saline Water Intrusion into Aquifers in Part of Eastern Dahomey Basin, Nigeria

This study was aimed at mapping the subsurface extent of saline water intrusions into aquifers at the eastern part of Dahomey basin, Nigeria. The study adopted geoelectric sounding methods. 108 vertical electrical soundings (VES) and 9 induced polarization soundings (IPS) data were acquired using Schlumberger array technique. Three aquifer units were delineated across the study area. The resistivity of the first, second and third aquifer layers varies from 0.2 to 1569 ohm-m, 0.5 to 904 ohm-m and 0.4 to 665 ohm-m respectively, while depth to the top of first, second and third aquifer varies respectively from 0.7 to 151.5 m, 1.4 to 305.5 m and 12.9 to 452.9 m. The depth to the first aquifer layer is shallow (less than 5 m) in the coastal area which makes this area to be highly vulnerable to anthropogenic pollution while their proximity to Atlantic Ocean makes them susceptible to saline water intrusion. In all the three aquifer units, the coastal area, Agbabu and other few locations in the mainland are characterized by low resistivity values (below 60 ohm-m) indicating possible presence of brackish or saline water. IP sounding results showed that all the low resistive layers in the mainland are characterized by clayey materials. The integration of VES and IPS results enabled the delineation of the saline water lateral extent across the study area. There is a strong direct correlation (r2 = 0.8564) between location distance from the saline water source and depth to saline water in the study area. This can therefore serve as a predictive model to determine depth to saline water at any location within the saline water zone in the study area.

Contaminations at the derma of Gaia: a case study in environmental geology

Crucial problems of natural resource conservation in Gaia are related to industrial, agricultural and urban pollutions of its atmosphere, water bodies and upper crust; today, one of the main human concerns is how to survive and to manage the environmental changes caused by widespread disasters related to improper disposal of large industrial and urban waste. During the last decade, we have developed hydro-bio-geological methods to quantitatively describe the soil and water contamination within a 4.0 km2 area, including two landfill deposits and a tannery industry of Alagoinhas city, Bahia state, Brazil. Geological reconnaissance works, and a detailed geophysical survey consisting of 59 Schlumberger resistivity and time-domain induced polarization soundings, were conducted around this urban–industrial disposal area to detect and delineate the 3-D extent of an underground contamination plume. These indirect results pointed out the presence of a strong electrical conductive anomaly within the aquifer, resulting from invasive fluids from the landfills, and from the surface disposal lagoons from the tannery. Complimentarily, permanent multi-electrode logging arrays were installed on three monitoring wells, constructed with thin rings copper electrodes of 1.0 cm width, externally mounted at 0.5-m distance interval, on the entire length of the rigid PVC well casings and filter sections of such wells. In each well, the ring electrodes are accessed in different electrode combinations at the ground surface, through electrical insulated multi-cables. Repeated apparent resistivity and chargeability logs were obtained for these wells using normal electrode arrays. Such measurements were also used to characterize the plume extent and to estimate changes in water chemistry below the water table and throughout the upper vadose section of the Marizal-Sao Sebastiao aquifer system. Chemical analyses of water samples, collected at available wells and along the neighboring Sauipe River, have shown drastic changes in the total dissolved solids, major cations and anions contents, total chromium and other heavy metals, inorganic macro-components, biochemical oxygen demand, chemical oxygen demand, nutrients and bacterial content. This work has conclusively shown that the subsurface is severely contaminated by liquids generated from these three surface sources. In addition, the monitoring technique using jointly electrical soundings and multi-electrode wells is an efficient and low-cost complement for an environmental evaluation of extensive aquifers.

Mapping of contaminant plumes with geoelectrical methods. A case study in urban context

Abstract During the past two decades, the diagnosis and monitoring of polluted sites have become more important. Urban sites are particularly difficult to study, because they are contaminated with various pollutants, and there is a large physical and chemical heterogeneity. The heterogeneity comes from the landfilling of various solid wastes and remolded soil (endogenous or exogenous) from which they were constituted over time. Traditional techniques such as wells monitoring, are often insufficient to evaluate the extension of soil contamination. This is why we proposed a geoelectrical methodology from the fastest to the most information rich technique, showing all carry out and acquisition times: electromagnetic low frequency conductivity mapping, electrical resistivity profiles, chargeability profiles and spectral induced polarization (SIP) soundings. This strategy has been successfully applied to an urban site located in the Paris Basin (France). A conductivity map in relation with geochemical and lithological informations should provide us information to implement electrical resistivity and chargeability profiles. The latter allowed us to differentiate 3 main anomaly zones that have been determined. As interpretation of chargeability profiles is difficult, because it integrates polarization mechanisms with different relaxation times, we add spectral induced polarization soundings that provide us information concerning the contaminant nature. We determined the extension of an organic phase, and of 2 highly mineralized zones that could be linked to biodegraded and/or with pyrite areas. That theory is consistent with groundwater analysis and SIP data. The conclusion is that the suggested methodology is well suited to the study of urban contaminated sites including several different pollutants.

Combined straightforward inversion of resistivity and induced polarization (time-domain) sounding data

It is proposed that the Straightforward Inversion Scheme (SIS) developed by the authors for 1D inversion of resistivity sounding and magneto-telluric sounding data can also be used in similar fashion for time-domain induced polarization sounding data. The necessary formulations based on dynamic dipole theory are presented. It is shown that by using induced polarization potential, measured at the instant when steady state current is switched off, an equation can be developed for apparent ‘chargeability–resistivity’ which is similar to the one for apparent resistivity. The two data sets of apparent resistivity and apparent chargeability–resistivity can be inverted in a combined manner, using SIS for a common uniform thickness layer earth model to estimate the respective subsurface distributions of resistivity and chargeability–resistivity. The quotient of the two profiles will give the sought after chargeability profile. A brief outline of SIS is provided for completeness. Three theoretical models are included to confirm the efficacy of SIS software by inverting only the synthetic resistivity sounding data. Then one synthetic data set based on a geological model and three field data sets (combination of resistivity and IP soundings) from diverse geological and geographical regions are included as validation of the proposal. It is hoped that the proposed scheme would complement the resistivity interpretation with special reference to shaly sand formations. ► We present an inversion scheme for Time-Domain Induced Polarization Sounding (TDIPS) data. ► It can be used in combination with Vertical Electrical Sounding (VES) data. ► A factious Chargeability-Resistivity Sounding is created from TDIPS and VES data. ► Inverted chargeability-resistivity and resistivity profiles are obtained through Straightforward Inversion Scheme (SIS). ► Quotient of the two profiles generates the desired chargeability profiles.

A Study on Anomaly Features Detected by Subaqueous Induced Polarization Sounding

Numerical modeling of subaqueous induced polarization sounding using the finite element method is discussed. Based on this, the anomaly features and the range of application of subaqueous induced polarization sounding are studied. Subaqueous DC resistivity and IP sounding can be used to detect geologic bodies in close quarters to improve the resolving power. But DC resistivity sounding is influenced by terrain of water-bottom. The results of numerical modeling of subaqueous induced polarization show that the topography does not have influence on the section of IP sounding. Subaqueous sounding of induced polarization is feasible and useful to detect geologic bodies. Calculating large numbers of model, we found that the resistivity of water is the primary factor to affect subaqueous IP sounding. The resistivity of seawater is so low that IP sounding cannot be carried out in the subaqueous condition.

Neural Network Based Interpretation Algorithm for Combined Induced Polarization and Vertical Electrical Soundings of Coastal Zones

The problem of fresh water availability in coastal aquifers is a reality. For in-situ and dynamic characterization of seawater encroachment into coastal aquifers, electrical geophysical methods are better suited. Vertical Electrical sounding (VES) when combined with induced polarization soundings (IPS) can resolve saline sands from moist clays. Our feed forward back-propagation neural network (BPNN) based approach automates the analysis of combined vertical electrical and induced polarization soundings to suit practical needs. Our method is initially tested on synthetic data computed from available geo-electric sections and geological information concerning coastal aquifers of the East Coast of India. The synthetic data comprised 18 combined Schlumberger IPS and VES soundings (504 apparent resistivity and chargeability samples) spread over five profiles in the study region. Fictitious apparent resistivity (product of apparent resistivity and apparent chargeability) soundings are derived from them. We used 118 carefully selected discrete fictitious apparent resistivity values from 210 sample sets gathered from 15 (420 samples) combined soundings to train the BPNN, while 33 samples from 3 separate combined soundings, and 26 random samples from 92 unused training samples of 15 soundings were used for testing. Our trained BPNN involved one input node and one bias-unit at the input layer stage, one node in the output layer, and 18 nodes and one bias-unit in hidden layer. The trained neural net showed an overall success rate of 83% in testing phase for distinguishing clays from saline sands in the synthetic example. Our method is also tested on real data concerning a shaly groundwater aquifer in Bahia, Brazil yielding an overall accuracy of 85%, quite comparable to that of synthetic case. Thus, both synthetic and field data analysis validate our neural network based algorithm.

Neural Network Based Automated Interpretation Algorithm for Combined Geophysical Soundings in Coastal Zones

The fresh water availability in coastal aquifers is an important problem faced by a major part of world's population dwelling there. For in situ and dynamic characterization of seawater encroachment into coastal aquifers, electrical geophysical methods are better suited. However, the resolving power of such data in distinguishing saline sands from moist clays in the subsurface is very poor. To meet this aspect and also the problem of analyzing voluminous data sets, we propose a feed forward back-propagation neural network (BPNN) based approach for the analysis of combined vertical electrical and induced polarization soundings. Our method is tested on synthetic data computed from available geo-electric sections and prevailing subsurface geological information of coastal aquifers of East Coast of India. The synthetic data are comprised of 18 combined soundings spread over five profiles. 15 out of 18 are used for training the BPNN, while 3 are used for testing. The trained BPNN (one node each in each of the input and output layers and 18 hidden nodes) showed 84.85% accuracy in testing phase for distinguishing clays from saline sands. Our method is also tested on real data concerning a shaly aquifer in Bahia, Brazil yielding an overall accuracy of 84.9%, comparable to that of synthetic case; thereby validating our approach.

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.

Applications of large-power induced sounding in surveying deep Jiaojia-pattern gold mines

Jiaojia-pattern gold mines could be surveyed by utilizing large-power induced polarization sounding. Firstly, a multi-layered numerical model of a polarization body was established in sense of physical and mathematical derivation. The polarization field generated from the model was the stack of individual fields generated from multi-layered polarization units. The equation for eliminating strong shallow polarization anomalies from the sounding data was derived based on the principle of multiply eliminating the fields of individual polarization units. As a result, the deep polarization anomalies were prominent. The processed sounding data was used to quantitatively calculate the depths to the tops of deep altered zones of golden mineralization.

1-D inversion of resistivity and induced polarization data for the least number of layers

An automatic inverse method has been developed for generating layered earth models from electrical sounding data. The models have the minimum number of layers required to fit a resistivity sounding curve or a combined resistivity and induced polarization sounding. The ground is modeled using a very large number of thin layers to accommodate arbitrary variations. The properties of the layers are optimized using as a constraint the [Formula: see text] norm of the vertical derivative of the resistivity distribution. The use of linear programming leads to piecewise smooth distributions that simulate traditional models made up of a few uniform layers. The process considers from the simplest model of a uniform half‐space to models of many layers, without fixing a priori the number of discontinuities. The solution is sought by iterating a new linear approximation, similar to the classical process of linearization, except that a reference model is not present in either the data vector or the unknown function. For induced polarization soundings, the problem is linear and the solution is obtained in a single iteration, provided an adequate resistivity model is available. The performance of the method is illustrated using numerical experiments and published deep resistivity data from Australia. The method also is applied to combined resistivity and induced polarization soundings from a local groundwater prospect in México.

Interpretation of I.P. time domain/resistivity soundings for delineating sea-water intrusions in some coastal areas of the Northeast of Spain

This paper deals with the application of simultaneous Induced Polarization and Vertical Electrical Soundings (IPVES) to delineate salt-water intrusions in two regions in the northeastern part of Spain. The combined interpretation of the Vertical Electrical Sounding (VES) and the Induced Polarization Sounding (IPS) in one of the areas (Moncofar) allows us to correlate the salinity and the mean chargeability of formations mainly composed of clays with intercalations of sand and gravel. In the other area (Gerona) the interpretation of the IPVES gives an idea of the different regional hydrogeological models to assist further exploration in the region.

MODEL STUDIES ON SOME ASPECTS OF RESISTIVITY AND MEMBRANE POLARIZATION BEHAVIOUR OVER A LAYERED EARTH*

AbstractElectrolytic model tank experiments to study resistivity and time domain induced polarization (IP) response over layered earth models were initiated primarily to facilitate the understanding of field results. Alternate layers of clay and sand (or clay‐coated sand) with, in some cases, a surficial layer of water were assembled in the tank and resistivity and IP measurements made for a range of electrode spacings using the Wenner configuration. Graphite and silver‐silver chloride electrodes were used as current and potential electrodes respectively.Clay‐coated (3% by weight) sand was found to generate stronger polarization than either clay or sand alone. Apparent chargeability ma was observed to be positive for a nonpolarizable surface layer. For a polarizable surface layer, the sign of IP was controlled by the polarizability, the thickness of the second layer, and the spacing of the electrode spreads. The apparent chargeability ma can theoretically change sign from positive to negative and vice versa with a gradual increase in electrode spacing, and such negative IP effects were obtained in a few observations. A simultaneous decrease in IP and an increase in resistivity, which is a qualitative diagnostic feature for the occurrence of clean freshwater sand aquifers, could also be generated in the model tank experiment.Combined resistivity and IP soundings were carried out near Fredericton Junction and Tracy, New Brunswick, Canada. Field curves are presented along with the model curves for qualitative comparison and understanding of IP behaviour over a layered earth. Twenty‐five out of twenty‐seven soundings show only positive apparent chargeabilities, whereas two show chargeability sign changes (positive/negative/positive). The model study gives reason to believe that surface soils and Quaternary gravel boulder deposits near Fredericton Junction are relatively non‐polarizable.As an auxiliary experiment, sand and clay were taken in different proportions by weight and mixed thoroughly with water in a cement mixer. The mixtures were then compressed with a suitable die and plunger under 3.6 Pa pressure to prepare cylindrical samples of height 18 cm and diameter 15.5 cm. IP measurements were done on the flat faces using the Wenner configuration with a= 2 cm. Chargeability was found to be negative for 100 and 90% clay mixtures. It reached a positive maximum for an 80% clay‐20% sand mixture and then decreased gradually with increasing sand and decreasing clay content.

DEFINITION AND APPLICATION OF THE FREQUENCY ‐EFFECT TRANSFORM FUNCTION TO THE INTERPRETATION OF IP SOUNDINGS*

AbstractThis paper deals with a new method of quantitative interpretation of induced polarization soundings in the frequency‐domain. From the general expression of the apparent frequency‐effect for soundings carried out on a multi‐layered earth the application of Hankel's inversion theorem allows to introduce a new function, called here the “frequency‐effect transform”. The new interpretation method consists of two steps: 1) the inversion of field data to obtain the frequency‐effect transform graph and 2) the analysis of this graph to derive the layering parameters. The first step is performed by means of a slightly revised version of a simple numerical procedure, previously suggested by the author for the inversion of d.c. resistivity sounding data. The second step is carried out by a complete curve‐matching procedure, applied directly on the transform graph. This implies suitable master curves, whose preparation doesn't meet all the mathematical difficulties which are present when preparing master curves of the apparent frequency‐effect function.