Apparent Chargeability Research Articles

Application of magnetic and induced polarization method for delineating gold-bearing vein zones at Cibaliung, Pandeglang Regency, Banten

Magmatic processes occurred during the Miocene period caused the formation of epithermal gold deposits in Cibaliung area. The deposit has previously been investigated through geological surveys which basically only covers the surface aspect, so in this study a subsurface analysis was carried out through magnetic and IP surveys to determine the distribution and continuity of the gold deposits. The magnetic survey was conducted over an area of about 3 km2 with sampling interval 20 m east and 100 m north. The magnetic data were processed using Oasis Montaj with magnetic intensity map as an output, which was then interpreted to determine the presence of structures and magnetite destruction zones as mineralization clues. The IP survey was conducted on 20 east-west oriented lines with length of about 1.2 km. The electrode configuration used is Wenner with 25 m spacing. IP data were processed using RES2DINV software to eliminate bad datum points and invert the apparent chargeability values into the true ones. IP data are then interpreted to clearly determine the position, direction, and distribution of gold mineralization body by detecting the presence of sulfide minerals as ligands carrying gold. Magnetic data analysis shows that gold mineralization tends to occur at low magnetic anomaly, ranging from 37 nT to 240 nT and generally associated with northwest-oriented structures. The mineralization zone is found in four main vein zones with resistivity and chargeability values < 51 Ωm and > 50 ms.

Induced polarization of volcanic rocks. 5. Imaging the temperature field of shield volcanoes

SUMMARY Kilauea is an active shield volcano located in Hawaiʻi. An induced polarization survey was performed in 2015 at the scale of the caldera. The data were acquired with a 2.5 km cable with 64 electrodes and a spacing of 40 m between the electrodes. A total of 6210 measurements were performed. The apparent chargeability data were inverted using a least square technique to obtain a chargeability tomogram. The normalized chargeability tomogram is obtained by multiplying cell-by-cell the chargeability by the conductivity. Once the conductivity and normalized chargeability tomograms are obtained, they are jointly interpreted using a dynamic Stern layer conduction/polarization model, which explains the low-frequency polarization spectra of volcanic rocks. This conductivity/polarization model is tested here on new laboratory experiments performed on 24 samples from a drill-hole located on the Kilauea East Rift Zone (Hole SOH-2). We show that for Kilauea, the ratio between the normalized chargeability and the conductivity is equal to a dimensionless number R = 0.10 ± 0.02 proving that the conductivity and the normalized chargeability are both controlled by the alteration products of the volcanic rocks with a minor role of magnetite except close to the ground surface. In turn, the degree of alteration is controlled by temperature and therefore normalized chargeability and electrical conductivity can both be used as a non-intrusive temperature sensor. This approach is then applied to the field data. Meaningful temperature tomograms can be produced from both electrical conductivity and normalized chargeability tomograms.

On negative induced polarization in frequency domain measurements

SUMMARY Induced polarization (IP) has been widely used to non-invasively characterize electrical conduction and polarization in the subsurface resulting from an applied electric field. Earth materials exhibit a lossy capacitance defined by a negative intrinsic phase in frequency-domain IP (FDIP) or a positive intrinsic chargeability in time-domain IP (TDIP). However, error-free positive apparent phase or negative apparent chargeability (i.e. negative IP effects) can occur in IP measurements over heterogeneous media. While negative IP effects in TDIP data sets have been discussed, no studies have addressed this topic in detail for FDIP measurements. We describe theory and numerical modelling to explain the origin of negative IP effects in FDIP measurements. A positive apparent phase may occur when a relatively high polarizability feature falls into negative sensitivity zones of complex resistivity measurements. The polarity of the apparent phase is determined by the distribution of subsurface intrinsic phase and resistivity, with the resistivity impacting the apparent phase polarity via its control on the sensitivity distribution. A physical explanation for the occurrence of positive apparent phase data is provided by an electric circuit model representing a four-electrode measurement. We also show that the apparent phase polarity will be frequency dependent when resistivity changes significantly with frequency (i.e. in the presence of significant IP effects). Consequently, negative IP effects manifest themselves in the shape of apparent phase spectra recorded with multifrequency (spectral IP) data sets. Our results imply that positive apparent phase measurements should be anticipated and should be retained during inversion and interpretation of single frequency and spectral IP data sets.

Mount Milligan alkalic porphyry Au--Cu deposit, British Columbia, Canada, and its AEM and AIP signatures: Implications for mineral exploration in covered terrains

Although alkalic porphyry Au--Cu deposits are much less common than their calc-alkalic counterparts, they represent attractive exploration targets both due to their occurrence as, locally concealed, clusters and their high Au contents. In this paper, the geology of the Mt. Milligan Au--Cu deposit, Canada, and its geophysical footprint on magnetic and AEM data is used as a case-study for the exploration and discovery of concealed alkalic porphyry Au--Cu deposits.The Cole-Cole parameters in the AEM data are extracted using an improved Airborne Induced Polarization (AIP) mapping algorithm. We propose that the subtle electromagnetic signatures of disseminated sulfides around the intrusive porphyry stocks at Mt. Milligan could be mapped by the mτ product, where m is AIP apparent chargeability, and τ the Cole-Cole time-constant, provided that the depths of the alteration zone does not exceed 75 m. The product is called Tau-Scaled Chargeability or TSC.In order to improve the exploration success in the search for concealed alkalic porphyry Au--Cu deposits, we propose to: (1) concentrate the exploration on known and well-endowed mineral districts; and (2) apply innovative digital data-processing techniques capable of extracting weak and subtle signatures from airborne geophysical data and, subsequently, to utilize DCIP ground surveys in the follow-up of anomalies, ideally in combination with enzyme-leach soil sampling.

Identifikasi Lapisan Pembawa Mineral Emas dengan Metode Resistivitas dan Induksi Polarisasi di Desa Paningkaban Kecamatan Gumelar Kabupaten Banyumas Jawa Tengah

The research has been done in the Paningkaban village, Gumelar District, Banyumas Regency, Central Java Province. This research aims to identify technically the presence of gold mineral distribution using the method of geoelectricity-resistivity and induced polarization (IP), to interpret the obtained resistivity and chargeability data from the research area, to determine the subsurface structure model in the reasearch area, and find out the potential of gold in the research area i.e Paningkaban. The method in this research includes field data acquisition, data processing, and data interpretation. The acquisition uses a syscal resistivitymeter tools by stretching 200 meters of cable on each track that has been connected to the electrode and porous pot. The results obtained from the acquisition are primary data which are still in the form of voltages, currents, and apparent chargeability values. Based on this data, the processing is carried out to obtain the resistivity and chargeability profiles so that can be interpreted and correlated. The correlation between the resistivity and chargeability data was carried out to identify the gold mineralization zone in two dimensions. The presence of gold mineralization is characterized by the emergence of rocks with a low resistivity value and high chargeability because mineralization in the geological area is seen in quartz veins. Quartz veins are common features of many deposits and are indicative of the presence of mineralization, especially gold. Based on the results of the interpretation, the subsurface rock which contain gold minerals are found on each line, with resistivity values ranging from 4,25 – 17,3 Ωm and the chargeability value ranges from 26,4 – 226 msec. These rocks which are estimated to contain gold minerals are the sandy clay.

A multiple parameter extraction and electromagnetic coupling correction technique for time domain induced polarisation full waveform data

ABSTRACTThe time domain induced polarisation (TDIP) method is characterised by a simple explanation and operational approach, and is used widely to prospect for various metal sulfide deposits. However, the existing TDIP method is parameterised with fewer interpretive output attributes than the frequency domain induced polarisation (FDIP) method, and there has been far less emphasis on electromagnetic (EM) coupling in the TDIP method. A new and simple method has been devised for the extraction of multiple parameters and the suppression of EM-coupling responses in full waveform TDIP data. The essence of this approach is the transformation of the TDIP rectangular wave into the fundamental wave and a series of odd harmonics using a discrete Fourier transform. By using real and imaginary components of the fundamental and odd harmonics, we can calculate various FDIP parameters, including the per cent frequency effect (PFE) and the phase. The decoupled PFE coefficients and decoupled phases can then be used to remove the EM-coupling response. This new method has been applied to a data set collected near a poly-metallic mine in the Tibetan Plateau. A comparison between the TDIP and FDIP methods with central gradient arrays was performed. The results indicate that the TDIP apparent chargeability results are affected greatly by the EM-coupling response. The PFEs and phases extracted from the TDIP full waveform data are in good agreement with those obtained from the FDIP data. Moreover, the locations of the high decoupled PFE coefficients and the large negative decoupled phases are consistent with the locations of known ore veins. This method can provide additional parameters for the subsurface media, improve the practical application effect and enrich the functionality of TDIP methods.

Induced Polarization and Resistivity of Second Potential Differences (SPD) with Focused Sources Applied to Environmental Problems

A novel technique is proposed to improve shallow induced polarization (IP) and resistivity survey results. We propose the apparent resistivity and apparent chargeability of second potential differences (SPD), employing two focused sources (FS) mathematically manipulated by superposition. To test the idea, a synthetic model is developed with two bodies. The first body is a small shallow heterogeneity which is above the second and larger body. This synthetic model illustrates the shape and response of the apparent resistivity and chargeability for FS under the random noise and masking effect. These processes reduce electromagnetic coupling, telluric noise, contamination in channel links, and small heterogeneous responses. A field test of the SPD for FS was carried out in an agricultural site irrigated with wastewater, where contaminated water laden with metals has been accumulating for years in the soil. Soil samples were collected and analyzed throughout the geophysical survey to correlate the resistivity-IP results. Soil laboratory analysis included metal content, moisture content and texture. The parameters computed after applying the SPD for FS depicted a better lateral resolution where vertical and horizontal boundaries of the anomaly zones were well defined. It was possible to determine the low permeability horizontal layer made of clay-soil (called tepetate), which is a barrier for water and metals.

Time Domain and Frequency Domain Induced Polarization Modeling for Three-dimensional Anisotropic Medium

Time domain induced polarization (TDIP) and frequency domain induced polarization (FDIP) synthetic models, incorporating three-dimensional (3D) anisotropic medium, were tested. In TDIP modeling, both resistivity and chargeability of the medium were anisotropic, and the apparent chargeability values were calculated by carrying out two resistivity forward calculations using resistivity with and without an IP effect. We analyzed the TDIP response of a 3D isotropic cube model embedded in the anisotropic subsurface half-space. In FDIP modeling, the complex resistivity of the medium at various frequencies was anisotropic. The complex resistivity was determined by a Cole-Cole model with anisotropic model parameters. We then analyzed the FDIP response of a 3D anisotropic cube model embedded in an isotropic subsurface half-space. Both of the TDIP and FDIP simulation results suggest that IP responses acquired in two orthogonal directions on the surface are different when the same arrays are used and acquisition in orthogonal directions helps resolve the presence of anisotropy. The anisotropy should be taken into account in practice for TDIP and FDIP exploration.

Grey wolf optimisation for inversion of layered earth geophysical datasets

ABSTRACTGeophysical observables using optimisation algorithms are interpreted in terms of physical properties defining the Earth. The field of optimisation is a dynamic field as no single algorithm can solve all optimisation problems. We implement the grey wolf algorithm, an apex predator‐based method, in optimizing geophysical datasets over a layered earth. The grey wolf optimiser is a swarm‐based meta‐heuristic algorithm and has two extremely interesting social practices, viz., social leadership hierarchy and hunting behaviour. The leadership hierarchy is simulated by employing different types of grey wolves where hunting strategies are implemented as optimisation methods. Global minimum from the grey wolf optimiser has been obtained with a pack of 7 wolves and 1500 iterations. To evaluate the efficacy of the grey wolf optimiser, we performed inversion of noise‐contaminated vertical electrical sounding (apparent resistivity), induced polarisation sounding (apparent chargeability), and magneto‐telluric apparent resistivity data. Subsequently, we implemented grey wolf optimisation on field apparent resistivity, apparent chargeability, and magnetotelluric apparent resistivity data adapted from published literature. Both noise‐contaminated synthetic and field data have been compared with popular population‐based algorithms, i.e., particle swarm optimisation and ant colony optimisation and with a local optimisation algorithm—ridge regression. The sensitivity analysis was performed by inverting the noise‐contaminated datasets using the grey wolf optimiser, particle swarm optimisation and ant colony optimisation with six different search space. It is observed that the grey wolf optimiser is least sensitive to the varied search space. The results obtained from the grey wolf optimiser as compared with other techniques are relatively more stable and the obtained normalised RMS deviation is either less or equal with ridge regression, particle swarm optimisation, or ant colony optimisation. This is due to the fact that the grey wolf optimiser does not converge prematurely and avoids getting trapped in a local minimum, as a balance between exploration and exploitation is maintained. We also compared the grey wolf optimiser using L1‐norm and L2‐norm as misfit functions for field data examples. The grey wolf optimiser using L1‐norm resulted in a more stable solution. The execution time of the grey wolf optimiser is least as compared with other population optimisation techniques. Grey wolf optimisation could be applied for routine interpretation of geophysical datasets.

Improved Data Preprocessing Algorithm for Time-Domain Induced Polarization Method with Digital Notch Filter

Time-domain induced polarization (TDIP) measurement is seriously affected by power line interference and other field noise. Moreover, existing TDIP instruments generally output only the apparent chargeability, without providing complete secondary field information. To increase the robustness of TDIP method against interference and obtain more detailed secondary field information, an improved data-processing algorithm is proposed here. This method includes an efficient digital notch filter which can effectively eliminate all the main components of the power line interference. Hardware model of this filter was constructed and Vhsic Hardware Description Language code for it was generated using Digital Signal Processor Builder. In addition, a time-location method was proposed to extract secondary field information in case of unexpected data loss or failure of the synchronous technologies. Finally, the validity and accuracy of the method and the notch filter were verified by using the Cole-Cole model implemented by Simulink software. Moreover, indoor and field tests confirmed the application effect of the algorithm in the fieldwork.

Induced polarization response of porous media with metallic particles — Part 3: A new approach to time-domain induced polarization tomography

The secondary voltage associated with time-domain induced polarization data of disseminated metallic particles (such as pyrite and magnetite) in a porous material can be treated as a transient self-potential problem. This self-potential field is associated with the generation of a secondary-source current density. This source current density is proportional to the gradient of the chemical potentials of the [Formula: see text]- and [Formula: see text]-charge carriers in the metallic particles or ionic charge carriers in the pore water including in the electrical double layer coating the surface of the metallic grains. This new way to treat the secondary voltages offers two advantages with respect to the classical approach. The first is a gain in terms of acquisition time. Indeed, the target can be illuminated with a few primary current sources, all the other electrodes being used simultaneously to record the secondary voltage distribution. The second advantage is with respect to the inversion of the obtained data. Indeed, the secondary (source) current is linearly related to the secondary voltage. Therefore, the inverse problem of inverting the secondary voltages is linear with respect to the source current density, and the inversion can be done in a single iteration. Several iterations are, however, required to compact the source current density distribution, still obtaining a tomogram much faster than inverting the apparent chargeability data using the classical Gauss-Newton approach. We have performed a sandbox experiment with pyrite grains locally mixed to sand at a specific location in the sandbox to demonstrate these new concepts. A method initially developed for self-potential tomography is applied to the inversion of the secondary voltages in terms of source current distribution. The final result compares favorably with the classical inversion of the time-domain induced polarization data in terms of chargeability, but it is much faster to perform.

Airborne Inductive Induced Polarization Chargeability Mapping of VTEM Data

SUMMARY Airborne inductive induced polarization (AIIP) effect has been widely recognized in airborne time domain EM system data. AIIP chargeability mapping opens new and exciting areas in mineral exploration for airborne time domain EM systems in the search for sulphides and clay minerals. An AIIP chargeability mapping tool based on CSIRO/AMIRA Airbeo is created for VTEM data, with examples from Mt Milligan, British Columbia, Canada and Tullah, Tasmania. Using the Cole-Cole frequency dependent resistivity, the tool examines the VTEM decay data spectrally and selects the decay associated with the lowest RMS error from a set of decays generated by varying chargeability m and time constant within specific ranges, giving a constant frequency factor c, while the background resistivity is inverted. The parameter mo used to generate the decay is the AIIP apparent chargeability.

Negative apparent chargeability in time-domain induced polarisation data

It appears to be relatively common to assume that negative apparent chargeability data in time-domain induced polarisation (IP) surveying is a sign of bad data quality. Negative IP data can however occur as a consequence of the distribution of chargeable zones in the ground, which is well documented in literature. A general mechanism behind negative IP data is proposed as follows; if the chargeable zones are mainly located in zones of negative sensitivity, and there is low or no chargeability in the positive sensitivity volumes in the investigated volume, it will result in negative apparent chargeability.Numerical modelling confirms that the phenomenon will typically occur for longer electrode separation if the chargeability is concentrated in a thin layer at the surface only, but that other distributions of the chargeable bodies can also cause negative IP data. Different electrode arrays differ in tendency to produce negative IP data, where dipole–dipole and pole–dipole arrays are more prone to generate negative data than nested arrays in the modelled examples. In addition to the relative location of the chargeable zone the resistivity is important for its impact on the apparent chargeability.Field data recorded in connection with the 3rd International IP Workshop on Ile d'Oléron in April 2014 confirm that negative apparent chargeability can be caused by a thin chargeable layer at the surface. The abundant negative IP data can be explained by an inverted model with low residuals, in which the chargeability is concentrated in a thin layer with modest chargeability close to the surface. Removing the data with negative apparent chargeability before inversion results in apparently poor resolution of the bottom layer and artefacts that are not present in the inversion results from the original data set. The results clearly demonstrate that negative apparent chargeability data can be a result of the distribution of chargeable zones in relation to the sensitivity distribution, and that such data should not be edited away on a routine basis since they contain important information.

Assessment of soil salinity using electrical resistivity imaging and induced polarization methods

2D imaging involving geoelectrical resistivity and time domain induced polarization has been used to assess the spatial variability of the physical properties of subsurface soil in Covenant University Farm, southwestern Nigeria. Apparent resistivity and chargeability of the induced polarization effect were concurrently measured along six traverses using Wenner array. The observed data were processed to produced 2D inverse models of the subsoil resistivity and chargeability. Soil samples were also collected and analysed for conductivity and salinity levels. The results show that the salinity level in the soil is within the range for normal soil and therefore healthy for plant growth. The inverse model sections were integrated with the laboratory test to qualitatively assess the salinity, degree of compaction, and thickness of the soil in the farm. Other petrophysical properties such as clay volume, moisture content and organic matter which are related to soil conductivity were also inferred. The study demonstrates that geoelectrical resistivity imaging can be a useful tool for effectively assessing the variations of soil condition in large tracts of land for precision agriculture. Key words: Soil salinity, 2D imaging, geoelectrical resistivity, induced polarization, precision farming.

A VES-IP survey near the geophysical station of Moscow State University, in the Kaluga region

Due to the progress in instrumentation and methods in the most recent decade apparent resistance curves have been supplemented by accurate apparent chargeability curves at some frequencies up to an electrode spacing of 500 m. The comparison of the induced polarization frequency characteristics makes it possible to arrive at additional conclusions on the nature of anomalies. The results of vertical electrical and induced polarization sounding survey (VES-IP) that were carried out under the geophysical field studies of the Moscow State University are described.

Interpretation of lithogeochemical and geophysical data to identify the buried mineralized area in Cu-Au porphyry of Dalli-Northern Hill

Geochemical and statistical analyses of 165 soil samples led to Cu-Au anomaly separation in Dalli-Northern Hill by using fractal geometry and U-spatial statistic. There was a good correlation between the anomaly areas and outcrops of quartz diorite porphyry (QDP) in the study area. Based on the interpretation of soil data and anomaly areas determination, one trench was suggested in NE-SW direction for follow-up exploration. Based on the rock sample analyses from the trenches in QDP rocks, the ratio of \( \frac{{{\mathrm{Ba}} \times {\mathrm{K}}}}{{{\mathrm{Ca}} \times {\mathrm{Zn}}}} \) clearly separated the mineralization area and considered as an index ratio for follow-up exploration stage. Dalli-Northern Hill anomaly includes pyrite/chalcopyrite/bornite, which is dominantly abundant in the ore body. Three IP profiles (IP04, IP05, and IP06) confirmed the conductivity of the sulfide zone. The background chargeability in the study area was about 5.4 mV/V and the average of maximum apparent chargeability in the study area was 44.4 mV/V, which could be related to the higher intensity of Cu-Fe sulfide minerals. The contact of andesite and quartz diorite has shown the strongest chargeability (71 mV/V) and high magnetic anomalies in the study area. DDH03 and DDH04 boreholes mostly confirmed that the IP/magnetic/geochemical anomalies were related to the contact of intrusive rocks and the wall rock of andesite. The combination of geochemical information from soil and rock together with geophysical data (induced polarization/resistivity) led to the subsurface geological cross sections and location of the mineralized zone enriched of sulfide. The mineralized zone in Dalli-Northern Hill distinct could be identified by potassic to weak sericitic and trace chloritic alteration, high values of the \( \frac{{{\mathrm{Ba}} \times {\mathrm{K}}}}{{{\mathrm{Ca}} \times {\mathrm{Zn}}}} \) ratio accompanied with high gold and copper values and enrichment from magnetite minerals. The high magnetic susceptibility, low resistivity, and high chargeability are considered as geophysical properties of the mineralized zones.

Induced polarization in airborne EM

A major impediment in the path toward airborne induced polarization (IP) is an effective method to quantify data from inductive sources, such as those used in airborne electromagnetic systems. We modeled inductive IP using a combination of Warburg and exponential decay models as a basis for fitting electromagnetic data from ground time-domain electromagnetic (TEM) and airborne versatile TEM (VTEM) surveys. Observed decays were deconvolved into electromagnetic and IP constituents by constrained least-squares fitting of basis functions modified to account for transmitter waveforms. The method was confirmed through synthetic modeling of 2D and 3D structures, and when applied to ground TEM or airborne TEM data, obtained an estimate of apparent chargeability at each station or fiducial. In the case of a VTEM survey in Africa, the apparent chargeabilities mapped graphitic sediments and provided spatially consistent indications of clay concentrations. A limitation on this airborne IP for airborne applications is motion noise, which places a lower limit on usable base frequency and begins to significantly affect the signal at the later delay times, when IP effects are most visible.

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.

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.

Mapping fly‐ash water pond leakage with TEM and IP data at Loy Yang coal‐mine (Australia)

The fly‐ash water pond at the Loy Yang power station (Latrobe Valley, Victoria, Australia) seeps and therefore presents a hazard to the local environment from its high total dissolved solids content. A transient electromagnetic survey (TEM) was completed to investigate the contaminant plume in more detail.The contaminate plume was successfully delineated; however, multiple contiguous soundings featured late‐time negative decays not typically recorded in coincident‐loop TEM surveys. This indicates that conductivity is frequency‐dependent. These anomalous soundings suggested the presence of a shallower contaminate plume than that mapped by the boreholes and therefore further analysis was required. An induced polarization survey revealed that the area where the late‐time negative responses were recorded coincided with a shallow conductive and extremely chargeable anomaly. A simple power‐law relationship holds between the negative transients decay rates and normalized apparent chargeability. Analysis on soil samples obtained from auger holes related the polarization anomaly to clay‐, organic‐ and metal‐rich layers of fill material.