Exploration Geophysics - Volume 36, Issue 2, 2005

Experimentation with Sub-Audio Magnetic (SAM) survey parameters over the Songvang mesothermal gold deposit demonstrates that this technology can be effective for identifying conductive, mineralised structures and differential weathering in the regolith at high resolution to a depth of 100 m, as long as the transmitter electrodes are placed sub-parallel to the strike direction of features to be detected. The equivalent magnetometric resistivity (EQMMR) response produced by horizontal current channelling between transmitter electrodes has a very similar pattern to gradient-array apparent resistivity results using a 50 m or less dipole spacing. However, SAM data are recorded using a magnetic sensor, thus avoiding electrical contact with the ground, and the 2 m along-line sample density provides much greater resolution. The same area was surveyed using the same parameters, except that the transmitter electrode directions were at a right angle, and this produced radically different EQMMR results. EQMMR anomaly trends running N–S were found to correspond to gold-mineralised shears, and dips of these shears estimated using gravity modelling methods agree with drilling results. The Songvang main lode is an elongated, shallow plunging, rod-like zone of gold-pyrite ore that is detected by a gradient-array induced polarization survey. SAM equivalent magnetometric induced polarization (EQMMIP) was ineffective at imaging the main lode using surface transmitter electrodes and 1 Hz transmitter frequency. However, the 4 Hz EQMMIP response of the main lode was later imaged in great detail when transmitter electrodes were placed down boreholes into the orebody. The downhole SAM EQMMIP results were very effective at highlighting zones of intense sulphide-related gold mineralisation ahead of drilling. SAM surveying provided EQMMR data that identified shallow, conductive shears that contained economic gold mineralisation, and the EQMMIP data helped to plan resource definition drilling by prioritising chargeable targets related to increased pyrite alteration and gold grade along the Songvang main lode.

The Sub-Audio Magnetics (SAM) equivalent magnetometric resistivity (EQMMR) response has been investigated along a single 2.2 km long transect in the East Victory area at the St Ives Gold mine near Kambalda. The integration of a number of highdefinition geophysical surveys, including electrical resistivity imaging (ERI), high-resolution gravity surveying, HeliSAM surveying, drill testing, and downhole logging along the same 2.2 km transect has shown the SAM EQMMR response here is due to variations in the regolith.

The thickness of the regolith, predominantly the depth to fresh rock, was accurately determined by inversion of apparent resistivity from the ERI survey. The depth of the regolith along the transect was later confirmed by drilling at six locations, with the base of saprock found to be within 5 m of predicted depth from the ERI profile.

From the high-resolution gravity surveying at 10 m station spacing, a residual was created to highlight the contribution of the low-density regolith to the gravity response. This gravity residual was found to have an excellent inverse correlation with the SAM EQMMR anomaly data. The high-amplitude, positive SAM EQMMR responses also show excellent correlation with the depth of regolith interpreted from the ERI profile. HeliSAM showed the same broad EQMMR anomalies as ground-based SAM, but the resolution and amplitude was much lower because of detector height, fewer along-line readings, and helicopter noise.

Gravity data were successfully modelled in 2.5D using the ERI inversion to constrain the depth of polygons. Using the gravity model, the SAM EQMMR response along the transect was also successfully modelled by assigning relative current densities to the polygons, as opposed to absolute density values. The SAM EQMMR response was also inverted in 3D from gridded data using the UBC Grav3D program with some degree of success, with the depth of the main conductor in the inversion in agreement with drilling information.

Downhole conductivity logging along the East Victory transect shows that the regolith is highly conductive, with peak conductivities ranging from 1200–2700 mS/m. It can be shown that the lateral differences in regolith conductivity do not play a major role in defining the SAM EQMMR response in this environment. The regolith produces SAM EQMMR anomalies that reflect wide zones of deeper weathering, where the conductive regolith contained higher current density compared to the underlying fresh bedrock. It is demonstrated that high-amplitude SAM EQMMR anomalies map out geological structures and rock units that have been preferentially weathered and have both high current densities and large geometric size.

The Bogada Bore prospect lies immediately to the northeast of the Jundee gold deposit, in the north-west margin of the Archaean Yandal Greenstone Belt, Western Australia. The prospect has a long exploration history and has undergone extensive but variably effective drilling. The gold mineralisation at Bogada Bore shares many common characteristics with the Jundee and Nimary deposits, and is controlled by predominantly brittle structures, with high-grade shoots occurring along the intersection of structures, or by the intersection of structures with dacitic porphyry intrusions.

Outcrop is sparse with more than 90% of the area under thin colluvial cover and regolith weathering commonly down to 50 m or more. Previous geological interpretations have been mainly generated from drilling information, with the use of geophysics limited to a single high-resolution aeromagnetic survey conducted in 1994. This survey was useful for refining the existing geological interpretation by highlighting regional structures, geological boundaries, and magnetic trends, but it failed to delineate the small-scale structures which control gold mineralisation at Bogada Bore.

Sub-Audio Magnetic (SAM) surveys were trialled over the prospect area, and have been able to delineate numerous regional and small-scale structures that are shown to correlate with gold mineralisation over several areas. These surveys have also provided the basis for a new and more detailed geological interpretation in conjunction with a gravity survey and the existing aeromagnetic and drilling data.

The first Sub-Audio Magnetics (SAM) trial survey ever conducted took place in May 1992 over the Orlando Au-Cu-Bi Deposit near Tennant Creek, Australia. The survey was conducted with prototype equipment and although the waveforms were adequately sampled, lack of synchronisation between the transmitter and magnetometer meant that data processing was extremely difficult. Furthermore, the software used to process the data was in the initial stages of development and was therefore quite primitive. Although it was always acknowledged that the main geological structures were being detected in the Orlando total field magnetometric resistivity (TFMMR) data, the quality of processing and presentation at the time was inadequate for any subtle detail in the data to be revealed. However, the survey provided the first of many invaluable SAM data sets from which more refined processing and presentation techniques have since been developed.

The Orlando TFMMR data were recently reprocessed with G-tek’s current SAM processing software in order to determine whether detail is present in the data beyond that which had been previously recognised. The reprocessed Orlando TFMMR data revealed many very subtle features, which were not evident in the original presentations. The survey was found to map the main Orlando Shear Zone and secondary shear extremely well. Offsets in stratigraphy, due to previously recognised faults, are clearly visible in the images. In addition, the correlation between the TFMMR data and the gradient-array apparent resistivity is extraordinary. However, because of its higher spatial resolution, the TFMMR data contains significantly greater detail than the conventional resistivity data.

Direct detection of bedrock structures using conventional electromagnetic and electrical surveys is very difficult in terrains covered by regolith saturated with hypersaline groundwater. The resistivity contrast between the brine-saturated regolith and the fresh, crystalline bedrock is usually much greater than any resistivity contrast within the bedrock itself. Mapping bedrock faults and shears by detecting their expression as changes in depth of weathering at the base of the regolith is considered to be the most effective application of electromagnetic and electrical methods to gold exploration in hypersaline environments. SAM is more effective than airborne electromagnetics (HoistEM) for mapping the regolith expression of structures, because the SAM transmitter dipole enhances current channelling into linear features, and SAM collects much higher resolution data. The two-dimensional current flow from the SAM dipole transmitter produces clearer images of linear, low-resistivity features than does the three-dimensional “smoke ring” current flow induced by a time-domain electromagnetic loop transmitter. SAM surveys have imaged the base of regolith beneath 100 m of brine-saturated lake sediments, whereas airborne TEM transmitters have not resolved structure deeper than 50 m over the same ground.

The results of Sub-Audio Magnetic (SAM) surveys and the relationship to other geophysical datasets and geological information are presented for three different styles of gold mineralisation within the Archaean Yilgarn Craton, W.A.

The Jericho prospect is located to the north of the Butcher Well deposit in the Laverton area and is hosted within a mafic unit consisting of predominately high-magnesium basalt. The source of a broad SAM total field magnetometric resistivity (TFMMR) response initially could not be identified as it did not appear related to the depth of cover or weathering from shallow drilling. Further analysis of the SAM result, following additional drilling and a dipole-dipole induced polarization (IP) survey, indicated that the source of the SAM TFMMR response in the southern portion of the survey area is due to a combination of variation in depth of the regolith and a number of narrow potassic alteration zones within the basaltic unit.

The objective of the SAM survey at the Bronco prospect in the Southern Cross region was to map perturbations within the massive pyrrhotite-pyrite horizon that hosts gold mineralisation. In order to maximise current flow within the sulphide horizon, the electrodes were placed within the sulphide units at the base of two open pits located along strike. The TFMMR data shows a response having a width that significantly narrows to both the north and south of the survey area and correlates with the width of the primary sulphide. In addition, a number of flexures and offsets have been delineated. High-intensity magnetic responses, due to maghemite in the regolith, have resulted in noise being introduced in the TFMMR data.

Gold mineralisation at the Triple ‘O’ lode within theCornishman Deposit is associated with quartz-pyrrhotite-pyrite alteration of banded-iron formation that is located below a thrust in a ‘blind’ location. The sulphide alteration can be semi-massive and very conductive (up to 3000 S/m), but the use of traditional electromagnetic techniques at similar deposits in the region has not proven particularly successful because of the small spatial footprint of the sulphidic alteration. Hence, the objective of the SAM surveying was to determine whether the technique could successfully map the narrow, sub-horizontal sulphidic alteration zones. The TFMMR data highlighted a broad conductive zone that is comprised of three discrete conductors, two of which have been confirmed by drilling to result from sulphide alteration of the banded-iron formation and to have associated gold mineralisation. The third anomaly is thought to be due to the preferential weathering of a non-mineralised shear zone.

Manganese (Mn) ore in the Woodie Woodie area occurs as podiform ore bodies in Archaean dolomite and chert breccia thought to be 2.5 billion years old. Electromagnetic conductivity mapping using time-domain helicopter electromagnetics (HoistEM) surveying successfully detected six new discoveries under regolith and host rock cover in the area. Sub-Audio Magnetic (SAM) surveys were carried out over known EM-responsive Mn deposits to test the ability of this technology to detect conductive, highgrade manganese deposits as equivalent magnetometric resistivity (EQMMR) anomalies. SAM survey data were compared to other geophysical methods over five EM-responsive manganese deposits, which varied in size and burial depth. The methods compared were high-resolution gravity, HoistEM, gradient-array induced polarization (GAIP), dipole-dipole induced polarization (DDIP), and ground time domain electromagnetic (TEM) surveying. The SAM technique demonstrates that this technology can be effective for identifying conductive, mineralised structures and regolith features at shallow depth, usually less than 40 m. Podiform ore zones below 40 m depth did not produce EQMMR anomalies. The SAM depth of penetration is limited by the ability for current to be channelled into conductive, podiform ore zones sitting in resistive host rocks, below a moderately conductive regolith cover.

The Lena Shear is a gold-anomalous structure that sits in an Archaean greenstone belt and extends for more than 7 km. It is partially obscured by regolith and an extensive salt lake system over its length. Identification of the shear under cover is hampered by the poor magnetic susceptibility contrasts of the host lithologies. A Sub-Audio Magnetic (SAM) survey over an exposed section of the Lena Shear recorded anomalous equivalent magnetometric resistivity (EQMMR) values proximal to the shear, but not directly associated with the geologically mapped location of the shear as expected. An interpretation of the Sub-Audio Magnetic survey highlights current channelling in structural and geological elements that were previously unknown. The relationship between the shear zone and the anomalous EQMMR response is unresolved because of the stronger response of an adjacent ultramafic unit. It is likely that deep weathering of rock units, contacts and shears has combined to produce the observed anomalies.

The Sub-Audio Magnetics (SAM) method is a high-resolution electrical technique that derives information on sub-surface electrical and magnetic properties by introducing an electric current into the ground and measuring the total magnetic field changes on the Earth surface at a sub-metre interval via an optically pumped magnetometer. One parameter that may be derived from any SAM survey is the total-field magnetometric resistivity (TFMMR). To date, there are few quantitative interpretational schemes for deriving resistivity from TFMMR data. This paper outlines the theory to calculate the 2.5 dimensional (2.5D) TFMMR response due to a point source of current in an otherwise two-dimensional (2D) Earth. The problem is formulated in the wavenumber domain by first solving for the electrical potential from the current source, and then deriving orthogonal horizontal and vertical components of magnetic field using the modified Biot-Savart Law. An inverse Fourier transform is then applied to yield vector magnetic field components in the spatial domain, and hence the scalar TFMMR response.

A 2.5D finite-element modelling approach is developed to model TFMMR responses from various resistivity structures. For an isotropic, uniform resistivity Earth the finite-element model gives good agreement with analytical results, with an accuracy of about 4% in each of the three vector components. The greatest error is for the horizontal magnetic field component along strike. Finally, we demonstrate that the TFMMR technique is very useful for defining basement structures in areas of conductive regolith cover. The presence of a regolith (10. Ω.m) has little effect on the TFMMR responses provided that its thickness is less than aboutone twentieth of the current-electrode separation. Thus, for a typical electrode separation of 1.2 km, the TFMMR response is sensitive to basement structures for regolith thickness of up to 60 m, and hence is an important geophysical method for exploration beneath conver.

Total field magnetometric resistivity (TFMMR) is a highresolution electrical technique that yields information on subsurface resistivity. In a companion paper (Fathianpour et al., 2005, Part I) the basis of a 2.5D TFMMR finite-element modelling approach was developed for a point current source in an otherwise 2D resistivity structure. In this paper (Part II), we use the 2.5D forward modelling algorithm as the basis of a numerical inversion for 2D resistivity structure using a Marquardt-Levenberg algorithm. Application of a quasi-Newton updating formula for approximating the Fréchet derivatives in the course of inversion results in a fast and reliable routine. To overcome the problems of the effect of the geomagnetic field direction and dependency of TFMMR data on all three vector-components, field data are initially reduced to the pole. By doing this, we require only Fréchet derivatives for the vertical Bz anomalous magnetic field, and in the case of the 2D structures, Bz is the most sensitive component to vertical boundaries separating lateral changes in resistivity.

TFMMR data collected across the Flying Doctor Deposit, near Broken Hill in New South Wales, Australia are inverted for 2D resistivity structure. The inverse models show the presence of a conductive zone in the central part of the surveying area corresponding to the known mineralisation and the Globe Vauxhall Shear Zone. Depth resolution is limited, but we demonstrate that the method can resolve lateral boundaries.

Sub-Audio Magnetics (SAM) is a method by which a total-field magnetic sensor may be used to simultaneously acquire both total magnetic intensity (TMI) and total-field electromagnetic induction (TFEMI) responses from buried metal. In situations where public safety is involved, regulators, such as the U.S. Environmental Protection Agency, can mandate the use of both magnetic and electromagnetic sensors for the detection of potentially hazardous items such as unexploded ordnance (UXO) and metal drums. SAM has now completed a program of evaluation and certification as a state-of-the-art technology, acceptable for application in the litigation-sensitive environmental hazard remediation market.

During its certification program, SAM was applied to controlled U.S. test sites at the Aberdeen Proving Ground, Maryland, McKinley Range, Alabama, Waikoloa, Hawaii, and at the Newholme facility in Australia. It was also applied to situations in Montana where live UXO was present. In these demonstrations, a direct comparison was made between SAM results and common, state-of-the-art alternative geophysical detecting technologies. Based on demonstrations at these sites, it has been possible to quantify the key performance parameters: level of effort, detection performance as a function of depth and target size, false-alarm rejection achieved through dual-mode sensing, and the cost-benefit ratio of SAM compared with contemporary sensor technologies such as the Geonics EM61 and EM63 electromagnetic detectors, and G-tek’s TM-5EMU electromagnetic detector and TM-4 total field magnetometer.

SAM has proven to be very successful in detecting a wide range of metallic UXO items to depths exceeding achievable detection by all contemporary sensors. In the Montana live sites, where the smallest item of UXO was a 76 mm projectile, false-alarm rates were reduced from 7 per UXO, using TMI only, to 0.7 per UXO using combined SAM TMI and TFEMI data, without diminishing the probability of UXO detection. Survey cost per hectare using SAM was determined to be less than one quarter that of conducting TMI plus EMI surveys separately.

The application of SAM to extremely magnetic lava flows at Waikoloa in Hawaii showed that SAM cannot yet compete with the ground-balancing capability of the TM-5EMU in this environment. Advanced signal-processing development may reduce this present limitation.

In June 1999, I undertook a series of heat-flow measurements in the Gascoyne Region of Western Australia. The measurements involved precision temperature logging of stratigraphic boreholes, and thermal conductivity measurements on core samples. Results indicate that the surface heat flow in the onshore Southern Carnarvon Basin is about 55 mW/m². This is 3.5 mW/m² less than the average heat flow in the Dampier Sub-basin, suggesting that the lithosphere beneath the Dampier Sub-basin may still retain heat from the Mid-Jurassic rifting event.

The precision temperature logs suggest that the ground surface temperature across the Gascoyne Region is about 6°C warmer than the average air temperature, implying a relatively high absorption of solar radiation or relatively low surface albedo. The logs also suggest that the thermal diffusivity of the surface alluvium is about 1.4 × 10^-6 m²/s. Observations near the eastern margin of the Southern Carnarvon Basin support the prediction of elevated heat flow resulting from heat refraction and extra heat input from an underlying, heat-generating body of granite.

This paper describes a research program at the Australian Resources Research Centre (ARRC) to establish and use an analogue model of a turbidite channel reservoir to gain insight into issues of uncertainty in reservoir simulations of channelised fields and their seismic expression. The project is unique in that it integrates seismic and reservoir engineering research in a controlled laboratory environment. The research is based around a cementation technique that allows synthetic sandstones to be fabricated with predetermined physical properties such as porosity, permeability, and impedance. These laboratory models provide real data that do not rely on assumptions, and are therefore useful to compare with numerical simulations of both the seismic and fluid-flow response.

The 1:1000 scale model comprises two intersecting sandstone channels within an impermeable acrylic matrix. Fluid communication between the channels exists in two separate intersecting areas with contrasting flow connectivity. Production from the initially oil-saturated model was performed via waterflood, and the injected water was dyed blue to allow the displacement process to be recorded on video. Production rates and data such as time to water breakthrough, produced water-oil ratio, and cumulative recovery were recorded and used to historymatch reservoir simulations of the production response.

Scaled time-lapse 3D data were acquired before and after production at frequencies of 50 kHz and 1 MHz. The 1 MHz data provides high-resolution images and accurate monitoring of the oil and water distribution. The 50 kHz data honours the correct scaling of the relative seismic wavelength-to-channel size, where the individual channels are within tuning thickness, and provide data for valid comparisons with seismic attributes in field data from similar reservoirs. An analysis of velocity dispersion indicates that the measured acoustic response to changes in saturation at 50 kHz is representative of the expected lower frequency seismic response.

During the Permo-Carboniferous the Gondwana supercontinent experienced the most extensive Phanerozoic glacial event, as recorded by widespread glaciogenic sediments from South America to Australia. Simultaneously the northern margin of the supercontinent was affected by the onset of the Neotethys rifting. During that period, several elements such as glacial surfaces or glacially eroded valleys suggest the presence of a terrestrial ice sheet in Western Australia, and the glacial Lyons Group has been reported from various locations in the Carnarvon Basin. Such deposits are proposed to form the lower part of the sedimentary cover of the Mermaid Nose area (Dampier Sub-basin, Northern Carnarvon Basin, NW Shelf).

Therefore, a classical seismic stratigraphic approach combined with recently developed seismic attributes and innovative workflows is used to define the morphology of the Late Palaeozoic sequences, despite severe limitations in the resolution of the seismic data.

Six stacked units are defined within a half-graben in a syn-rift setting. They are interpreted as a series of basal moraines followed by deposits associated with deglaciation periods.

Six stacked units are defined within a half-graben in a syn-rift setting. They are interpreted as a series of basal moraines followed by deposits associated with deglaciation periods.

In addition, the glacially related hydrocarbon-rich deposits of the Al Khlata Formation in east-central Oman provide a most interesting parallel with the Permo-Carboniferous syn-rift deposits observed on the Australian Northwest Shelf.

The Applied Potential method finds important application in mineral exploration. To better understand the detection and delineation capabilities of the technique, especially where (conductive) mineralisation is discontinuous, we have carried out a series of laboratory experiments using a 3D tank model system. Crosshole electric measurements were made with a bipole-bipole array, with one current electrode located in each simulated borehole. Both scanning (both bipoles moved together) and profiling measurements (using one fixed bipole and one mobile bipole) were conducted rather than full tomographic imaging. Five different classes of cylindrical model were studied, which were distinguished on the basis of (1) cylinder orientation relative to the plane of the boreholes, (2) whether the target was continuous, terminating, or broken between the points of measurement, and (3) whether the conductor was intersected by one or both boreholes. Each class of model produces a characteristic albeit complicated response, which can be used to identify the conductor and roughly locate its upper and lower boundaries. Data were collected for different bipole separations. Pronounced anomalies are produced for the scanning arrangement for models involving conductor intersection, although it is difficult to always distinguish the models and interpret the data. The character is very dependent on the electrode configuration relative to the target. Electric profiling produces a subtler and less diagnostic pattern, which is hard to discriminate from a background variation. The examples presented provide useful guidance when designing mise-á-la-masse or Applied Potential surveys. Moreover, they indicate that full tomographic imaging is required for all but the simplest cases.

Seismoelectric methods are based upon physical properties of the earth that produce electrical signals from seismic waves. Electrokinetic sounding (EKS) is one such method that has great potential for hydrogeological studies as the signal arises from the movement of pore fluids under seismic excitation. In principle, the method should be able to directly map changes in hydraulic permeability, rock porosity and fluid-chemistry. A number of researchers have recently tried to exploit the phenomenon in groundwater problems where conventional methods worked poorly. However, publications of successful case histories to support the theory are rare. This may be due to the very weak amplitudes of the electrical signals generated from the seismic wave, which are millivolts to nanovolts in magnitude, and the presence of cultural noise, which is usually much greater in magnitude.

We demonstrate electrokinetic responses from formations more than 50 metres deep in two test areas in Western Australia. One is over a saline palaeochannel and the other is over a freshwater aquifer. The seismic waves were generated from a sledgehammer source and acoustic and electrical data were recorded by a seismic acquisition system. Seismic refraction and reflection data provide seismic velocity information for depth conversion and support the seismoelectric data. The data were then compared to borehole logs to find what physical contrasts were detected. Significant hydrogeological boundaries were detected up to 50 m deep in saline groundwater conditions, and to at least 100 m deep in freshwater aquifers.

The total intensity aeromagnetic image of a part of peninsular India, published recently (GSI, 2000) by the Geological Survey of India, covers the area between the latitudes 8° N to 25° N and the longitudes 74° E and 89° E, exclusive of a major portion of the Deccan Volcanic Province, which is yet to be covered by aeromagnetic surveys. The aeromagnetic coverage over most of the area was at a nominal line spacing of 400 m and at a flight height varying between 1500 m and 2700 m above mean sea level. Over areas such as the Cuddapah Basin and parts of the Singhbhum Craton, drape-flown data with 1000 m and 250 m line spacing were included in the image. Because of the varying flight heights, we continued all the data either upward or downward to a common altitude of 2100 m to produce a consistent contiguous image.

As the inclination of the Earth’s field varies between 0° and 35°, we produced a Reduced to Pole (RTP) image, which gives a better understanding of the magnetic anomalies, by transforming the total magnetic intensity data by parts and then stitching them together. We also provide the first vertical derivative image of the RTP data, which provides a better insight to near-surface structures. Our interpretation, based on the images of total magnetic intensity, RTP, and first vertical derivative data is presented in this paper.

In general, the total magnetic intensity image brought out the regional geology and structure very well. The southern granulite terrain differentiates itself from the rest of the area by means of high amplitude and high frequency anomalies. The boundary between the western and eastern Dharwar Cratons appears to be the eastern boundary of the Chitradurga Schist Belt. The boundary of the Karimnagar Granulite Terrain could be established from the image. Several major lineaments that were previously unknown could be identified from this image. A major lineament, starting from the west coast and extending about 1500 km to Bhuvaneswar in the east, cuts across other major trends and might be of tectonic significance. The image supports the proposition that the Peninsular Shield is an agglomeration of land masses that came together and were sutured in the course of geological time. The image also provides clues to the intracratonic tectonic elements or subdivisions with characteristic cross trends. The Godavari, Mahanadi, and Damodar Rift Systems appear to have been connected together and joined to the Narmada Rift, and this is another new inference from the image.

Based on the assumption of parallel conduction between laminated and dispersed shaly sand patterns, and the theory of a symmetrical anisotropic effective medium for resistivity interpretation, a generalised equivalent resistivity model is proposed for predicting water saturation in complex shaly formations. This model assumes that the shaly formations contain laminated shales, conductive rock-matrix grains, nonconductive hydrocarbons, dispersed clay particles, and water. It shows that shale distribution has the most important effect on water saturation calculations. The curvature of a formation conductivity Ct versus water saturation Swt curve is greatly affected by clay conductivity Ccl, but is less affected by formation matrix conductivity Cma. Also, this curvature is affected by water percolation rate λw or percolation exponent γ, but it is less affected by matrix percolation rate λma. When Ct is kept constant, Swt increases as λma or γ increases, and decreases as λw increases. Rock sample data in dispersed shaly sands, and well logging data in laminated shaly sands, show that this model can be applied in both of these environments. Experiments with artificial samples with conductive rock grains also show that the model may be applied in clay-free porous rocks, if water conductivity is larger than rock grain conductivity. Therefore, the proposed resistivity model can be considered to be a generalised equivalent resistivity model for laminated and dispersed shaly sand formation evaluations.