Ground Electromagnetic Surveys Research Articles

Electromagnetic resistivity pseudo-log as a new instrument for estimating reservoir properties beyond boreholes

A new approach to the reservoir properties' prediction using so called “electromagnetic resistivity pseudo-log” built in the target location is suggested. It is based on artificial neural network continuation of resistivity well logs beyond boreholes using electromagnetic resistivity profiles determined from adjacent ground electromagnetic survey. As a result of this transformation, we obtain a resistivity profile which possesses high resolution of the borehole resistivity log in the reservoir scale. We demonstrate advantages of this approach by neural network modelling of the porosity prediction below boreholes and in the interwell space. To this end we use electrical logging data collected in two boreholes located in the Northern Tien Shan and results of magnetotelluric survey in their vicinity. It is shown that porosity prediction using the developed algorithm provides better results than its estimation using resistivity logs or electromagnetic resistivity alone. In particular, the mean relative error can be as low as ∼2% in the case of predicting at double borehole depth and around 8% in the interwell space.

Using Low Frequency Ground and Downhole TDEM to explore for Massive Sulfide Mineralisation in the Carajás Mineral Province

SummaryThe Carajás Mineral Province in Brazil hosts the highest concentration of major IOCG deposits in the world. Airborne and ground electromagnetic (EM) methods are frequently applied when exploring for these deposits using relatively high operating frequencies (25Hz - 30Hz). This frequency range limits the survey to effectively detect shallow, weakly to moderately conductive sources.An airborne EM survey (VTEM) using a 30Hz base frequency has been flown by Falconbridge Ltd over the Pedra Branca deposit in 2005, identifying two discrete anomalies in the area. The high frequency nature of the VTEM system did not provide good depth resolution of the anomalies and follow-up ground EM surveys were conducted.An orientation fixed loop EM (FLTEM) survey was carried out at Pedra Branca to compare the effectiveness of using lower frequency (2Hz) ground EM against existing VTEM and previous high-frequency ground EM surveys. The application of EM methods using frequencies of 2Hz and below enables the detection of late time conductors by recording much later in time compared to high frequency systems. These typically detect stronger, deeper conductors which are commonly related to higher grade and/or thicker massive sulfide mineralisation.The results of this new survey significantly increased the resolution of the conductor. Modelling of the FLTEM data shows excellent correlation with the high-grade mineralisation surface, down to a depth of approximately 550m.An FLTEM survey was also completed over the Azevedo prospect and provided a significant improvement in resolution compared to the historical, poorly defined VTEM anomaly. The FLTEM data provided an 80m lateral adjustment to the initial anomaly, allowed detailed modelling, and reduced drill testing uncertainty.Ground and downhole EM surveys at lower frequencies are now being used extensively to effectively explore for massive sulfides with a greatly increased depth of investigation compared to historical airborne and ground EM surveys.

Strategic electromagnetic geophysical prospecting across a belt - an example over the Albany Fraser Orogen

SummaryNickel exploration routinely utilises electromagnetic (EM) techniques as a direct detection tool. Independence Group is currently exploring a large tenement holding in the Albany Fraser Orogen in Western Australia that contains its flagship Nova magmatic nickel-copper-cobalt deposit. As a first-pass exploration stage, large-scale high-resolution airborne EM is being flown over the project using the Spectrem system. Interpreted discrete basement conductors are then followed up with ground EM surveys. In areas of conductive regolith cover identified from the airborne EM surveys, other datasets are used to determine if subsequent EM is warranted. Such methods include regional aircore geochemistry and/or through favourable lithostructural settings interpreted from high resolution potential field datasets. This program of regional data collection is nearly complete, and a follow-up drilling program is starting in earnest this year. This paper includes a couple of examples of the geophysical targets drill tested to date.

Application of near-surface geophysics as part of a hydrologic study of a subsurface drip irrigation system along the Powder River floodplain near Arvada, Wyoming

Rapid development of coalbed natural gas (CBNG) production in the Powder River Basin (PRB) of Wyoming has occurred since 1997. National attention related to CBNG development has focused on produced water management, which is the single largest cost for on-shore domestic producers. Low-cost treatment technologies allow operators to reduce their disposal costs, provide treated water for beneficial use, and stimulate oil and gas production by small operators. Subsurface drip irrigation (SDI) systems are one potential treatment option that allows for increased CBNG production by providing a beneficial use for the produced water in farmland irrigation.Water management practices in the development of CBNG in Wyoming have been aided by integrated geophysical, geochemical, and hydrologic studies of both the disposal and utilization of water. The U.S. Department of Energy (DOE) National Energy Technology Laboratory (NETL) and the U.S. Geological Survey (USGS) have utilized multi-frequency airborne, ground, and borehole electromagnetic (EM) and ground resistivity methods to characterize the near-surface hydrogeology in areas of produced water disposal. These surveys provide near-surface EM data that can be compared with results of previous surveys to monitor changes in soils and local hydrology over time as the produced water is discharged through SDI.The focus of this investigation is the Headgate Draw SDI site, situated adjacent to the Powder River near the confluence of a major tributary, Crazy Woman Creek, in Johnson County, Wyoming. The SDI system was installed during the summer of 2008 and began operation in October of 2008. Ground, borehole, and helicopter electromagnetic (HEM) conductivity surveys were conducted at the site prior to the installation of the SDI system. After the installation of the subsurface drip irrigation system, ground EM surveys have been performed quarterly (weather permitting). The geophysical surveys map the heterogeneity of the near-surface geology and hydrology of the study area. The geophysical data are consistent between surveys using different techniques and between surveys carried out at different times from 2007 through 2011. This paper summarizes geophysical results from the 4-year monitoring study of the SDI system.

Integrated application of heliborne and ground electromagnetic surveys for mapping EM conductor for uranium exploration and its subsurface validation, North Delhi Fold Belt, Rajasthan, India: A case study

A fracture-controlled uranium deposit was identified in Proterozoic Ajabgarh metasediments of the North Delhi Fold Belt within the Khetri subbasin at Rohil, Sikar district, Rajasthan, India. Uranium mineralization in the area is associated with geologic structures, albitization, and pyroxenization of metasediments and conductors such as metallic sulfides and carbonaceous phyllites/graphitic schists. To locate uranium mineralization akin to Rohil in nearby thick soil covered areas, this association was targeted through heliborne geophysical surveys. High-resolution heliborne magnetic and time domain electromagnetic (TEM) surveys were conducted around Rohil. The survey delineated several targets with favorable geologic structures and conductors such as graphitic schist for further uranium exploration. One favorable target near Chappar village was taken up for follow-up exploration work. The EM conductor mapped from heliborne survey was subsequently validated through ground time-domain electromagnetic surveys and subsurface exploration. Modeling of heliborne and ground-based electromagnetic data revealed the presence of subsurface conducting bodies with comparable model parameters. Drilling established the presence of a subsurface conductor up to a depth of 300 m, which was attributed to the presence of graphite and sulfides (pyrrhotite) along foliation plane of carbon phyllite/graphitic schist/quartz-biotite schist and calc-silicate rock. Further detailed laboratory investigations (petrology/X-ray diffraction) of selected core samples from the conductive zones confirmed the presence of pyrrhotite and graphite responsible for EM signature. This study, carried out by using multiparameter data sets, proved the efficacy of heliborne surveys in locating favorable targets for uranium exploration in Ajabgarh group of rocks.

A case study of deep electromagnetic exploration in conductive cover

This paper describes the survey design, data collection and results from a geophysically driven exploration program in Northern Australia. The exploration project was a joint venture between Falcon Minerals NL and Anglo American. It was focussed on finding massive to net textured nickel sulphides associated with a mafic intrusive body. The extent of the intrusive body was inferred from interpretation of a significant 14mgal gravity anomaly. Limited previous exploration indicated any drill targets were likely to be covered by 400 to 500m of mesozoic and recent sediments. These sediments are estimated to have a conductance of 200Siemens, making it difficult to use conventional electrical geophysics for effective exploration. Anglo American, together with its research partner IPHT of Germany have developed a low temperature superconducting quantum interference device (LTS) that can be used as a sensor in transient electromagnetic (TEM) surveys. The LTS is a ?B field? sensor with significant advantages in noise levels over other ?B field? sensors and the ability to detect targets with time constants up to 3 orders of magnitude larger than conventional dB/dt coils (Le Roux et al 2007). The LTS sensor is particularly suited to finding highly conductive targets beneath conductive cover. After forward modelling and survey design, the JV completed some 270km of reconnaissance moving loop ground electromagnetic surveys. Selected anomalies identified with this survey were then followed up with more detailed moving loop and some fixed loop surveys. These results were then modelled using a variety of software packages to try and fit the late time TEM field data. Drilling of selected targets and some downhole TEM surveys are in progress.

Stratigraphic and Structural Constraints on Limestone Exploration: A Case Study from Northern New Brunswick, Canada

Industrial-grade limestone is found in both the Lower Silurian La Vieille Formation and Upper Silurian LaPlante Formation of the Chaleurs Group in northern New Brunswick. Currently, between 150 000 and 200 000 tonnes of limestone are produced per year from the proximal facies of the LaPlante Formation at the Sormany quarry of Elmtree Resources Ltd., located west of Bathurst. The proximal facies of the LaPlante Formation was deposited on the margins of tectonically uplifted Ordovician terranes. This facies comprises stromatoporoidal-algal bindstone intercalated with wackestone, packstone, and floatstone in variable proportions. The distal facies comprises calcareous shale and minor limestone deposited deeper offshore. Folding and faulting related to Middle Devonian Acadian tectonism have caused an increase in the apparent thickness of the limestone sequences, especially adjacent to the regional Rocky Brook–Millstream fault. Structural and stratigraphic observations indicate that some of the limestone bodies in the area have been tectonically displaced from their site of deposition. A variety of prospecting techniques was used to locate new limestone resources, including geological mapping, airborne and ground electromagnetic surveys, and satellite remote sensing. Clastic rock units above and below the LaPlante Formation have distinctive properties that help to trace the intervening limestone along strike. Because of water-saturated glacial cover, thick vegetation, and the small size of targets, airborne geophysical methods did not prove effective in delineating limestone beds, but aeromagnetic surveys helped map the underlying clastic unit. The remote-sensing data and especially high-resolution digital elevation models helped in identification of karst topography related to limestone.

Airborne electromagnetics supporting salinity and natural resource management decisions at the field scale in Australia

Airborne geophysics has been used at the catchment scale to map salt stores, conduits and soil variability, but few studies have evaluated its usefulness as a land management tool at the field scale. We respond to questions posed by land managers with: (1) comparison of airborne and ground-based electromagnetic surveys in the Lower Balonne catchment, Queensland, and (2) comparison with historical and anecdotal knowledge of landscape response in the country around Jamestown in mid-South Australia.In the Lower Balonne, direct comparison between ground electromagnetic survey (EM) and airborne electromagnetics (AEM) showed a strong relationship for both the absolute values and spatial patterns of conductivity. The penetration of AEM to greater than 100m is valuable in defining hydrological barriers. In the Jamestown area, AEM conductivity corresponded well with specific outbreaks of salinity and observed variability in crop response; local inconsistencies at the ground surface could be resolved when sub-surface data were considered. AEM can provide valuable information at the field scale that is relevant to salinity management. Farmers can have confidence in any of these techniques (historical information, EM and AEM) and they may directly compare or integrate the results.

Electrical Structure of the Regolith in the Lawlers District, Western Australia

The weathered rock and transported overburden, which covers much of Australia, reduces the effectiveness of electromagnetic (EM) surveys. This material, termed the regolith, is variably conductive, both laterally and vertically, and reduces the sensitivity and penetration of EM systems.AMIRA (Australian Mineral Industries Research Association) project 407 was set up to improve the effectiveness of EM and especially airborne EM (AEM) in Australia. One of the specific aims was to study the electrical structure of the regolith in the expectation that a better understanding might lead to a significant reduction, or even removal, of its effect on deep penetrating AEM methods. As part of that study, several sites around Australia were selected for a detailed study of the electrical structure of the regolith and its effect upon AEM systems. This report discusses the results from a SALTMAP survey over the Lawlers area, which lies within the Eastern Goldfields region of the Yilgarn Craton in Western Australia.To test the accuracy and resolvability of the AEM data, a number of AEM responses were followed up with ground EM surveys. Both sets of data were interpreted using layered earth inversion algorithms. It was appreciated that a stitched set of 1D layered structures would not accurately model the physics of all of the responses, however it was expected that the vast majority could be usefully treated this way. The results show that although there may be up to six or even seven geologically identified layers with distinct electrical properties in the regolith, neither the airborne or surface EM data can effectively resolve more than three layers. However, good fits to three layer models can be obtained and much can be inferred about the nature of the regolith. At Lawlers, these layers can be broadly represented as follows: (1) the (mostly) resistive surface layer of colluvium/alluvium; (2) underlying conductive saprolite, mottled clays, etc.; and (3) resistive basement.

Full 3-D inversion of electromagnetic data on PC

Three-dimensional (3-D) electromagnetic (EM) inversion might be believed to require high-performance computers. However, with the rapid progress of recent computer technology, running 3-D inversions on personal computers (PC) is becoming a rational choice. This paper describes an attempt to carry out full 3-D inversions of synthetic frequency-domain EM data on a PC. In the inversion, a staggered-grid finite difference scheme is used to solve for the secondary electric field. The system of equations is solved using the incomplete Cholesky biconjugate gradient (ICBCG) method. By applying the static divergence correction proposed by Smith [Geophysics 61 (1996) 1319–1324], the rates of convergence are dramatically improved, and the forward calculation per source location on a medium-size grid takes only a few minutes on a PC. The sensitivities of the EM responses to subsurface resistivity changes are calculated from forward solutions using the reciprocity relation. An inverse problem is formulated so that a model is found that has a smooth structure and at the same time is close to an initial model, and is solved with an iterative least-squares method. A synthetic example for an airborne EM survey shows that the lateral extents of 3-D bodies are well resolved, and the vertical coil coaxial system gives a better lateral resolution than the horizontal coil coplanar system. An example for a ground EM survey shows that expanding the survey coverage or aperture is needed to improve the resolution at depth.

Petrophysical characterisation of parna using ground and downhole geophysics at Marinna, central New South Wales

Parna, an aeolian sediment of dominantly clay size, forms a significant component of the regolith developed in western New South Wales. These materials blanket parts of the contemporary landscape and, in places, they form surficial deposits in excess of 8 m thick. Recent studies have suggested that parna constitutes a significant store and source of soluble salts, hence its significance to land management. This paper describes the mineralogical and petrophysical characteristics of parna, making specific reference to materials adjacent to a recognised parna section at Marinna, near Junee, which is located in the Murrumbidgee catchment. Rising water tables and dryland salinity have been identified as major concerns to the overall health of this catchment. Therefore information on the distribution and movement of salt within the upper reaches of this catchment is important, as is the role of parna in controlling these dynamics. Results from multiparameter borehole geophysical studies indicate that parna is characterised by sub-horizontal variations in conductivity and natural gamma activity. These properties are attributed to contrasts in the porosity and permeability of these materials and to variations in their composition. Significant vertical conductivity contrasts have been noted between parna and underlying saprolite, with the results from EC1:5 measurements and downhole induction logs suggesting that conductivity is primarily controlled by textural variations rather than the soluble salt and moisture content of these materials. Ground electromagnetic surveys (EM31) suggest that areas covered by parna may exhibit marked lateral variations in average conductivity attributable to differences in the thickness and proportion of parna in these materials.

Methods of exploration for strata-bound mineral deposits in the Appalachian-Caledonian Orogen

In response to a questionnaire on the topic of strata-bound mineral deposits in the Caledonian-Appalachian orogen circulated in 1980-1981, 82 contributions were received: United States (20), Canada (17), Denmark--for Greenland (3), Ireland (3), United Kingdom (10), Norway (6), and Sweden (23).On the basis of 68 strata-bound mineral deposits found in the past, the most successful methods of exploration have been geologic mapping and ground electromagnetic surveys, followed by geologic interpretation and prospecting. Boulder tracing found Laisvall and many other deposits in Sweden. The Brunswick 12 deposit in Canada was discovered by airborne magnetic and ground electromagnetic methods.Reports from 58 districts illustrate that a wider range of methods is currently being used for strata-bound mineral exploration in the Appalachian-Caledonian orogen than in the past. Geologic interpretation and ground electromagnetic techniques are proving to be most effective, followed by mapping and soil and till sampling. Photogeology, general prospecting, boulder tracing, drainage geochemistry, and airborne techniques are also achieving considerable success. Outstanding results are being obtained from magnetic and gravity surveys in the Bathurst area of Canada.