Geophysical Survey Data Research Articles

Mineral potential evaluation based on airborne geophysical data

A prediction model, based on the differences between the distribution functions of geophysical survey data from mineralised and non-mineralised areas, has been developed to identify exploration targets. Exploration for mineral resources is conducted in a step-by-step approach, and the proposed prediction model is applicable to each of these steps. Empirical probabilities of discovering new deposits are estimated by a cross-validation technique. The cross-validation technique is central to the proposed methodology.The first study area, located in north-east Guinea, West Africa, covers 46000 km2. Magnetic and radiometric data from a 1 km line spacing airborne survey were used to create a mineral prediction map for lateritic-type gold deposits. This can be considered a reconnaissance study to identify further exploration areas likely to contain undiscovered deposits. These areas should be small enough to carry out further geologic study. From the prediction model, we have identified target areas, covering approximately 2300 km2 or 5% of the study area. We expect that the target areas contain 55% of all undiscovered lateritic gold deposits in the study region. The best prediction results are obtained when the total magnetic field, potassium abundance and uranium to thorium ratio data are used.The second example is from the Bathurst Mining Camp in New Brunswick, eastern Canada. Geophysical data are from a high-resolution helicopter-borne magnetic, electromagnetic and radiometric survey flown during the summer of 1995. This prediction study can be considered as a second step after a first reconnaissance study. The area covers approximately 4100 km2 and many volcanogenic massive sulphide (VMS) deposits are found in this region. The best prediction results are obtained using magnetic and electromagnetic data only, without radiometric data. The target areas delineated in the prediction map cover 41 km2 or 1% of the study area. From the cross-validation analysis these target areas are expected to contain 40% of all undiscovered VMS deposits in the Bathurst Mining Camp.

Concluding remarks and recommendations for practice

The concluding remarks lead on to some general recommendations for practice, particularly in the way that the geophysical investigation should be planned, staffed and managed integrally with the whole scheme of investigation. 11.1 CONCLUDING REMARKS A conceptual ground model should be developed for every site under investigation. The model should be based on sound geological appraisal of the site and its surroundings carried out by a geologist and should take account of the geotechnical and geo-environmental objectives of the investigation. The geophysical survey should be designed on the basis of this model, which should be continually reappraised in the light of new information. The geophysical survey data, and their interpretation, will form an integral part of the development of the ground model. Hence, the interpretation of the geophysical data should be carried out within the context of the model, and not independent from it. This implies that the geophysicist has to be regarded as a member of the site investigation team. 11.2 RECOMMENDATIONS FOR GOOD PRACTICE 11.2.1 Planning As one of the first activities of a construction project, there is often too much pressure applied to mobilise equipment, so that “something can be seen to be happening”. As a consequence, insufficient effort and skill are applied to the planning of many investigations. Work that commences in a poorly planned, haphazard way often continues in that way. The following, represents both an overview of the broader requirements for good site investigation practice and recommendations for when geophysics

An integrated approach with DighemV to groundwater exploration – Tsabong, Botswana

Exploration for potable groundwater to supply settlements in the southern Kalahari Desert of southwestern Botswana is currently in progress. A DighemV survey was flown over outcrops of Precambrian rocks after target areas were selected in an initial desktop study. The study included some preliminary ground geophysics, reviewed all available information in the area and completed an interpretation of remotely sensed (Landsat TM and SPOT) and aeromagnetic data. Two target areas were recommended for DighemV airborne electromagnetic (AEM) surveying near outcropping basement. The DighemV survey was designed to map faults and fractures intersecting resistive quartzite of the Olifantshoek Sequence, the most productive groundwater host in the area. Outcrop geological mapping, ground geophysical traverses to test previous successful boreholes and to locate new ones, exploration drilling and borehole logging were then used to follow up and refine targets generated from the AEM survey. Airborne and ground geophysical survey data were carefully integrated with outcrop geology and borehole information. An improved understanding of the geology and structure in Precambrian Olifantshoek quartzite was based on interpretation of the DighemV and aeromagnetic data. Exploration drilling success is attributed to an integrated multidisciplinary approach, which is strongly recommended for other potential groundwater projects in the Kalahari Desert.

Neotectonics of Puerto Rico and the Virgin Islands, northeastern Caribbean, from GPS geodesy

The boundary between the North American and Caribbean plates is characterized primarily by left‐lateral motion along predominantly east‐west striking faults. Seismicity and marine geophysical survey data are consistent with at least two, and possibly three, microplates in the diffuse boundary zone in the northeastern Caribbean: (1) the Gonave, (2) the Hispaniola, and (3) the Puerto Rico‐northern Virgin Islands (PRVI). We discuss results from GPS geodetic measurements acquired since 1994 to test the microplate hypothesis, define PRVI translation and rotation within the boundary zone, and constrain PRVI neotectonics. GPS‐derived velocities are analyzed with respect to both North American and Caribbean plate reference frames. Integrated displacements across PRVI are limited to a few millimeters per year, consistent with a rigid PRVI and permitting calculation of an average velocity for PRVI. The motions of PRVI relative to North America and the Caribbean are 16.9±1.1 mm/yr toward N68°E±3° (1σ) and 2.4±1.4 mm/yr toward S79°W±26° (1σ), respectively. In contrast with some recent models, ongoing rotation of PRVI about a nearby (< 25° distant) vertical axis is not supported by the geodetic data. In addition, we argue against eastward tectonic escape of PRVI and favor a simple, progressive increase in velocity across the plate boundary zone, requiring that the summed magnitude of strike‐slip fault slip rates will equal the total plate motion rate between the Caribbean and North America. GPS data are consistent with components of left‐lateral strike‐slip faulting along the Muertos trough south of Puerto Rico and shortening across the Puerto Rico trench. Comparison of GPS velocities for PRVI with respect to North America with total North America‐Caribbean relative motion suggests up to 85% of North American‐Caribbean plate motion is accommodated by the Puerto Rico trench and offshore faults north of Puerto Rico. Differences in GPS‐derived velocities from Hispaniola and PRVI yield east‐west extension across the N‐S trending Mona rift of a few millimeters per year when estimated elastic strain accumulation effects along the north Hispaniola deformed belt and the Septentrional fault zone are considered. The opening rate implies an age of the Mona rift of 2–3 million years, agreeing with marine geophysical data that support a young age for the structure.

Wroxeter hinterland project and geophysical survey at Wroxeter

This paper provides the overall archaeological context of a geophysical survey carried out at the Roman town at Wroxeter, including both excavation carried out within the town and survey and excavation within its hinterland. The geophysical survey at Wroxeter was carried out within the remit of the Wroxeter Hinterland Project. The archaeological rationale behind this project is explained and the contribution of geophysical survey data to achieving the goals of this project is discussed. Copyright © 2000 John Wiley & Sons, Ltd.

Identifying large-scale erosion and deposition processes from airborne gamma radiometrics and digital elevation models in a weathered landscape

Patterns of erosion and deposition are difficult to identify and measure at catchment and regional scale but it may be possible to infer their distribution from remote sensing using easily measured surrogate variables. Airborne geophysical surveys provide data on gamma ray emissions from surface and near-surface material and allow estimation of K, Th and U content. Gamma ray signatures are largely determined by lithology but also change with weathering and with erosion and deposition, and may be used as a partial surrogate for those processes. Comparison of gamma ray signatures with topographic characteristics closely related to sediment transport capacity, downstream sorting of sediments, and the extent of erosion and deposition shows strong linkages. Studies in four small catchments in southeastern Australia indicate that K consistently increases as slopes become steeper while Th and U may either increase or decrease. This suggests the presence of fresh rock rather than weathered material and implies removal of material by erosion, although some patterns may result from systematic changes in lithology across catchments. Analysis by lithology confirms the increase in K with slope in granites, metamorphosed sediments and basalt, and also shows a tendency for U to decrease with slope in the granites and basalt. Gamma ray emissions vary only slightly with catchment area (a surrogate measure of water discharge) suggesting that water erosion is limited or that discharge is not closely related to area. Gamma radiometric profiles down hillslopes, averaged across the full range of gradients, show that, in most cases, radioelements initially decrease, probably reflecting increased weathering, but then increase close to valley floors because of accumulation of fine sediments. Analysis by lithology confirms the increase in radioelements close to valley floors in granites and metamorphosed sediments but the trend is less clear in basalt. Gamma ray profiles down floodplains are variable and indicate the amount of deposition and accumulation of weathered material in valley openings and exposure of bedrock in valley constrictions. Simple erosion and deposition models, based on the conservation of mass equation, and applied to the four catchments, show that all radioelements increase as the potential for deposition increases. They reach a low point in zones of no net erosion or deposition and subsequently increase as erosion becomes more intense and weathered material is lost from slopes. Analysis by lithology largely confirms this pattern. The usefulness of airborne geophysical survey data is limited by flight line spacing with most data being flown at a 200–400 m spacing in Australia. However, general trends in erosion and deposition can still be distinguished and there is capacity for calibrating long-term erosion and deposition models once better approaches to interpretation of gamma ray data have been developed. Copyright © 2000 John Wiley & Sons, Ltd.

Bayesian Method for hydrogeological site characterization using borehole and geophysical survey data: Theory and application to the Lawrence Livermore National Laboratory Superfund Site

A stochastic Bayesian approach for combining well logs and geophysical surveys for enhancing subsurface characterization is presented. The main challenge we face is in creating the bridge to link between ambiguously related geophysical surveys and well data. The second challenge is imposed by the disparity between the scale of the geophysical survey and the scale of the well logs. Our approach is intended to integrate and transform the well log data to a form where it can be updated by the geophysical survey, and this tends to be a convoluted process. Our approach starts with generating images of the lithology, conditional to well logs. Each lithology image is then used as the basis for generating a series of shaliness images, conditional to well log data. Shaliness images are converted to resistivity images using a site‐specific petrophysical model relating between shaliness, resistivity, and lithology, to create the necessary interface with the cross‐well resistivity survey. The lithology and resistivity images are then updated using cross‐well electromagnetic resistivity surveys. We explored the limits of the approach through synthetic surveys of different resolutions and error levels, employing the relationships between the geophysical and hydrological attributes, which are weak, nonlinear, or both. The synthetic surveys closely mimic the conditions at the LLNL Superfund site. We show that the proposed stochastic Bayesian approach improves hydrogeological site characterization even when using low‐resolution resistivity surveys.

Crossing the borders: from plains to rugged terrains

We often assume that the Earth is flat and that all geophysical surveys are conducted on a flat survey plane, just as many geophysical text books suggest. However the surface of the Earth is often rugged, and the effect of this rugged surface depends directly on the height of the survey above the ground. Mountainous areas have always provided a challenge for both airborne and ground surveys, and for the processing and interpretation of the survey data. For example, airborne surveys in these terrains suffer variable terrain clearance between adjacent survey lines as the aircraft attempts to 'hug' the terrain. This combined with the undulating survey surface, causes data levelling problems and distorted anomalies that often lead to erroneous interpretations of the survey data. Problems associated with airborne surveys conducted in rugged terrains are, in recent times, becoming evident in the low-level surveys now being conducted with agricultural aircraft over seemingly undulating terrains. The close proximity of the survey aircraft to the gently undulating terrain produces the same terrain induced problems as experienced in mountainous terrains.All ground geophysical methods are strongly affected by rugged terrain. For these surveys, it is often possible to use the rugged terrain to advantage by obtaining multiple 'views' of a buried source by surveying several slopes of the terrain around the target source. In addition, EM loops and galvanic electrodes can also be judiciously located on the terrain to obtain a more accurate definition of a buried anomalous source.Explorers are also demanding more accurate interpretations of data from rugged terrains in order to resolve target responses from the distorted responses measured in these terrains. To this extent modern 3-dimensional computer modelling software can now accommodate survey data from an undulating survey surface and the geophysical responses of geologically complex undulating terrains can now be computed. Rugged terrains present new challenges to survey design, survey operations, data processing and interpretation, and modelling of the survey data. As a consequence of this there is now an urgent requirement for high resolution digital terrain models for survey design and for accurate reduction, interpretation and modelling of all geophysical survey data. Techniques developed for rugged terrain environments can also be applied to underground and open-pit mine environments.

Study on the fractal character of magnetic anomaly fields and the delineation of tectonic elements in the South China Sea

This is a quantitative method for studying the fractal character of magnetic anomaly fields and delineating tectonic elements using fractal theory. For an area with different geological and geophysical features, the definition of Hausdorff dimension is used to calculate the fractal dimension of the area’s magnetic field, then delineate geological tectonic elements by comparing this value. Use of 1978–1986 geophysical survey data in this method applied to the South China Sea yieled three first grade tectonic elements and nine second grade ones, in six of which subsecond grade elements were found.

Some features of the arc‐continent collision zone in the Ryukyu subduction system, Taiwan Junction area

Abstract To the northeast of Taiwan, northwestward subduction of the Philippine Sea plate is occurring beneath the Eurasian plate along the Ryukyu Trench. The Ryukyu Trench, which is well defined along the northeastern part of the Ryukyu arc, cannot be easily defined west of 123° east. This is an area where the Gagua Ridge (whose origin is controversial) enters the trench from the south. On the basis of the marine geophysical survey data the following results have been obtained. The structural elements associated with the Ryukyu subduction system deform and partially disappear west of 123° east. Among other things the Ryukyu Trench terminates close to the western slope of the Gagua Ridge. The Gagua Ridge is the result of tectonic heaping and is likely to be an uplifted sliver of oceanic crust. The interaction between the Ryukyu subduction system and the Taiwan collision zone encompasses a wide region from Taiwan to the longitude 124.5° east. The Gagua Ridge is a boundary between the active deformation zone related to the collision in Taiwan and the West Philippine Basin. It is proposed that there is a tectonic zone that can be traced from the Okinawa Trough on the north to the southern termination of the Gagua Ridge on the south.

Offshore Maryborough Basin, southeast Queensland: Extent, Structure and Petroleum Potential from New Geophysical Survey Data

Offshore Maryborough Basin, southeast Queensland: Extent, Structure and Petroleum Potential from New Geophysical Survey Data

Kane Fracture Zone

The Kane Fracture Zone probably is better covered by geophysical survey data, acquired both by design and incidentally, than any other fracture zone in the North Atlantic Ocean. We have used this data to map the basement morphology of the fracture zone and the adjacent crust for nearly 5700 km, from near Cape Hatteras to the middle of the Mesozoic magnetic anomalies west of Cap Blanc, northwest Africa. We use the trends of the Kane transform valley and its inactive fracture valley to determine the record of plate-motion changes, and we interpret the basement structural data to examine how the Kane transform evolved in response to changes in plate motion. Prior to about 133 Ma the Kane was a small-offset transform and its fracture valley is structurally expressed only as a shallow ( < 0.5 km) trough. In younger crust, the offset may have increased to as much as 190 km (present offset 150 km) and the fracture valley typically is up to 1.2 km deep. This part of the fracture valley records significant changes in direction of relative plate motion (5°–30°) near 102 Ma, 92 Ma, 59 Ma, 22 Ma, and 17 Ma. Each change corresponds to a major reorganization of plate boundaries in areas around the Atlantic, and the fracture-zone orientation appears to be a sensitive recorder of these events.

A collocation method to fit geophysical survey data

A self adaptive collocation method has been applied to the quantitative interpretation of geophysical survey data. Examples of interpretation of vertical electrical soundings (VES) and magneto-telluric soundings (MTS) have been chosen for this work due to the high nonlinearity of the response functions and to the high degree of correlation of the parameters involved. In both cases horizontally isotropic multi-layered models have been adopted. For comparison two different computing algorithms have been considered. In the first, using a generalized inverse formulation a new different decomposition into characteristic values has been used. In the second the Marquardt procedure has been modified in an original way to give a faster convergence using a strategy based on the contemporary adjustment of the step length and the direction of the regression. The confidence limits of the results have been evaluated. Results are given for interpretation of both MTS and VES curves using the two different algorithms. Model data and field data have been considered for an effective comparison of speed of convergence and accuracy.

Computer aided contour plotting of geophysical survey data

Geophysical survey data represent a class of information currently subjected to computer contouring techniques. The gridded network is a feature inherent to a large portion of these surveys. Taking advantage of this feature, a simple and highly efficient computer program has been developed, minimizing the contouring effort in terms of the money, time and programming involved in the interpretation process. The program includes interpolation between data points in two directions, determination of contour line coordinates, and subsequent plotting.

Automatic Plotting of Airborne Geophysical Survey Data

When geophysical measurements are made from the air, it is important to know the position of the aircraft. The position at any time is usually known from a combination of position-fixing and dead-reckoning systems. In-flight optimization of this combination is a well-studied subject. When post-flight track plotting is required for geophysical survey, use can be made of both the initial and terminal position fixes to correct the dead-reckoning track and hence improve on the in-flight version. This technique is applied to compute the track of aircraft conducting ice thickness measurements in the Antarctic, and the effect of certain errors is evaluated. The algebraic results are equally applicable in parts of the world where better navigational aids are available. A relation is derived between the track plotting errors and die errors in the geophysical measurement such that the precision of one is not degraded by errors in the other.