Application of the F-approximation to gravity data interpretation. II. The results of testing with the use of detailed gravity and magnetic surveys

Summary This paper presents results of regional and detailed geophysical investigations used in an integrated exploration strategy to find diamondiferous kimberlites in the North Kimberley region of Western Australia. This work is part of an ongoing exploration program by Striker Resources N.L. Indicator-mineral sampling and grain analysis are used to define prospective areas for follow up study. Fixed wing aeromagnetic and helicopter electromagnetic/magnetic surveys are used with detailed ground-based magnetic, gravity, and electromagnetic surveys. Landsat and digital terrain data are also used to identify structures that may be important as sites for kimberlite emplacement. Regional airborne magnetic, electromagnetic and radiometric datasets are useful for lithological and structural mapping. Regional gravity data has shown the presence of two large positive anomalies suggesting deep sources that may influence the location of kimberlite pipes. However, the small (<2.0 ha) aerial extent of the known diamondiferous kimberlite pipes and the presence of other magnetic and electromagnetic sources demonstrates it is vital to conduct detailed ground exploration to explain the sources of anomalies. The current exploration program is proving successful in discovering new diamondiferous kimberlite. Physical properties of kimberlite pipes and host rocks Kimberlitic rocks are formed from the emplacement of potassic, ultramafic, incompatible element rich, CO2-bearing magmas from the upper mantle. The rocks usually have higher densities than the surrounding crustal rocks and commonly contain some magnetic material that may not be evenly distributed in the pipe. Remanent magnetism is commonly observed. Near the surface, the kimberlite can weather to clay which is commonly less dense and more conductive than the surrounding rocks. The combination of low density and moderate conductivity from the weathered zone produces suitable targets for geophysical exploration. In addition, if weathering is extensive, then the thickness of the clay is sufficient to produce a gravity low which can exceed the gravity high from fresh kimberlite in the pipe. Geophysical anomaly detection from kimberlite is therefore dependent on the extent of weathering and the resulting magnetic, electromagnetic and gravity contrasts with the surrounding rocks. Examples Examples are presented for the uses of geophysical techniques in the discovery and delineation of diamondiferous kimberlites in the Ashmore and Seppelt clusters. In both areas, which are separated by approximately 20 km, pipes are weathered to around 50 m and intrude Proterozoic sandstones. Trial mining is in progress at Ashmore to establish whether the cluster contains economic concentrations of diamonds. The Ashmore and Seppelt areas are located on the southern side of a regional positive gravity anomaly that is interpreted as being due to a source with considerable depth extent. It is interesting to note that the Argyle diamond mine, located 200 km south east of Ashmore, is also sited close to an extensive positive gravity anomaly. Gunn and Meixner (1998) suggested that this feature is due to a major intrusion with approximately 20 km of depth extent. Aeromagnetic coverage is available at both Seppelt and Ashmore, while helicopter electromagnetic (HEM) surveys have been acquired for the Ashmore area using Dighem and Geophex/UTS systems. Airborne data at Ashmore showed a local magnetic and conductivity high. Close-spaced electromagnetic, gravity and magnetic ground-based surveys at Ashmore and Seppelt have contributed to discovering and delineating new pipes. Geonics EM 34 and Geophex GEM 2, frequency domain electromagnetic equipment has detected conductive anomalies associated with the preferential weathering of the kimberlite in the more resistive sandstone backgrounds. Gravity surveys have been carried out with Scintrex CG 3 gravity meters and delineated gravity lows associated with the deeper weathering of the kimberlitic material. Magnetic surveys using Geometrics G856 magnetometers have identified

A detailed magnetic and gravity survey was done on portion of the Gulgong Goldfields, New South Wales. The geophysical survey was undertaken to study the geology in an area blanketed by a thick alluvial cover and in particular to investigate a buried, Tertiary river system. This system is known to be present and was once the object of active gold mining operation. The magnetic survey revealed a complex anomaly pattern containing both normal and reversed anomalies. These features were difficult to interpret due to the paucity of geological information and did not appear to be related to the channel structures. The gravity survey was successful in distinguishing the generally broad bedrock depressions associated with the gold-bearing channels. Owing to the presence of a complex regional feature, qualitative interpretation was simplified by applying a number of second derivative operators to the gravity data. Quantitative interpretation wer known ground suggested that lateral density variations within the bedrock or alluvium did not significantly contribute to the measured anomalies and that a density contrast of 0.5 gm/cc exiats between the bedrock and alluvium.

Summary. Correlation of gravity and magnetic anomalies combined with other geological and geophysical data is useful for enhancing the quality of geological interpretation of potential anomaly fields. Maps produced by equivalent point source inversion are used to investigate visual-spatial correlations of surface free-air gravity and POGO satellite magnetic anomalies and regional heat flow and tectonic data for North America and adjacent marine areas. A quantitative analysis of regional potential field anomaly correlations at satellite elevations is also considered utilizing Poisson’s theorem in a moving-window linear regression between derivatives of the anomalous gravity and magnetic fields. An inverse relationship is observed between long-wavelength gravity and magnetic anomalies over continental terrain. Negative gravity and positive magnetic anomalies are areas characterized by relatively thick crust and high magnetization. An example is a prominent magnetic high which corresponds to a trend of gravity minima extending from the Anadarko Basin to the Cincinnati Arch. Negative magnetic and positive gravity anomalies characterize thinner crust and regions of higher heat flow such as the Cordillera of North and Central America and specifically the Yellowstone geothermal region. Although gravity and magnetic anomalies over oceanic areas show poor correlation, the sign of the statistical correlation generally is positive. 1 Crustal correlation considerations The utility of long-wavelength gravity anomalies, covering several degrees of surface area, for geological analysis is well recognized. Magnetic anomalies which have significant energy in wavelengths of several hundred kilometres also have been identified as originating from within the Earth’s crust and perhaps the uppermost mantle. The geological interpretation of these regional anomalies is hindered by the effect of anomaly superposition and source ambiguity which is inherent to the analysis of potential fields. An approach to minimizing these limitations, especially in continental terrains, is to evaluate the correlation between anomalous gravity and magnetic fields. The basis of correlation analysis is the hypothesis, commonly validated in continental areas, that variations in lithology and physical properties of the crystalline crust are reflected

The purpose of the work is development of a complex seismic-density-magnetic model of the geological cross-section for the geotraverse SG-1 (67) (Nadvirna―Otynia―Ivano-Frankivsk) and appreciation of oil and gas bearing perspectives in the Kolomyia Paleovalley. The research methodology is based on the analysis of anomalous gravity and magnetic fields and the creation of density and magnetic models of geological cross-section. Local anomalies are defined by methods of transformations. Modeling of the geological cross-section structure is performed by methods of solving direct and inverse problems of gravity and magnetic survey, which implemented the ideas of the criteria approach to creating optimal models of geological environments that are consistent with the observed geophysical fields and do not conflict with the data of drilling and seismic survey. The reliability of interpreting gravimagnetic data method is achieved by geological subordination: the condition of maximum application of seismic survey and geological and tectonic maps. The conceptual density model and magnetic model of the geological environment along the seism-geological profile of CG-I (67) was developed. The distribution of densities and magnetization of rocks in sedimentary cover and basement blocks to a depth of 20 km is detailed. As a result of modeling within certain stratigraphic complexes, zones with anomalous in density and magnetization of rocks were defined. The decompression zones, which are within positive structures, are identified as perspective ones. It is also confirmed that there is a high probability of deep intrusions that are predicted from materials of seismic survey within the limits of the hidden tectonic zone and reaching the bottom of sedimentary cover. The analysis of morphology of gravity and magnetic anomalies and gravimagnetic modeling allowed clarifying the geological structure of the Kolomyia Paleovalley cross-section and to appreciate oil and gas perspectives of its separate areas.

Detailed gravity and magnetic surveys in and around the Godavari basin have helped in delineating the subsurface structures which, in turn, facilitated reconstructing its evolution. The general NW-SE trend of the magnetic and the gravity anomalies towards north takes a swift turn along an E-W line south of Bhadrachalam. This may be attributed to a deep-seated fault dividing the basin into Godavari and Chintalapudi sub-basins. The same fault may be responsible for limiting the southern extent of Pakhal sediments. A similar deep-seated fault north of Bhadrachalam along the Mailaram 'high' is also indicated. Modelling of some critical gravity and magnetic anomalies indicates the maximum thicknesses for Gondwana and Sullavai sediments as 4.5-5 km and 1.5-2 km respectively. A large gradient in the Bouguer anomaly on the eastern side of the Gondwana basin suggests the existence of a long continuous normal fault which is the master fault of the Gor.dwana rift valley. The other side of the basin is characterised by variations in the gradient of the Bouguer anomaly suggesting enechelon faulting. The Gondwana basin is flanked on either side by heavy rocks of density 2.95 g/cm3 between the depths of 6km to 13 km which might be due to the block uplift of the lower crust forming the 'shoulders' of the rift valley or large sub-basic intrusions in the region. The definite magnetic anomalies in this region providing a high susceptibility of 0.1-0.2 emu almost at the same depth as obtained from the Bouguer anomaly further substantiate them. The subsequent uplifts of the basin is apparent from 'highs' in Bouguer anomaly inside the basin such as around Chinnur which shows sharp gradients on either side suggesting faulted margins. The Regional Bouguer anomaly separated from the observed field along a few representative profiles suggests the presence of a high density (3.1 g/cm3) material along the Moho between the depths of 30km to 38km. The magnetic data reveals several basement ridges and intrusions as far west as. Manthani and Sirpur-Kagaznagar which are transverse to the general trend of the Gondwana basin and almost parallel to the eastern ghat trends suggesting that they may be the subsurface reflections of eastern ghat orogeny. A well-defined magnetic anomaly in the Chintalapudi sub-basin suggests the depth to the basic intrusion in the basement as 3.1 kilometres. There are high magnetic anomalies along the Precambrian faults defining the Purana basin suggesting basic intrusions along them. The basin as a whole depicts a high heat flow specially its southern part which might be due to several intrusive bodies as inferred from the magnetic and the gravity studies. The Godavari basin is a typical rift structure formed during the Gondwana period. The high density material along the Moho and in the upper crust suggests the diapiric upwelling of the asthenosphere as the main cause for the development of a typical continental graben in this region and subsequent uplifts along the shoulders.

During May, 1944, detailed gravity and magnetic surveys were made at the Grand Saline Salt Dome to secure additional information on the physical properties of this typical East Texas salt dome. The results of the surface gravity and magnetic surveys, and the subsurface gravity survey in the Morton Salt Mine are illustrated and discussed. Densities and the available subsurface data were compiled and were utilized in a quantitative evaluation of the observed gravity data. The theoretical mass distribution which was determined by this quantitative evaluation is not intended to represent the unique solution of the geophysical and geological data; instead, it is offered as a possible solution based on relatively simple assumptions.

The effect of parallel pipeline parameters on the characteristics of gravity and magnetic surveys

As part of the Department of Energy’s Underground Structure Detection Program, personnel from Los Alamos conducted a detailed surface gravity survey over a buried cloud chamber located in Yucca Flat, Nevada Test Site (NTS). The primary purpose for conducting the gravity survey was to measure small gravity anomalies due to near-surface density variations not directly associated with the cloud chamber itself. Such density variations are considered noise sources for the purposes of this study. Few published measurements of near-surface noise over underground structures exist. Those studies that have been made (Butler, 1984) have not focused upon near-surface noise, perhaps because the overall signal-to-noise ratios in such studies were relatively high. The motivation for measuring the microgravimetric noise due to near-surface density inhomogeneities is that such sources are believed to be one of the dominant sources of microgravimetric noise. As microgravity methods are used to detect subsurface facilities or cavities, one of the limiting factors will likely be the amplitudes and wavelengths of gravity anomalies due to naturally-occurring, near-surface density changes. Thus, measurements of such effects are required in order to estimate the utility of microgravity as a practical methodology for detecting underground facilities.

The Canadian Cordillera in central British Columbia has seen the Mesozoic subduction of an oceanic terrane; the amalgamation of volcanic-arc terranes; continued intermittent Mesozoic compression and magmatism; and Tertiary wrenching, extension and magmatism. Except in its northernmost mountain ranges, the area is extensively covered in glacial drift and thin veneers of Tertiary volcanic rocks. In 1994, a group of scientists and technologists believed they could understand that cover, see through it, and discover the components of that collision and extensional orogen. They would apply modern techniques of isotopic and paleontological geochronology; lake-sediment, till, and plant geochemistry; detailed gravity, magnetic, radiometric, paleomagnetic, and electromagnetic surveys; and isotopic and trace element lithochemistry, as they conducted extensive bedrock and surficial mapping. This special issue summarizes a cross-section of the scientific contributions derived from that mapping conducted under the auspices of the Nechako NATMAP Project. It demonstrates the absolute necessity of applying modern isotopic and paleontologic geochronology to understand the Phanerozoic geology of the Cordillera. It emphasizes the necessity of detailed aeromagnetic surveys (500 m or less line spacing) in looking through covered terranes at anything more than 1 : 250 000 scale. And, it shows the immense utility of applying various geochemical techniques to solve geological problems and establish baselines for future research and economic development. Bedrock and surficial mapping in the central Cordillera, using these and other techniques, have established the nature and timing of Mesozoic crustal growth, Tertiary crustal thinning, and the associated formation of mineral deposits.

The Benagerie Ridge Magnetic Complex is located in eastern South Australia. The Complex, a component of the Curnamona Craton, comprises highly altered Palaeproterozoic meta-sediments of the Willyama Supergroup, and is concealed by tens of metres of sedimentary cover. The cover includes Neoproterozoic, Cambrian, and particularly, Tertiary strata. Exploration in the region has led to the discovery of Proterozoic iron oxide-style copper-gold mineralisation, plus related elluvial gold in the overlying Tertiary strata. The mineralisation is associated with zones of deeper weathering, resulting from intense feldspathic alteration and sulphidisation of the mineralised stratigraphy. Faulting also influences the distribution of mineralisation. Semi-regional to detailed aeromagnetic and gravity surveys have all been extremely effective for mapping stratigraphy, alteration and structure within the Benagerie Ridge Magnetic Complex. However, airborne EM data did not contain responses from the basement. The intense weathering associated with mineralisation is the cause of residual negative gravity anomalies with amplitudes of 1 to 2 mGal. Consequently, detailed gravity surveys have been very effective for identifying prospects within the region. IP/resistivity and time-domain EM surveys were largely unsuccessful, with conductive overburden preventing responses from the mineralised stratigraphy being detected. However, complex IP/resistivity surveys detected responses that coincide with known areas of mineralisation. A lack of electrical continuity, due to poor development of massive sulphides, means downhole EM methods have been largely ineffective in the area.

Computation of anomalous gravity and magnetic fields generated by various models is a necessary step if techniques of curve‐matching are to be used for quantitative interpretation of potential field data. Recently developed methods show that anomalous magnetic and gravity fields are completely determined by the divergence of magnetization and the first vertical derivative of density, respectively. Using these methods, efficient algorithms can be developed for computing potential field anomalies caused by arbitrary distribution of magnetization and density in an irregularly shaped body. Automatic iterative procedures are normally employed in the space domain for estimating parameters of the selected model that yield a best‐fit anomaly curve for a set of discrete observed data. Examples of application of the Newton‐Raphson method, Marquardt method, and the Powell algorithm to the interpretation of magnetic data are presented and discussed. Amplitude and energy spectra of the anomalous fields are also used conveniently in many cases for systematic estimation of average and individual depths, horizontal and vertical extents, and density or magnetization contrasts of causative bodies in a bounded region. Some of these frequency‐domain approaches are found to have many useful applications.

This work reports the progress on collecting existing gravity data in a rectangular area covering the Richmond and Taylorsville Basins and its vicinity. The area covers one-degree latitude and one degree longitude, starting at 37 North, 77 West and ending at 38 North, 78 West. Dr. David Daniels of the United State Geological Survey supplied us with more than 4900 Bouguer gravity anomalies in this area. The purpose of this report is to present the data in form of several maps and discuss its relation to the geology of the Triassic Basins and its vicinity. Johnson and others (1985) also presented a map of the Bouguer gravity anomaly of this area. However, their map covers a smaller area, and it is based on smaller number of observations.

This work reports the progress on collecting existing gravity data in a rectangular area covering the Richmond and Taylorsville Basins and its vicinity. The area covers one-degree latitude and one degree longitude, starting at 37 North, 77 West and ending at 38 North, 78 West. Dr. David Daniels of the United State Geological Survey supplied us with more than 4900 Bouguer gravity anomalies in this area. The purpose of this report is to present the data in form of several maps and discuss its relation to the geology of the Triassic Basins and its vicinity. Johnson and others (1985) also presented a map of the Bouguer gravity anomaly of this area. However, their map covers a smaller area, and it is based on smaller number of observations.

A detailed gravity survey was carried out for the entire Carson Sink in western Nevada (Figure 1) through a subcontract to Zonge Engineering, Inc. The Carson Sink is a large composite basin containing three known, blind high‐temperature geothermal systems (Fallon Airbase, Stillwater, and Soda Lake). This area was chosen for a detailed gravity survey in order to characterize the gravity signature of the known geothermal systems and to identify other potential blind systems based on the structural setting indicated by the gravity data. Data: Data were acquired at approximately 400, 800, and 1600 meter intervals for a total of 1,243 stations. The project location and station location points are presented in Figure 14. The station distribution for this survey was designed to complete regional gravity coverage in the Carson Sink area without duplication of available public and private gravity coverage. Gravity data were acquired using a Scintrex CG‐5 gravimeter and a LaCoste and Romberg (L&R) Model‐G gravimeter. The CG‐5 gravity meter has a reading resolution of 0.001 milligals and a typical repeatability of less than 0.005 milligals. The L&R gravity meter has a reading resolution of 0.01 milligals and a typical repeatability of 0.02 milligals. The basic processing of gravimetermore » readings to calculate through to the Complete Bouguer Anomaly was made using the Gravity and Terrain Correction software version 7.1 for Oasis Montaj by Geosoft LTD. Results: The gravity survey of the Carson Sink yielded the following products. Project location and station location map (Figure 14). Complete Bouguer Anomaly @ 2.67 gm/cc reduction density. Gravity Complete Bouguer Anomaly at 2.50 g/cc Contour Map (Figure 15). Gravity Horizontal Gradient Magnitude Shaded Color Contour Map. Gravity 1st Vertical Derivative Color Contour Map. Interpreted Depth to Mesozoic Basement (Figure 16), incorporating drill‐hole intercept values. Preliminary Interpretation of Results: The Carson Sink is a complex composite basin with several major depocenters (Figures 15 and 16). Major depocenters are present in the south‐central, east‐central, and northeastern parts of the basin. The distribution of gravity anomalies suggests a complex pattern of faulting in the subsurface of the basin, with many fault terminations, step‐overs, and accommodation zones. The pattern of faulting implies that other, previously undiscovered blind geothermal systems are likely in the Carson Sink. The gravity survey was completed near the end of this project. Thus, more thorough analysis of the data and potential locations of blind geothermal systems is planned for future work. « less

This paper compares the usefulness of several types of detailed airborne and ground magnetic surveys, and a detailed gravity survey, in mapping the geology of the Corsair area, about 10 km east of Kalgoorlie. The gravity and magnetic surveys when interpreted together map out the distribution of mafics, ultramafics and sediments. The mafic rocks are only weakly magnetic, but they generate an easily observable gravity anomaly of about 20 µm.s-2. Surficial deposits are associated with both elongate and high-amplitude short-wavelength magnetic anomalies; their distribution can best be mapped as the area of short-wavelength anomalies in the unsmoothed observations (dappled on pixel maps). The magnetic field due to basement is best derived by attenuating the high-amplitude short-wavelength anomalies; this is most reliably done by estimating the mode, rather than the mean, of the observations. The airborne gradiometer survey gave superior results to both the aeromagnetic total intensity survey, and the ground magnetic survey, in that it contained the short wavelength information required to map the surficial deposits, and it had better detail of the anomalies due to the basement. The image of the gravity gradient provided the best representation of linears. Many of these linears are in part coincident with workings, so they are gold carriers. Known gold mineralization is preferentially developed along a sheared zone abuting the boundary between the two major basement rock associations; this zone corresponds with the major gravity gradient.