J. Derek Fairhead

New insights into magnetic derivatives for structural mapping

Magnetic and gravity derivatives can be likened to seismic attributes in that they can help define/estimate the physical properties of the source structure causing the anomaly. This contribution looks at the tilt derivative, first reported in 1994 and more recently used to derive the local wavenumber (1997). We will show that the combination of the tilt derivative and its total horizontal derivative are highly suitable for mapping shallow basement structure and mineral exploration targets and that they have distinct advantages over many conventional derivatives. We provide the simple theory behind the derivatives, use a range of simple 2D models to illustrate their response, and apply them to mapping a mineral target in Namibia.

Euler deconvolution of gravity tensor gradient data

Tensor Euler deconvolution has been developed to help interpret gravity tensor gradient data in terms of 3-D subsurface geological structure. Two forms of Euler deconvolution have been used in this study: conventional Euler deconvolution using three gradients of the vertical component of the gravity vector and tensor Euler deconvolution using all tensor gradients. These methods have been tested on point, prism, and cylindrical mass models using line and gridded data forms. The methods were then applied to measured gravity tensor gradient data for the Eugene Island area of the Gulf of Mexico using gridded and ungridded data forms. The results from the model and measured data show significantly improved performance of the tensor Euler deconvolution method, which exploits all measured tensor gradients and hence provides additional constraints on the Euler solutions.

Tilt-depth method: A simple depth estimation method using first-order magnetic derivatives

Aeromagnetic data are routinely presented as contour or color-shaded maps of the total magnetic intensity (TMI). An interpreters task is to identify features (anomalies) contained within the map and qualitatively and/or quantitatively interpret them into geologic structures at depth. If the map contains anomalies that have large magnetic intensities, the bodies might be considered to have large magnetizations, or to be at shallow depths. Small amplitude anomalies superimposed on these anomalies could be masked or even missed by an interpreter. Thus the task of the interpreter is to use the spectral content of the anomalies to try and resolve these ambiguities. Part of this process is also to obtain estimates of the depth and shape of the body causing the anomalies.

Tectonic evolution of the west Scotia Sea

Joint inversion of isochron and flowline data from the flanks of the extinct West Scotia Ridge spreading center yields five reconstruction rotations for times between the inception of spreading prior to chron C8 (26.5 Ma), and extinction around chron C3A (~6.65.9 Ma). When they are placed in a regional plate circuit, the rotations predict plate motions consistent with known tectonic events at the margins of the Scotia Sea: Oligocene extension in Powell Basin; Miocene convergence in Tierra del Fuego and at the North Scotia Ridge; and Miocene transpression at the Shackleton Fracture Zone. The inversion results are consistent with a spreading history involving only two plates, at rates similar to those between the enclosing South America and Antarctica plates after chron C5C (16.7 Ma), but that were faster beforehand. The spreading rate drop accompanies inception of the East Scotia Ridge back-arc spreading center, which may therefore have assumed the role of the West Scotia Ridge in accommodating eastwards motion of the trench at the eastern boundary of the Scotia Sea. This interpretation is most easily incorporated into a model in which the basins in the central parts of the Scotia Sea had already formed by chron C8 contrary to some widely accepted interpretations and which has significant implications for paleoceanography and paleobiogeography.

The flexural rigidity of Fennoscandia: reflection of the tectonothermal age of the lithospheric mantle

Abstract Estimates of effective elastic thickness ( Te ), a measure of the strength of the plate, have shown that older and cool cratonic regions are characterised by a stronger lithosphere (higher Te values) than areas which have been tectonically reworked. We use geophysical data and information from xenoliths in volcanic rocks from Fennoscandia and show that the strength of the lithosphere reflects the tectonothermal age of the lithospheric mantle. Fennoscandia is made up of the Baltic shield (1.9–3.0 Ga) and the Caledonian Orogen of Norway and northern Sweden (400–500 Ma). The shield is characterised by an old, thick and cold lithosphere in contrast to the Caledonides where the lithosphere is hotter and thinner. Our geophysical analysis results show a regional variation in elastic plate thickness from 8 km to 70 km, or equivalently, a flexural rigidity between 0.4×10 22 and 3×10 24 Nm. These results suggest that the lithosphere is strongest in the relatively stable Archaean Province, weaker in the regions characterised by Proterozoic crustal formation, and lowest in the tectonically reworked and deformed Caledonian belt. Within the study area, there is a direct correlation between lithosphere strength, the age of the last major tectonothermal event registered in the crust and lithospheric mantle composition. These broad correlations reflect thinner and more fertile lithosphere, and higher geothermal gradients, beneath regions of progressively younger crust.

CHAMP satellite and terrestrial magnetic data help define the tectonic model for South America and resolve the lingering problem of the pre-break-up fit of the South Atlantic Ocean

In July 2000, a new potential fields satellite was launched and is currently measuring the earths gravity and magnetic fields (the latter down to 0.1 nT). The satellite, called CHAMP (CHAllenging Mini-satellite Payload), will be providing high accuracy crustal magnetic field data of the earth for wavelengths of about 300–3000 km until about 2005. Previous magnetic satellites have been reviewed by Ravat and Purucker ( TLE , 1999) who found that an excellent comparison could be made between the long wavelength component of the DNAG magnetic field (>500 km) covering North America and the Magsat satellite field. Magsat was limited to a six-month operating period due to its low orbital altitudes (between 352 and 578 km) before burning up in the atmosphere in June 1980. Its large variation in orbital altitude was one factor that made generation of the crustal field component difficult. The CHAMP crustal field, on the other hand, is derived from solar night data, for which the magnitude and complexity of the transient variation corrections are significantly reduced—making for a more stable crustal field solution with significantly greater resolution. As we will show in this article, resolution of the satellite magnetic field over the Atlantic Ocean is sufficient to map subtle coherent features (<1 nT in amplitude) which are interpreted as bulk changes in the remanence of the oceanic crust associated with the linear magnetic reversals pattern of the geomagnetic field during and since the Mesozoic period. Such resolving power of these N-S striking anomalies, which subparallel the orbital tracks, provides confidence that the larger magnetic variations imaged over the continents are being reliably mapped and can aid identification of geologic provinces of interest in petroleum and mineral exploration. Comparisons of the satellite and terrestrial aeromagnetic data over Brazil show that the Trans Brazilian Lineament, a major continental …

Satellite-derived gravity having an impact on marine exploration

Converting sea-surface height variations, derived from satellite altimetry, to free air gravity is not new. In the early 1980s William Haxby (Lamont Doherty Geological Observatory) produced the first global marine gravity map from SeaSat satellite altimeter data using interorbital track spacing of about 180 km. Haxbys map had a significant impact on plate tectonic theory because marine free-air gravity data were able for the first time to uniformly image the tectonic fabric of the earths oceanic crust. Since that time, much effort has been applied to improving satellite-derived gravity resolution. A major advance occurred in 1995, when the altimeter data from Geodetic Missions (GM) of GeoSat and ERS-1 satellites were released. Table 1 gives details of these satellites.

Comparison of grid Euler deconvolution with and without 2D constraints using a realistic 3D magnetic basement model

We describe the application of a 2D-constrained grid Euler deconvolution method which is able to determine for each solution window whether the source structure is two dimensional, three dimensional, or poorly defined and to estimate the source location and depth. In each solution window, eigenvalues and eigenvectors are derived from the Euler equations and compared to threshold levels. A single eigenvalue below the given threshold and lying in the x–y-plane is shown to indicate a 2D source, while the absence of such an eigenvalue indicates a 3D source geometry. Two small eigenvalues indicate the field in the window has no distinct source. Applying these criteria to each solution window allows us to generate a map of source-geometry distribution. We evaluate the effectiveness of 2D-constrained grid Euler deconvolution using synthetic magnetic data generated from a 3D basement model based on real topography from an area with surface-exposed faulting. This modeling strategy provides a complex, nonidealized data set that compares Euler depth estimates directly to the known basement surface depth. Our results indicate that noninteger structural indices can be the most appropriate choice for some data sets, and the 2Dconstrained grid Euler method images magnetic basement structure more clearly and unambiguously than the conventional grid Euler method.

The South American Gravity Project

The South American Gravity Project (SAGP) was a three year oil industry funded project undertaken by the University of Leeds Industrial Services Ltd (ULIS). It was designed to help the long term exploration objectives of the oil industry by collecting all available public and proprietary gravity data for the South American continent and its margins. The land and marine point gravity data were converted to a uniform digital form and reprocessed to a common datum prior to being interpolated onto a regular 5′ × 5′ grid (~10 km x ~10 km) from which a series of maps have been generated. This contribution describes the reprocessing of existing gravity data, highlighting the need for an accurate digital terrain model to permit accurate outer zone terrain corrections to be calculated. Such corrections are both large in amplitude and extend over large areas making their determination important in generating accurate Bouguer anomaly maps. One immediate result of SAGP is that the large gravity data gaps in Brazil identified by this study are now being filled.

The use of GPS in gravity surveys

Global Positioning System (GPS) technology provides a cost-effective surveying method that is replacing traditional methods of precise leveling in exploration. Using satellite-derived GPS coordinates can, however, generate problems, because they will need to be translated to national systems, if this is the preferred local system being used by the explorationists. However, consistent coordinates should be used to compute gravity corrections. This contribution reviews how GPS-derived coordinates are used for different types of gravity surveys and how different coordinate systems generate significant differences in the location, height, and derived gravity values. The use of GPS technology also leads to the term “gravity disturbance” which may be new to many but which turns out to be conceptually the more straightforward expression for the anomalous part of the Earths gravity field. This article also draws attention to recent detailed articles on this subject and on our GPS experience in South America. GPS was designed to provide an instantaneous absolute positioning using two codes, P and C/A, transmitted by a constellation of satellites. The P code has certain characteristics that allow an accuracy of decimeters in the coordinates but is restricted by the United States to military applications. The C/A code, a free civil code, provides an accuracy of a few tens of meters in the worst case. An alternative to these codes, now used extensively in geophysical surveys to determine precise 3D position, is the measurement of the phase of the carrier wave which, as such, does not require the need to know or to use the modulations of the signal or the codes transmitted by the satellites. Each satellite transmits two frequencies with the terrestrial receiver designed to receive either one or both frequencies. In the second case, the receiver system can correct for ionospheric refraction, by using the correlation of this …