EGM 2010 International Workshop

In the last five years, considerable progress has been made in airborne gravity gradiometry with Fugro’s proprietary FALCON technology. Noise levels have nearly halved and the routine incorporation of regional gravity data has led to very wide bandwidth gravity data. The successful development of a digital AGG has made it possible to deliver cost-effective helicopter-borne gravity gradiometry, providing increased sensitivity and spatial resolution. In addition, joint gravity, magnetic and electromagnetic surveys have been demonstrated from both helicopter and fixed-wing aircraft. The heli-FALCON system is credited with the discovery of new kimberlites in the Ekati diamond field.

This paper considers the simple case of a water layer overlying a 1 ohm-m half space to study the signal properties of a horizontal current dipole source in controlled source electromagnetic surveying. Although a conventional CSEM source has much more energy at certain frequencies than a transient PRBS signal with the same strength, processing of the PRBS data provides tremendous gain in signalto- noise ratio. Deconvolution alone gives two orders of magnitude increase in signal-to-noise ratio; correlated noise, including magnetotelluric noise, can be reduced by a further order of magnitude; and the air wave becomes separable from the earth response.

Subsurface resistivity mapping based on Controlled Source Electromagnetic (CSEM) measurements is an attractive technology for exploration as it offers the possibility to distinguish between hydrocarbon and brine bearing prospects where conventional seismic methods prove inconclusive. In Shell, we have applied the CSEM method on a worldwide scale since 2003 to both de-risking and portfolio polarization. Early on in the development of the CSEM technique some compelling results were obtained with single 2D profiles over prospects. Unfortunately, the lack of subsurface coverage of this type of acquisition often leads to ambiguous results because the Earth rarely satisfies the 2D assumption at the scale of the CSEM experiment. To reduce such ambiguities, we have focused efforts on the development of 3D processing and inversion capabilities as well as interpretation workflows that take the complexity of the Earth into account. In this paper, we share some of our motivations behind our approach and illustrate its effectiveness with both real and synthetic data examples.

Multiple surveys using a Full Tensor Magnetic Gradient (FTGM) signal instrument from IPHT, have been made in Southern Africa. Traditional Fourier domain and minimum least squares residual of the linear differential tensor relationships have been adapted. The result is a full tensor grid representation of the curvature gradients that is coherent and compliant with the physics at all points. Superior anomaly interpretation regarding the full magnetic history and inferences can then be made. There is more directly inferable structural geology in this tensor signal than can be found in a conventional TMI signal.

Magnetic measurements most often correspond to the measurement of the magnitude of the earth’s magnetic field while the magnetic field is a vector. Assuming that the magnetic anomaly has low amplitude, the theory shows that it is possible to calculate the vector field. But this assumes to know the earth’s magnetic field direction and, for some applications, that of the magnetization. These assumptions can be severe limitations to interpretations. It is very likely that inertial measurement units will be available soon and combined to three-component fluxgate magnetometers, measurement of the earth’s magnetic vector will be possible with sufficient accuracy. Superconducting quantum interference devices also need to be combined with inertial measurement units. They allow the measurement of the nine gradient components of the total magnetic field with an excellent accuracy. But difficulties of implementation must be overcome so that their use becomes general.

Although the theory involved is well-understood, practical real-world computation of terrain corrections for airborne gravimetry and gradiometry (AGG) surveys poses challenges in several areas, which affect not only the processing of the data, but also the survey operation itself. This paper suggests answers to several of these challenges, accompanied by computational results on realistic simulated surveys.

This paper describes the development of a magnetic tensorial gradiometer system and its capabilities with emphasis on the design, testing and operation. The system has been constructed by employing two independent cubic coil sensors at a distance of 50cm. Each magnetometer is supplied with six ring core metallic glass ribbon (metglass). The frame of the the ring core was made in a glass epoxy resin. The low temperature expansion of glass resin epoxy is about 14ppm/°C. This lowering the temperature
drift of the sensitivity and the offset drift. Thanks to the accuracy in the mechanical construction of the cubic head (0.05 mm) the non orthogonality of the three axis is better than 0.1°. Both magnetometers are calibrated separately in a three axes coil system. The calibration system consists of three sets of two square coils with diameter of 50 cm It can produce a uniform field with an error less than 10 ppm in a volume of diameter of about 5 cm at the center of the system. The gradiometer prototype consists of five components: an analogue electronics unit; the data processing unit and mass memory, two triaxial fluxgate magnetometer sensors and an inertial platform.

Major parts of Mozambique were flown a few years ago to acquire high resolution magnetic and radiometric data. The National Geology Directorate of Mozambique intends to interpret these data generating value-added maps that are easier to handle by the exploration and mining industries than mere airborne geophysical data and maps. The National Geology Directorate of Mozambique and the Council for Geoscience have joined to conduct an example study case in the Alto Ligonha pegmatite fields, northern Mozambique. A valueadded map of the area was generated using crisp exploratory K-means clustering. Subsequent aposterior discriminant analysis served to establish the trustworthiness of the classification. The map is the result of grouping 850,000 four-element samples (Th- and K-surface concentrations, apparent magnetic susceptibility and the first vertical magnetic gradient) into a number of classes. These were associated with the major geology units known from the study area. Focus has been on amphibolitic magnetic gneisses which host mineralized pegmatites in many cases. The area covered by this kind of gneiss is clearly enhanced by our classification map expanding the area with increased potential of mineralised pegmatites.
These results were confirmed on site. Ground truth control served to verify the role of amphibolites as geophysical marker, inspect the areas newly classified as having increased potential of mineralized pegmatites and try to locate occurrences not contained by the records of the Mozambique Geology Directorate. The automated integration of airborne geophysical data using well known K-means algorithm proved to be a fast, objective and effective tool to generate a value-added integrated map. The experience made in the Alto Ligonha pegmatite fields encourages the adoption of this methodology over other parts of the Mozambique Fold Belt.

The eigenvectors of the symmetric gravity gradient tensor can be used to estimate the position of the source body as well as its strike direction. For a given measurement point, the eigenvector corresponding to the maximum eigenvalue points approximately towards the center of mass of the causative body. For a collection of measurement points a robust least squares procedure is used to estimate the source point as that point which has the smallest sum of square distances to the lines defined by the eigenvectors and the measurement positions. The strike direction of the source can be estimated from the direction of the eigenvectors corresponding to the smallest eigenvalue for quasi 2D structures. Assuming that the magnetization direction is known, the pseudo gravity gradient tensor derived from magnetic field data has the same properties.

This paper demonstrates the value of co-interpretation of seismic with potential field data in the evaluation of exploration acreage. Specifically, the seismic interpretation has been complemented by integration of gravity and magnetic data with the primary objective of assessing depth to basement and to identify the presence and extent of critical lithologies, such as carbonates. The regional gravity and magnetic data over the area of interest provides an updated geological model and the gravity effect of the thickness variation of the carbonates has been assessed. Depth to basement interpretation has been carried out along 2D seismic lines. An integrated depth to basement map has been created by 3D inversion of the available magnetic data in close cooperation with the seismic interpreter.