Interpretations of gravity and magnetic data are non-unique, meaning that sometimes a number of different geological models can fit the observed data. With constraints from geological and other geophysical data types (e.g. seismic and well data), the interpretations can provide critical insights into the geometry of the subsurface (e.g. depth-to-basement).
Magnetic anomalies are mainly generated by variations in the crystalline basement and igneous material. Most sediments can be thought of as being magnetically ‘transparent’, i.e. insignificant susceptibility. Thus, magnetic data interpretation can be a useful way to investigate the deepest and often the most fundamental structure of a basin. Gravity anomalies originate from any subsurface density contrast, .e.g. intrusions, faults, basin boundaries, salt diapirs, etc.
We have over 15 years of expertise in undertaking interpretations of gravity and magnetic data, and, more recently, FTG (or gravity gradiometer) surveys over the following regions:
- Kwanza Basin, Angola
- Coastal Benin
- Papua basin, Papua New Guinea
- Offshore Tanzania/Kenya
- Red Sea Margin, Saudi Arabia
- Offshore Liberia
- Makassar Straits, Indonesia
- Offshore South Africa
- Ogaden Basin, Ethiopia
- Sub-Andean basins of South America
- Offshore Faeroes/Shetland
- Offshore Brazil
- Nevada, USA
- Cenozoic rifts, Tanzania
- Muglad Basin, Sudan
- Offshore Vietnam
- Lower 48, USA
Derivatives and transforms of gravity and magnetic data (e.g. filtering, reduction to the pole of magnetic data and tilt derivative), all based upon mathematically-defined algorithms, help highlight significant and sometimes subtle features within the datasets. A suite of carefully chosen derivatives can shed light upon the following:
- Structural trends
- Fault kinematics
- Crustal architecture
- Regional tectonics
- Sedimentary lithology
- Igneous bodies
2D and 3D profile modelling, magnetic depth estimation and 3D inversions, help revise the qualitative interpretation, test plate tectonics models and define basement depths and other significant subsurface horizons.
One of the most frequently requested pieces of work we are asked to do is to generate a depth-to-basement map of a basin or sub-basin. These are important for the New Ventures explorationist, since they help to identify the extents of potentially productive sedimentary basins. They will provide a vital constraint for source rock maturity modelling and for understanding the local thermal regime in both conventional and unconventional exploration settings. Our staff have been at the forefront of developing new techniques, with numerous publications and conference presentations on the subject.
At a global scale, we have embarked on the Global Depth-to-Basement Study, utilising our global gravity and magnetic datasets and supporting geological, well and seismic control.
At a basin scale, the workflow for any specific study will be modified depending on the quality and extent of the datasets available and the nature of the problem to be solved.
A typical interpretation will involve the following:
- Specialist processing of the gravity and magnetic data, such as tilt derivative, pseudo-depth slicing, reduction to pole of magnetic data, and lineament enhancements.
- Review of the geological basin setting
- Qualitative interpretation
- Estimations of depth to magnetic basement, such as spectral analysis, tilt-depth, 3D Euler Deconvolution and local wavenumber.
- 2D profile modelling
- 3D inversion of gravity and/or pseudo-gravity data
- Basement depth map