From !khure

Project B5 : The tectonic framework of Southern Africa interpreted from gravity and aeromagnetic data

French pi: A. Galdeano (with J.L. Le Mouël)
South African pi: M. Doucouré


Project Participants

  • South Africa: Rodger Hart, Susan Webb, Moctar Doucouré
  • France: Armand Galdeano, Luis Gaya-Piqué, Jean-Louis Le Mouël, Erwan Thébault
  • Other: Stuart Gilder (Munich), Valentin Mikhailov (Moscow)

Aims and objectives

  1. . To identify the major terrain boundaries (edges) including the limits of effects large impacts in Southern Africa.
  2. . To identify and understand the relationship between felsic (crustal) and mafic (dominantly mantle) rocks in the Earth‟s crust.
  3. . To apply magnetic and gravity imaging techniques to coherently map out the major structures in Southern Africa.
  4. . To Understand the relationship between major sedimentary basins and depth to MOHO
  5. . To understand the horizontal and vertical distribution of Cretaceous age magmatic/volcanic features.
  6. . To provide MSc training for at least one black South African student (still to be identified).


An understanding of the development of both crustal magnetization and gravity features in the crust is essential in interpreting continental scale terrain boundaries which manifest themselves either as major magnetic or gravity anomalies. In order to determine the major gravity and magnetic features in the upper 30 km of Southern Africa, we intend to apply a number of transformations e.g. block leveling and low pass filtering of the gravity and aeromagnetic data of Southern Africa. These crustal features include kimberlites resulting from magmatic events. The distribution of kimberlites will be analysed through anisotropy associated with magnetization directions.

Aeromagnetic Data Set

Aeromagnetic surveys over Southern Africa were flown by the Geological survey of South Africa during the period from 1966 to 1981 [1]. Total field data were collected along north–south flight lines at a nominal terrain clearance of 150 m, flying at 240 km/h with a sampling interval of two seconds. Flight line spacing was 1 km and perpendicular tie-lines were flown every 10 km. In order to facilitate digital transformation, the data were flight line leveled and block leveled, thus establishing a common datum for all aeromagnetic surveys across South Africa. The interpolation of the aeromagnetic map was done with a grid lattice of 250 x 250 m (fig). It should be noted that the post-acquisition treatment of the data has not been published and is unknown.
Because of the way the magnetic data set was collected and leveled, the longest wavelength anomalies do not match up across the subcontinent. We want to develop a procedure to do large scale block corrections in order to obtain unity in the data across the subcontinent. One possible way to achieve unity across the different survey areas is to compare aeromagnetic data with satellite data by upward continuing the aeromagnetic data.
Finally once the data is corrected we plan to do a interpretative study that will include structures and geometry definition, source depth clustering and separation of crustal/mantle features.

Bouguer Gravity Data Set

The gravity data were collected from a number of regional and detailed surveys conducted by the University of the Witwatersrand, the Geological Survey of South Africa, the institute of Geological Sciences of Great Britain and various mining and exploration companies. Most of the data were gathered along roads with maximum station intervals of approximately 3 km. The majority of station elevations were determined barometrically and the measurements reduced using the Geodetic reference system 1967 formula. A total of 13500 stations were finally accepted for the compiled data set with an average error of ~1,62 mgal (Wilsher 1987). The data set were gridded at 100m intervals and the UTM projection utilized.

The gravity data will be used to identify large (>100 km) crustal anomalies (dense rocks).
To do this we have to devise techniques to effectively remove topographic and mantle contributions to the gravity field.

One example of the processing we can perform across Southern Africa is to correlate the gravity map and the pseudo-gravimetric map (derived from the magnetic map). This can be computed in a moving window of few kilometers. The resulting map will give us the distribution of the coherence (or the “anti-coherence”) between the 2 anomaly fields (gravity and magnetic) which in turn will provide information about the nature of responsible sources.

Mode of co-operation between the French and South African research teams

The project will involve bilateral travel between France and South Africa for both research teams, including post-graduate students, for the exchanges of ideas, the interpretation and publication of results. Both South African and French research teams will serve as supervisors of the post-graduate students. Tangible items to be shared between France and South African research teams, to the benefit of both countries, will include digital data (including databases), computer programs and other software, maps, and written works including scientific publications, reports and theses.

Capitalizing on other geophysical studies

  1. . Magnetotelluric data have recently been collected in the region that have the potential to image to upper mantle depths.
  2. . Deep (16 sec) seismic data of the region collected during the Kaapvaal seismic program is also available.
  3. . Recent basic 3D model of the Wits basin has been developed in gOcad which can be used in modeling programs to test ideas.


  • B. Corner and W.A. Wilsher, Structure of the Witwatersrand basin Derived from interpretation of Aeromagnetic and Gravity Data. In: Exploration ‟87. Geol. Surv. Can. Spec. 3 (1989) 523-546.
  • D. Gibert & A. Galdeano, A computer program to perform transformations of gravimetric and aeromagnetic surveys, Comp. & Geosciences, 11, 553-588