Interpretation of Gravity and Magnetic Data from Namibia

Report of the project:

• Namibia - Human Resources Development for the Geological Survey of Namibia, Engineering & Environmental Geology Subdivision
• Namibia – Beratung des Geologischen Dienstes (Phase II)

Gravity Filed Work in Namibia Source: BGR

Within the framework of Technical Cooperation with Namibia "assistance to the Geophysics Division of the Geological Survey GSN and training of personnel" was provided in order to strengthen the capability of the geophysical and remote sensing units and to stimulate the interdisciplinary cooperation within the GSN.

Location Map Source: BGR

In particular counterpart personnel was trained and qualified in methods of geophysical exploration, especially in the area of gravity.

As an example of such project activities the gravity survey performed over a pronounced magnetic anomaly is presented here. The target area was the Grootfontein Mafic Body GMB, located in the North Eastern part of the country. It excels as a pronounced, approx. 60 km long geophysical anomaly. Because of its supposed mineral potential too it continues to be an attractive target.

The magnetic anomaly of the GMB with strong variations of over 2000 nT shows a clearly developed maximum to the NW and corresponding weaker minimum to the SE. The anomaly was continuated upward by 5 km and reduced to the pole. A clear separation into two parts becomes well visible; the western part is more prominent and stronger in amplitude. The outlines of the geologically mapped mafic body drawn in coincide well mainly with this western part.

Original aeromagnetic data: 3d presentation viewed from the SW. Source: BGR

Magnetic anomaly continued upward by 5 km and reduced to the pole Source: BGR

A gravity survey was performed in order to obtain additional information on the structure and physical parameters of the GMB. A total of 291 new gravity points within the area under consideration could be added to the existing country wide data base. The distribution of these new points and the Bouguer anomaly calculated are drawn down left, as well as the body’s outlines (purple polygon line). Gravity varies by approx. 35 mGal within the survey area.

Bouguer anomaly Source: BGR

A direct concordance of the GMB and an expected gravity high does not exist. A strong positive anomaly shows up in the NE of the mapped area; the values decrease then rapidly to the south. This decrease is definitely caused by the thickening of the Kalahari sediments which are outcropping there. The mapped mafic body seems to be located below the strongest horizontal gravity gradient.

Gravity and magnetics along profile 1 were interpreted jointly using a 2D model (below). Two separated anomalous bodies (purple) were assumed of which the northern (left) one only possesses a stronger susceptibility S. It is the main cause of the magnetic anomaly observed. Both bodies have somehow increased densities D compared to the conventional density of crustal material (green), however, the main effect in gravity is the decrease by approx. 20 mGal towards the south (right) due to the thickening of the less heavy sediments (yellow).

Grootfontein Profile 1 Source: BGR