Geoelectric Investigation of a Waste Disposal Site for the City of Blantyre, Malawi

Report of the project:

• Malawi - Support of the Geological Survey in environmental geology
• Malawi - Unterstützung des Geologischen Dienstes bei umweltgeologischen Aufgaben

Field party Source: BGR

Within the frame of a technical cooperation between the Republic of Malawi and the Federal Republic of Germany a site investigation for an environmentally friendly waste deposit for the city of Blantyre was implemented in 2003. Partners of this cooperation are the Geological Survey Department (GSD) of Malawi and the Federal Institute for Geosciences and Natural Resources (BGR) of Germany.

At a pre-selected potential site at Chigumula, approx. 15 km SSE of the city centre of Blantyre, we contributed a geophysical survey. The objective was to investigate the inhomogeneous weathered surface layers over the gneissic basement. An appropriate site should have a thick impermeable soil cover of more than 5 m.

The geoelectric method seemed to us the most promising. So we performed multi-electrode measurements and a few vertical soundings.

Setup in the Field Source: BGR

We used the equipment IRIS/SYSCAL R2, which allows the setup of 64 electrodes simultaneously. These are addressed by so-called intelligent nodes and the measuring procedure is automatically executed by a computer with the help of a pre-prepared sequence file.

The survey was performed along four SW-NE profiles, 100m apart of each other, and one perpendicularly to them.

We experimented with the dipole-dipole configuration and applied then routinely the Wenner array with a dipole length a of 5 and 2 m. (A and B are current electrodes and M and N potential electrodes.)

An example of field results Source: BGR

An example of field results is shown above. The section was measured using the Wenner configuration with a dipole length a of 5 m and a separation factor n of up to 8. The second panel below displays a model of true resistivities of the subsurface. It was calculated by inverting the data measured, using the program RESIXIP2DI© of INTERPEX.

Depth Slices of Resistivity Source: BGR

The layering already visible in the field data becomes much clearer in the calculated model, and the resistivity contrasts are more pronounced than in the measured data. Below the high resistive ( > 500 Ω*m) top layer only a few meters thick, a sharp drop in resistivity to less than 100 Ω*m is conspicuous. bviously this is the highly weathered moist gneiss known in the area. Below that the fresh basement crops up with again high resistivities.

3D presentation of the most conductive zones Source: BGR

The interpreted models of the five profiles can well be displayed as horizontal cuts. The distance of 3.3m between them is caused by the grid width of the model used. The highly resistive overburden has almost disappeared in the second slice and down to 15 or 20m depth the weathered low resistive layer is well visible (red and magenta colours) then. At one spot we see a hole of profound weathering, and at others the fresh gneiss is bulging up to the surface. These areas are inappropriate for the waste deposit planed. Another possibility of visualising the results is the 3d presentation of the most conductive zones shown to the right. The bodies defined by resistivities below 100 Ω*m are coloured in magenta and the zones below 50 Ω*m in blue.