Aeromagnetic, airborne electromagnetic
surveys and Transient Electromagnetic Soundings were carried out in the Lower Okavango
Delta in northwestern Botswana to evaluate the groundwater potential of a 12,500 square
kilometers area. The area is covered by thick sediments of the Kalahari Beds and underlain
by various bedrock types including the Karoo Supergroup of Carboniferous to Jurassic Age
and Damara Supergroup of Palaeozoic to Proterozoic Age. Kalahari Beds sediments and
Bodibeng sandstone of Karoo Supergroup were the prime targets for exploration.
The geophysical investigations were carried
out in two phases during 1995-1997. The Phase 1 investigations concentrated on evaluating
the regional geology and structure. This was done by interpreting aeromagnetic data
(70,447 line Kilometers) along with deep TEM soundings. Phase I also included testing the
applicability of airborne and ground Transient Electromagnetic (TEM) methods for
evaluating the ground water potential of two distinct targets i.e. shallow unconsolidated
sediments and the deep underlying bedrock.
The airborne magnetic survey provided
information on sediment thickness, depth to the top of the magnetic basement, location of
major faults, dykes and sills. This information along with data from existing deep
boreholes, all of which were located outside the study area, was used to develop a
geological model for the area. This model defined the spatial and vertical distribution of
Kalahari Beds and various bedrock types that could be postulated to occur below the
sediments. The interpretation also suggested a complex geological and structural
environment of the area. Lack of borehole data within the project area constrained the
aeromagnetic interpretation model.
A test airborne electromagnetic (AEM) survey
over the existing Shashe wellfield along with ground TEM soundings were completed and
showed that AEM clearly defined the spatial distribution of fresh water within the
sediments and ground TEM precisely defined the vertical extent of fresh water. It was thus
recommended that a regional AEM survey be conducted over the western part of the area
(15.871 line kilometers that is underlain by thick Kalahari sediments and receives active
recharge from the delta outlet distributaries. Phase 1 surveys concluded that the regional
AEM interpretation in conjunction with ground TEM soundings are the most pertinent methods
for exploring groundwater resources of the Kalahari Beds. For bedrock exploration,
aeromagnetic interpretation in conjunction with TEM soundings was used.
Phase 2 commenced with incorporating the
results of the AEM survey in a reassessment of the eight exploration areas that were
previously delineated on the basis of TM imagery interpretation hydrogeological data and the flow characteristics of the delta
distributaries. Using the AEM data the eight exploration areas were redefined into six
areas; four of the previously defined areas were retained, one was discarded, one was
reconfigured and a new area was added. These six areas were demarcated for ground
geophysics. In each area TEM soundings were conducted and a series of geoelectric sections
were drawn to infer the lateral and vertical extent of sediments containing fresh water.
The maximum thickness of the fresh water within these sediments was interpreted to be
close to the river channels and the thickness decreased towards the edges of channels /
valleys. The thickness of fresh water in the sediments was interpreted to range from 50 to
140 meters and the resistivity of these sediments ranged from 10 to 70 ohm - meters.
The drilling results confirmed the interpretation of AEM and ground
TEM soundings. TEM soundings enabled the delineation of the lithologies within the
Kalahari Beds (as units of sand, clay) and as well as characterizing the water quality in
these units in terms of fresh, brackish and saline water. The method was proven to be
capable of resolving the thickness of fresh water sediments within 5 meters at depths to
100 meters, or to an accuracy of 95%. The resistivity of sediments to a depth of about 100
meters (ignoring the top 5- 10 meter laver) decreases with depth from 70 ohm - meters in
the shallow aquifers to 10 ohm - meters in the middle aquifers. Less than 5 ohm - meters
resistivity was interpreted for the lower saline aquifers. The change in formation
resistivity with depth is attributed to increased water salinity with depth, as a high
degree of correlation was ascertained between water salinity and formation resistivity.
In the investigation for bedrock aquifers, the
primary target was the Bodibeng Sandstone Formation. Two areas were selected for locating
such targets. One in the northeast of the area where aeromagnetic interpretation suggested
that basalt could occur below Kalahari Beds and it was assumed that the Karoo Age Bodibeng
Sandstone would be underlying Stormberg Basalt. The second was between Thamalakane and
Kunyere Faults where previous drilling had indicated the possibility of encountering water
of relatively low TDS in Bodibeng Sandstone below a zone having high TDS water.
Interpretations of magnetic data and deep TEM soundings were used to locate a deep drill
The first deep exploration borehole was completed to
a depth of 980m in the northeastern part of the project area. Beneath 720m of basalt the
Proterozoic Meta-arkose Ghanzi Group was encountered indicating that the Karoo Age
sediments were not deposited in this area. The second deep borehole drilled to a total
depth of 247 meters encountered Bodibeng sandstone below 154 meters of Kalahari Beds. All
water strikes in both boreholes were saline indicating that prospects of fresh water in
the bedrock were poor.
The results of this exploration program have
demonstrated the usefulness of the aeromagnetic method for mapping the bedrock structure
and the airborne EM method for determining water quality in the overlying sediments in a
large area. The airborne EM provided the conductance variations within the Kalahai Beds
that were related to fresh and saline water sediments. The regional coverage by airborne
EM provided the delineation of the most promising areas for ground geophysics and ensured
that no potential area was omitted. The TEM sounding method very precisely demarcated the
zones bearing fresh, saline and brackish water in the six exploration areas. It was also
better able to decipher the vertical distribution of resistivity than the airborne EM
method. Drilling pumping tests and modelling results have proved large reserves of fresh
water in these exploration areas. Potential wellfield areas have been delineated for
In conclusion the Airborne EM survey provided an
effective geophysical technique for delineating the lateral extent of shallow aquifers.
Using this data in conjunction with ground TEM soundings accurately defined the lateral
and vertical extent of fresh groundwater. The results provided the basis for definition of
aquifer geometry and water quality for quantification of resources. Aeromagnetic data and
deep TEM soundings were used to better understand the bedrock. For additional
understanding of the bedrock, exploration methods such as seismic reflection and
magnetotellurics are recommended.