What methods do you use to site a borehole?

Our surveyors use horizontal resistivity profiling (HRP) and horizontal loop electromagnetic (HLEM) traverses for the identification of geological anomalies. Once promising hydro-geological targets have been identified, each target is further evaluated using vertical electrical soundings (VES) to give some indication of the suitability of the target for drilling.

Methods are chosen as appropriate for the hydro-geological setting -- in crystalline basement settings, HRP's are usually more suitable, whereas in sedimentary settings, HLEM is usually more suitable.

How much will a geophysical survey cost?

Our charges depend upon the distance travelled to the locality and the amount of work carried out. In restricted localities, such as an urban domestic stand, we charge US$200 for a single survey whereas in unrestricted rural localities, multiple surveys may be necessary.

Item US$/Item
Transport and travel -- paved road 0.75/km
Transport and travel -- dirt road 1.00/km
Survey -- urban domestic 200.00/stand
Survey -- urban commercial/rural single 300.00/survey
Survey -- rural multiple sureys 500.00/day
Establishment and administration fee 150.00

Please contact us -- a more accurate quotation will be provided once we have details of the setting and once the distance from Bulawayo to your location has been determined.

Do you guarantee water?

Despite claims to the contrary, the only current method of directly 'finding' water uses magnetic resonance imaging (MRI.) Whilst MRI is now fairly commonplace in the medical field, instruments exploiting this phenomenon in the geophysical field are experimental and expensive.

The geophysical methods used for the siting of boreholes exploit the varying electrical properties of the rocks and soils in the vicinity of the proposed well point. Dry undecomposed igneous rock, such as granite, is effectively an insulator (much like a ceramic) whilst moist clay is a fairly good conductor.

Lateral anomalies in the underlying terrain are identified from the changing values of these properties and from the changing depth of the decomposed and fractured overburden.

Once a promising anomaly has been identified, vertical changes in these values may be used to resolve the thicknesses of the various strata through which the borehole will be drilled. Once this geological profile has been constructed, an estimate may be made of whether a borehole drilled at that point will satisfy your expectations.



Why are some boreholes dry?

A borehole may be thought of in terms of a leaky pipe -- water will only ingress into the borehole when various conditions are met:

  • A reservoir of water from which to feed the borehole -- this occurs only where the overlying rock is sufficiently porous to accomodate water. Porosity may occur as a primary feature in sedimentary rock, or may develop from fracturing and subsequent decomposition of igneous and metamorphic rock.
  • A path for the water from the reservoir to reach the borehole -- this occurs only where fissures or porosity of the rock allow water to flow freely. The presence of clay or talc may impede the flow of water, in which case yields will be reduced.
  • A way for the water to enter the borehole -- the best 'leaks' are fissures or 'breaks' in the rock through which the borehole passes. These breaks must be present below the standing water level (SWL) -- the greater the depth of the break below the SWL, the greater the pressure of the water, and the greater the yield.

A dry borehole thus occurs when dry consolidated basement rock is encountered before these conditions are met.

What is a resistivity profile?

Resistivity profiles may be abstracted from data taken either directly from resistivity traverses (HRP) or from electromagnetic data taken from horizontal loop traverses (HLEM.) Software is used to invert the apparent resistivies to create cross sectional models of the terrain over which the survey passed.

Anomalies in these cross sectional profiles are then identified and promising targets are selected for further investigation.

What is a vertical electrical sounding (VES)?

An electrical sounding is conducted over all promising targets to evaluate the apparent resistivity of the rock strata through which the borehole will pass. The depth of investigation is increased by increasing the distance between electrodes -- these must be placed approximately twice as far apart as the depth of investigation. Since the depth of a typical borehole in a crystalline basement area is 50-60m -- this requires electrode spacings in excess of 100m (which often renders survey in restricted urban localities difficult.)

Once a reliable set of data has been collected -- an inversion of the data using suitable software creates a model of the underlying rock strata based upon changes in the apparent bulk resistivity at different depths of investigation.

Where have you conducted successful surveys?

We have, over the years, been contracted to conduct surveys and design drilling programs in many areas of Zimbabwe -- this satellite image of Zimbabwe has been marked with most of the districts to which we have been called.

Do I need permission before sinking a borehole?

The situation is a little confusing with respect the sinking of a borehole for primary purposes and abstraction of water thereof. For any other purpose, a permit is required.

For details pertaining to your situation, I can do no better than to refer you to this blog post: