Michael, as Fidelis said, ...
Published by Andrew McKenzie, Groundwater Information Specialist at British Geological Survey
Michael, as Fidelis said, the starting point is understanding the geological environment, and whether you are seeking water in an extensive aquifer with intergranular flow, or in a hard rock, fracture dominated, environment. In either case actual observations in existing wells and boreholes, and identifying any flowing springs or baseflow supplied rivers. should be the first stage of an investigation. In many cases if there are enough existing data points you can extrapolate a water table with reasonable confidence and also gain understanding of the effects of existing abstractions on the aquifer. In environments without too much abstraction a good conceptual model of groundwater flow, coupled with estimates of recharge and aquifer properties, can let you 'model' likely water tables with remarkable accuracy.
If there aren't nearby data points you can consider geophysical methods - Vertical electrical soundings (resistivity or conductivity) can 'see' water tables in intergranular aquifers, but accurate interpretation of the results requires a good knowledge of the geology and water quality, which brings one back to needing existing data points. Resistivity is probably the most widely used geophysical method, partly because it is relatively cheap. More expensive options include seismic methods and ground penetrating radar. In the latter case unless the area of interest is very arid, or the radar is very powerful, it is limited to relatively shallow depths.
2 Comments
Precisely the answer that I would have given - thanks for saving me the time Andrew (glad I read the comments before answering!)
Published by Jonathan Abra, Knowledge Transfer Manager, The Knowledge Transfer Network
Thank you Mr Andrew McKenzie for your detailed contribution.
Yes, water table in arid zones are not shallow. Can you shed some light on the seismic method to map underground water table? Resistivity method is comparative. What other approaches can be used if there is no prevailing geological profile for a location?
Published by Michael Adeyemi
1 Comment
The water table can act as a seismic reflector, so a shallow seismic reflection survey can be used, but there are a number of limitations to the method - it needs the water table to lie within a relatively homogeneous stratigraphy, otherwise lithological reflections will make interpretation difficult. Seismic reflection surveys are expensive - in equipment, in data acquisition and in interpretation, and imaging with seismic reflection at depths of < 30 metres gets progressively harder. So it can be done, but whether it's economically worth doing is questionable.
I think I would sum up as saying that there isn't a single ideal non-invasive method to determine depth to the water table, all (?) geophysical methods require interpretation within a geological context. It's fairly rare for there not to be some data from a borehole or well in a similar geological and topographic context, even if not in the same location, and it's probably more valuable to make a real effort to find pre-existing data or survey existing water points than to conduct speculative geophysical surveys without geological context. If it truly is an 'unexplored' area, at some early stage it will be necessary to drill an exploration borehole to prove the lithological sequence, aquifer properties and water quality At which point you have the opportunity to design and calibrate an appropriate geophysical survey.
Published by Andrew McKenzie, Groundwater Information Specialist at British Geological Survey