(Non-technical.  Click link for a technical webpage on aquifer exploration probability.)

By Darrel Dunn Ph.D., PG, Hydrogeologist.  

(View Résumé 🔳)


Water resources management planning often involves estimating how many test wells would be required in an aquifer to complete at least one successful test well.   Test wells are used to search for sources for water via exploration drilling.  Estimates of the number of test wells may be needed for budgeting.  The accuracy of such groundwater exploration cost estimates may be improved by treating the uncertainty mathematically by making use of statistics and probability theory.  If enough data is available from previous testing in the aquifer, exceedance probability might be used.  Exceedance probability is the probability that one value obtained at random from a large distribution of values will equal or exceed a certain value.   In aquifer exploration (groundwater exploration), the value of interest might be hydraulic conductivity, which is a measure of the ability of an aquifer to transmit water to wells (similar to permeability).  A hydrogeologist can use groundwater modeling (including relatively simple calculations) to estimate what the critical value of hydraulic conductivity would be in a successful test well (or the subsequent production well).  Once this critical value is obtained, existing data on hydraulic conductivity from previous testing of the aquifer may be used to estimate the number of test wells that would be required to find one or more sites with adequate hydraulic conductivity.  The mathematical technique used for this estimate is described in the associated technical version on this topic.   Basically the more rare (higher) the critical value and the less uncertainty one is willing to tolerate, the greater the number of test wells one should be prepared to construct.

The mathematical technique assumes that the testing is random.  One can increase the chance of success and reduce cost if test wells can be placed at geologically advantageous locations.  Thus it is beneficial if the hydrogeologist performing and/or supervising the investigation has a broad background in geology, groundwater hydrology, geophysical methods, and advanced mathematics.

The concept of exceedance probability is widely used in flood hazard reporting (100-year flood, et cetera).   It is also potentially useful in petroleum exploration by the oil and gas industry and mineral exploration by the mining industry (how many test holes should company be prepared to drill).

Posted:  May 2013