Comparison of a decade of examples of AEM vs Drilling results for Groundwater Exploration: The Good, the Bad and the Ugly

Abstract:

As AEM has gained more acceptance and use in groundwater exploration a library of examples of the success and failures of using AEM to locate and predict targets for groundwater exploration has been amassed. Many government, commercial, and published examples show the successes. What about the failures? What about the near misses? A successful groundwater exploration well is a well with acceptable water quality with acceptable yield and sustainability. In many situations this translates to water of 750 or less TDS and yields on the order of 500 to 2,000 gpm. However, in some regions water for oil field fracking operations is the goal with water quality between 2,500-4,000 TDS and yields greater than 50 gpm. The obvious key to AEM’s ability to be used in groundwater exploration is the electrical resistivity contrasts between the areas of aquifers with the desirable aquifer characteristics and the areas of non-desirable aquifer characteristics or aquicludes. Investigation depth, hydrogeological layer geometry, and required resolution all impact the ability of AEM to successfully image the hydrogeological structure. An additional complication in the use of AEM is the logistical limitations of gathering AEM data around and near anthropogenic features (i.e. industrial areas, cities, pipelines, electrical power grids, highways, and confined animal feeding operations). Best practices typically include the gathering of downhole geophysical, lithological, and hydrogeological data, and performing forward modeling to examine the ability of the one of the many AEM systems to provide a useful image of the targeted aquifer. This is limited by assumptions of the geological layering, the electrical properties, the parameters of the AEM system, and the electrical noise levels. The type of forward and inverse model used can also impact the assessment. Moreover, after all this planning, the goal is still finding groundwater within the required specifications. Several examples are presented showing the comparison of AEM results with targeted drilling and the actual results of drilling and aquifer testing. As with all exploration of the geological environment, there are Good, Bad, and Ugly examples. By examining the examples, a better understanding of the limitations and successful application of AEM can be achieved.