Step 1: Engage with the local agency
There can be various starting points in an AEM project. Two examples:
- The agency has limited understanding of the area and is considering the use of the AEM method to conduct a renaissance survey to understand the large-scale structure of an area. This typically involves AEM flight lines spaced 500 m to 5 km apart, with selected areas for denser lines to be flown at a later date. The line separation is determined using existing knowledge of the subsurface hydrogeology. As the line spacing increases, so does the possibility that the interpretation will be more challenging and more uncertain.
- The agency has an existing conceptual model and/or groundwater model but is considering the use of the AEM method to obtain additional information to build and refine improved models. This typically involves dense AEM flight lines spaced 100 m to 500 m apart. The line separation is determined using existing knowledge of the subsurface hydrogeology.
The first step in the workflow is to determine how/if AEM data can assist with groundwater management. As examples of the types of questions that can be addressed using AEM data, we have provided links to the questions defined for Butte and Glenn Counties, for Indian Wells Valley, and for San Luis Obispo. While an initial set of questions were posed by the agencies, what you see here are the questions that were identified after considerable back and forth between the agency and the geophysicists, sometimes involving modeling steps later in the workflow. This iterative process ensures that there is a clear understanding of the information that can be derived from the AEM data.
At the start of the iterative process, a feasibility study is conducted. This study is designed to determine whether the predicted resolution and depth of investigation of the AEM method, in the study area, is sufficient to provide useful information. The data typically used for this feasibility study include geologic cross-sections, lithology logs and electrical logs. It is important to note that a lithologic model, displaying sediment type/texture and rock type, is the product that can typically be directly derived from the AEM data alone. Integration with other data sources is needed to obtain a model of hydrostratigraphy.
Engagement with the local agency continues throughout the project, as incorporating local knowledge at all steps improves the quality of what is done and ensures that the final product supports the agency’s groundwater management needs.
Agencies often want to explain the purpose of the AEM survey to residents of the area and to their technical advisory groups. The linked power point presentation can be used for that purpose.