Survey Design for Acquisition of Airborne EM
The Place
Butte and Glenn Counties, located in the northern part of the Central Valley, California.
The Problem
The airborne electromagnetic (AEM) method provides a cost-effective approach for mapping out the resistivity structure of the subsurface over a large area. However, without co-located well data providing direct information about hydrology, it is very difficult to interpret the resulting resistivity structure from the AEM data. Therefore, designing an AEM survey in a way that can obtain co-located well data (e.g., within 100 m separation distance from AEM flight lines) is critical. A major roadblock to flying an AEM system over the wells is a noise source due to man-made objects (e.g., powerline, steel well casing).
Our Approach
We designed an AEM survey to fly over the available wells in the study area. This resulted in 41 water wells located within 100 m of the AEM flight lines, which provided a unique opportunity to investigate the effect of water wells on AEM data. We designed systematic numerical analyses and examinations of the field data.
Results
We found that the only noise source related to the wells that affects the AEM signals in our study area was the power lines; the effect of the steel casings in the study area was negligible. From the numerical simulations, we found that the effect of the powerline will be observed in the AEM data until there is a separation distance of ~170 m from the power line. However, we found that, in the field data, an average separation distance between the powerline and high-quality AEM data (i.e., no impact from the power lines) is only 60% of this predicted distance. We attributed this to the high ground contact resistance caused by the degradation of the grounded electrodes located at the bottom of the power-line poles. For our 41 wells, after removing any soundings contaminated by powerline noise, we were left with high-quality soundings located 0-160 m from the water wells, with an average distance of 77 m.
Project Sponsors
Stanford Woods Institute for the Environment, The Gordon and Betty Moore Foundation
Project Publications and Presentations
Kang, S., Dewar, N., & Knight, R. (2020). The effect of power lines on time-domain airborne electromagnetic data. GEOPHYSICS, 86(2), E123–E141. https://doi.org/10.1190/geo2020-0089.1