Frequency selection method and resistivity measurements for understanding groundwater in arid regions: Case study of the El Carrizal aquifer, Baja California Sur, Mexico

The evaluation of small aquifers in arid regions, like those located in Baja California Sur, Mexico, poses challenges due to limited data on bedrock and inadequate depth coverage from traditional geophysical methods. In many of these regions, there is a lack of deep-drilling data related to bedrock, which could be utilized for interpreting the aquifer's geometry. Additionally, geophysical electric methods such as vertical electric soundings (VES) do not provide adequate depth coverage. The method measuring the potential difference using the Frequency Selector Method (FSM) detector holds great potential in determining the aquifer geometry, however, caution should be taken when we want to analyze the results if the method is not completely understood. The FSM method relies on an instrument that independently selects low audible-range frequencies, plotting them one by one in a horizontal graphical form. This produces a rapid qualitative profile of lateral resistivity changes. Combined with point-based methods, FSM offers a low-cost alternative to complex techniques, with minimal time investment. Its mathematical principles can be interpreted both statically and dynamically. Here, we highlight the efficacy of this methods in understanding aquifer geometry, even in scenarios where the precise location of groundwater is challenging. We present results of a study case of the Carrizal aquifer, situated in an arid region of Baja Peninsula, Mexico. The FSM method is adequate for studying the aquifer basement. Contrary to previous studies, our results indicate that FSM is not suitable for precisely quantifying depth or delineating aquifer boundaries, nor for accurately locating aquifers. However, it can be a good choice when is adequately analyzed in conjuction with other methods and geological techniques. Qualitative analysis supplemented with in - depth verification, is suggested due to FSM's speed.

Plain language summary

Studying small water sources in arid regions such as Baja California Sur, Mexico, presents significant challenges due to limited on-the-ground data and insufficient depth coverage from conventional methods. Many areas lack deep-drilling data, and traditional techniques do not penetrate deeply enough. The Frequency Method (FSM) shows promise in determining the shape of these water sources, but the results require careful analysis. Our study of the Carrizal aquifer demonstrates that FSM is effective for examining the aquifer's base, even though it cannot precisely measure depth. For optimal results, it is recommended to combine FSM with other techniques.