Electrical Resistivity Surveying

--- tags: geol 420 website page --- # Electrical Resistivity Surveying Note: while I will post some of my own materials on this topical page, your primary resource are the linked pdf from various book chapters posted on the [Electrical Resistivity Reading Resources](/VFWf-JocRlGtXCWgwqb2Vw) page. You should expect to put in significant time reading this material to absorb it, and formulate questions about it. There is a lot of overlap between the various pdf files but you might find one "speaks" to you better than the others. I will provide guidance in class to what the most important parts are for our discussions. ## General Concepts Keep in mind: - Basic circuit concepts. We make a circuit with the earth as the resistor (many resistors actually). - Current $I$, and (change in) Voltage or electric potential $\Delta V$. Resistance: $R=\frac{\Delta V}{I}$. - Note that when writing $\Delta V$, it is commonly just written as $V$ with the $\Delta$ symbol as "understood". - *Resistivity vs. Resistance*. Resistivity is denoted by $\rho$ and is the material property. Resistance is an integrated measurement including geometry - length, $L$ and cross-sectional area $A$. The relationship between resistivity and resistance is $$ \rho=R \cdot \frac{A}{L} $$ or, equivalently $$ R= \rho \cdot \frac{L}{A} $$ - Note that field measurements of resistivity are really *apparent resistivity*, denoted by $\rho_{a}$ - What is meant by *apparent resistivity*? - We use software such as EarthImager to *invert* apparent resistivity field data into a *resistivity model* of the subsurface. The inverted model then contains estimated *true resistivity* values rather than *apparent resistivity values*. The way such software works is interesting, but at least initially not the focus of our course. - How does the software judge whether its inverted model is good? An error measurement is provided (usually RMS misfit but EarthImager also provides other types of misfit measurments). The misfit is generated by taking the current resistivity model of the subsurface and *simulating your exact survey details over it* to generate *synthetic data* to compare to your field data. - The difference between the two is turned into the misfit estimate. The software continues iterating until the misfit is below a set percentage or the specified number of iterations in completed. - This process is called **forward modeling**. In other words, if you specify the subsurface and the survey style, then you can generate what the "field data" would look like. - **Inversion** takes the actual field data in, and produces a model of the subsurface that "best" fits the data (in this case an electrical resisitvity model of the subsurface). - We must be careful with subsurface models: more than one subsurface could generate the same (or essentially the same) data. Which is correct? Which model did the software find? This is the priciple of **equivalence**. - Converting field measurements or $\Delta V$ and $I$ from resistant $R$ to apparent resistivity $\rho_{a}$ - This depends of the array style you used, the electrode spacings of each measurement, etc. Generally called the **geometric factor**. Each array has its own (see the reading). - Some geologic boundaries or material contrasts are not sensitive to electrical resistivity. What does this mean? More than one geologic material can have a similar electrical resistivity - thus the boundary between these geologic materials will not be detected - see [Course Introduction](/gJ8XoBcxTE-fwdoDOxG0qA). ## Electrical Resistivity Surveying for Groundwater Exploration Groundwater exploration and aquifer characterization is a major theme of our course. Electrical resistivity surveys and research into advancing its methods are very common in the fields of hydrogeology and hydrogeophysics. **Relevant Blog Posts from AGI** - [1D Geophysical Resistivity Survey: Vertical Electrical Sounding](https://www.agiusa.com/1d-resistivity-survey-vertical-electrical-sounding). Describes the common array styles of 1D-VES (Wenner and Schlumberger) and discusses limitations. - [What ERI Survey Is Best For Groundwater Exploration?](https://www.agiusa.com/blog/what-eri-survey-best-groundwater-exploration). - [How to find groundwater using Electrical Resistivity Imaging](https://www.agiusa.com/blog/how-find-groundwater-using-electrical-resistivity-imaging). This is a very good resource to help you think about interpreting electrical resistivity data for groundwater exploration regardless of if your data is 1D, 2D, or 3D. **Study it!!**