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tags: geol 420 website page
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# Course Introduction

## Applied Geophysics in Action
Mineral exploration, groundwater exploration, critical zone studies, and geologic structural features.

- (2022-08-18) Infrastructure law helps USGS [map undergound geology](https://www.usgs.gov/news/state-news-release/bipartisan-infrastructure-law-helps-map-underground-geology-across-maryland) across Maryland,
Pennsylvania, Virgina, West Virginia.
- Earth Mapping Resources Initiative (Earth MRI - must have a good acronym!). [Earth MRI (USGS)](https://www.usgs.gov/special-topics/earth-mri)
- [Imaging Saltwater Intrusion](https://gemcenter.stanford.edu/imaging-saltwater-intrusion-using-electrical-resistivity-tomography) using electrical resistivity tomography.
- Nearby Los Osos has long had a seawater intrusion problem into its coastal aquifer. The [Los Osos Basin Management](https://www.slocounty.ca.gov/Departments/Groundwater-Sustainability/Groundwater-Basins/Los-Osos-Basin-Management-Committee-(BMC).aspx) website contains information about the aquifer remediation efforts and its groundwater management plans under the [Sustainable Groundwater Management Act](https://water.ca.gov/Programs/Groundwater-Management/SGMA-Groundwater-Management)

## Geophysical Methods Vary ...
... in what they can "image" about the subsurface. Each geophysical method remotely senses some subsurface geophysical property of the geologic materials.

In the case of seismic methods we collect information about the speeds at which various seismic waves travel in the subsurface.

In the case of electrical methods, we measure subsurface electrical resistivity of the geologic materials.

What makes things difficult is that very different geologic materials might have a similar value of seismic velocity or electrical resistivity. This means that a given method may not be able to distinguish between a contact or material contrast in the subsurface :(

Here is an illuminating example:


Figure above taken from [CH 1 of Introduction to Applied Geophysics](https://cpslo-my.sharepoint.com/:b:/g/personal/jjasbins_calpoly_edu/EeHr_D6kWRZFumLi1AIowY0B9dYMrA4rQJF9L6PY_o3JTg?e=IitHie) by Burger, Sheehan, and Jones. Here are the [CH 1 Figures](https://cpslo-my.sharepoint.com/:u:/g/personal/jjasbins_calpoly_edu/EYHyGK9W9eJDi17oFxiKcTcBhU0meDMbx_Wr3dGaop3MOw?e=WIAiHe) - but, there is only one figure in CH 1.

The first interface would not be well detected by a seismic method, but would be an electrical method. In the case of the second interface, it is the revers.

Ideally you have some idea of the geology of the area (maps, outcrops, yur surface observations) to help you understande your geophysical data. Even better, you have boreholes/logs. Those are expensive - so one great use of geophysics is to "interpolate" between borehole locations. Still, sometimes you know almost nothing, and this makes it very difficult to interpret your data.

🌟 Please read [CH 1 "Approaching The Subsurface" (pdf)](https://cpslo-my.sharepoint.com/:b:/g/personal/jjasbins_calpoly_edu/EeHr_D6kWRZFumLi1AIowY0B9dYMrA4rQJF9L6PY_o3JTg?e=IitHie) - it's just 4 pages - from the Book Introduction to Applied Geophysics by *Burger, Sheehan, and Jones*.

## Terminology
What is *applied geophysics* vs. *exploration geophysics*?

Both terms are commonly used. *Exploration geophysics* refers to the use of geophysical methods specifically in the "traditional" energy industry, i.e. oil and gas exploration.

*Applied geophysics* can refer to lithospheric scale questions, such as studying the North American [mid-continent rift](https://www.usgs.gov/centers/geology%2C-geophysics%2C-and-geochemistry-science-center/science/geophysics-midcontinent-rift).

Sometimes applied geophysics may refer to *environmental geophysics*, such as
[critical zone studies](https://www.nsf.gov/news/newsmedia/ENV-discoveries/CZO-discovery-series.jsp), [groundwater exploration and managment](https://enviro.stanford.edu/), faults and other geologic hazards, and [mineral exploration](http://quantecgeo.com/mineral-exploration/).