# Course Introduction - II

*What can (or cannot) a geophysical method detect*?

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If the subsurface is completely **homogeneous** then the **value** of the **physical parameter** you are measuring will remain constant.

However, to detect **variations** in the subsurface  (change in state of the geologic material, or a contrast to a different  geologic material such as layering, or a fault) then the value of the **physical parameter** must change.

> [!WARNING]
>
> Different geologic materials may have the same (or strongly overlapping) value of a given **physical parameter**. If so, then that interface between the material (e.g. a layer boundary) is **invisible to that geophysical method**.

The above concepts are nicely illustrated in the figure below taken from the I2AG (*Introduction to Applied Geophysics*) textbook. In this case using only one geophysical method would not allow you to detect both interfaces.

![3layers](./i2ag-fig-1-1.jpg)

*Three layers of different geologic materials and two interfaces as "seen/detected" or "not seen/not detected" by two different geophysical methods*.

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