The best way to understand the materials that make up the earth and the arrangement and/or organization of those materials is by direct observation. This is the realm of the field geologist, who carefully describes the physical (and sometimes chemical) properties of materials as they occur in place, noting their location and relationships to materials in other places.
It is relatively easy to get a good 2-D view of earth’s materials and structure as exposed at earth’s surface. Sometimes steep topography (as we have here in the mountains), vertical cuts into a hillside, or drill holes (such as those poked in the ground for water or oil exploration) provide us with some perspective on the third dimension. Geologists use surface observations, 3-D clues such as drill holes, and knowledge of material properties and behaviors to predict the character of the subsurface.
Geophysics provides us with tools to better constrain our understanding of what is down beneath our feet, but out of sight. Geophysics is not a magic wand, but it can be a powerful tool when wielded with a proper respect for the known (observations). Waves and forces that are in the realm of measurement by physicists behave in predictable ways when they encounter contrasts in the physical properties of materials.
- Variations in the magnetic character of earth materials (referred to as susceptibility) can add to or subtract from the earth’s magnetic field as measured at the surface.\
- Variations in density of materials can very measurably modify the local pull of earth’s gravity.
- The way that different materials conduct or resist the flow of electrical current is variable.
- The density and elastic properties of materials modify the velocity and path of seismic waves passing through.
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