WHY GEOPHYSICS ???
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 like 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.
For instance…
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.
