#siesmic

Beneath your feet

How do geoscientists see beneath the ground without digging it all up? Being able to look at hidden rock layers is very useful; it can spot buried faults, buried resources, and buried geologic structures. An image like this is one way that geophysicists look at rocks without looking at them.

This image shows seismic data across rocks in the Mediterranean folded into an anticline. This type of seismic data highlights layers and structures of rocks beneath the surface.

Seismic waves, even weak ones, travel through rocks and interact with different layers and different rock types. A seismic or geophysical survey of an area can use this property to sample rock layers.

To do this analysis, a series of seismic receivers (geophones) are placed in a line covering the area to be surveyed. Energy is then released at the starting point; this can be a small explosion using gunpowder, or something smaller such as striking the ground with a sledgehammer.

That energy travels into the ground in all directions and begins to be received by the geophones, but it doesn’t come in a single wave. Energy travels outward into the ground and begins bouncing off the layers beneath. Each layer will bounce a bit of this energy back upwards, giving the geophones a long signal of dozens, hundreds of wave arrivals.

The time it takes each wave to arrive is related to how deep the layer is. Using data of when each wave arrives along a linear survey line, computers can perform an inversion to determine the structure below: the depth, orientation, and even some details about the composition of the units.

Buried beneath the Mediterranean is a thick layer of salt, produced when the sea itself dried up (see here: http://tinyurl.com/kelhq2m). That salt is easily folded and also can travel upwards as domes rising up after burial. The salt layer is noted on this image and there are flat layers above highlighted, possibly the remnant of salt moving upwards from one of these layers.

-JBB