Raw refraction data

Energy in the ground

Seismic body waves travel away from the source, into the ground. The energy spreads in all possible directions away from the source, so wavefronts expand as hemispheres under a source point, if the ground is uniform. The energy will arrive at geophones at times depending upon the velocity of the waves in the subsurface materials. When energy reaches a change in material, some energy will be reflected from the interface, and some will pass through it. The geometry of this situation is shown in the next figure.

If seismic signals travel at higher velocity in the lower layer, then some of the seismic energy travels along the interface, returning to the surface as a "head wave" along a wave front similar to the bow wave of a ship (figure below). These are refracted waves, and for geophones a long way from the shot point, they represent the first arrival of seismic energy. In other words, because head waves travel along the interface at the velocity of the "faster" material, they eventually overtake the direct waves (green in the figure below) travelling in the slower surficial materials.

Of course energy is both reflected and refracted, so ground motion detected at a geophone is a caused by the combination of direct, refracted and reflected energy arriving at the geophone's location. The different types of energy are distinguishable only because they have travelled along different pathways. Refraction surveying takes advantage of the fact that refracted waves arrive before reflected energy, so long as the geophone is at a great enough distance from the shot point.

Signals that get recorded

Each geophone produces an electrical signal that is proportional to ground motion (in one direction - usually vertical, but horizontal with special geophones used for shear wave work). That signal is recorded for a short period of time starting the moment the source of energy begins. We observe these signals by plotting them as one wiggly line for each geophone's signal. These signals are plotted next to each other so that ground motion at each geophone can be seen as a function of time. An example showing ground motion at 24 geophones is shown to the right.

On this plot, one geophone was not working properly. Geophones are labled with the first closest to the source. As expected, ground motion occured earlier at geophones closest to the source. For geophones 1 through 10, it seems as if there was no ground motion at later times, however this is an artifact of the "gain" (amplification) applied to these traces. Gain is lower for geophone signals near the source because signal amplitudes are larger. If the signals within the blue box are amplified and replotted, the adjustable figure below results.

Original First breaks picked Straight line segments

This figure shows shorter segments of traces from 12 geophones. The signal amplitudes have been amplified, so all ground motions are visible. There are clear beginings of ground motion for each trace, which appear later in time for traces farther from the source. Finding exactly what time the ground first moved at each geophone is called first break picking. It is not difficult in this case, but it can be challenging if signals are weak. Use radio buttons next to the figure to see the result of picking first breaks (also known as first arrivals) on this figure. Once the picks are chosen, it becomes evidend that there is a definate pattern to the arrivals - there are straight lines joining the first breaks of several adjacent traces. The third radio button reveals straight line patterns emphasized by drawing red lines.

We will learn that first arrivals are either direct signals (for near geophones) or refractions that have travelled along subsurface interfaces. The objective of seismic refraction surveys is to determine the geometry of subsurface interfaces, and this can be derived by analysis of the pattern of first arrivals.