Seismic refraction explorations consist of measuring the time required for a seismic impulse to travel from a seismic source to a receiving transducer.  An explosive device, an air gun, a weight dropped from a truck or a hammer striking a steel plate is typically used for the seismic source and twelve or more vertical geophones are used for the receiving transducers.  The choice of the seismic source depends upon the length of the seismic line, the degree of resolution desired, and the environmental suitability of a particular source.

      A signal enhancement seismograph records signals from the geophones.  By analyzing the arrival time of the seismic wave as a function of distance from the seismic source, the seismic velocities of the underlying soil/rock units and the depth to geologic contacts can be determined.  The geophone spacing and the distance between the seismic source and the first geophone are designed to obtain the needed penetration and resolution.  Application of the method is generally limited to areas where seismic velocity increases or is constant with depth.

      The seismic data are analyzed by plotting the first arrival time of the compressional wave at each geophone versus the distance from the seismic source to the geophone.  These graphs are commonly known as travel-time plots.  In the most basic analysis, the data are fit with straight-line segments.  Each line segment corresponds to a different stratigraphic layer.  The reciprocal of the slope of the line segment is the apparent compressional wave velocity of the layer.  Current state-of-the-art analyses use forward and inverse modeling and ray tracing that seek to minimize discrepancies between field measured arrival times and corresponding times traced through the velocity model.  A typical seismic refraction model is shown below.