First break:  a term used to describe the first "break" from background when recording seismic waves.  After the seismograph is triggered to record by the source, it "listens" to the array of geophones.  It records all incoming signals during a set period of time usually on the order of hundreds of milliseconds.  Analysis of the recording shows that direct waves or refracted waves are the first to appear in time on each trace of the record.  By noting exactly when in time these waves appear, or (as it is more commonly referred to) the first breaks occur, and the distance from each geophone to the source, it is possible to determine the velocities of the refracting layers.   

Seismic refraction data can be interpreted in several ways.  The simplest approaches assume a series of plane, dipping layers.  While effective in many instances, this method is not suited to irregular or undulating layers.  The Generalized Reciprocal Method (GRM) goes beyond the plane-layer assumption, producing a profile which allows for irregularities in the refracting surface.  Geosphere makes use of ViewSeis from Viewlog Systems and SeisOpt from the University of Nevada to analyze seismic refraction data.  When possible, we combine GRM results with reflection data to produce the most comprehensive seismic interpretation available.
12-Fold CDP Stacked Seismic Reflection Profile showing
Dipping Layers and Faults
GRM Seismic Refraction Line showing Depth to Bedrock and Associated Velocities.
Vertical Exaggeration is roughly 10:1.

124 north auburn road auburn, mi 48611 
tel: (989) 662-6149  fax: (989) 662-7701
copyright ©1990-2007 geosphere inc

displayed as a single seismic cross-section. A seismic trace may contain from hundreds to thousands of individual samples.  With each shot generating 24 or 48 traces, a typical seismic line will contain several million samples.  Geosphere processes these data with the "WinSeis Turbo" package, developed by the Kansas Geological Survey for PC workstations.  Augmented by several programs developed by Geosphere, we now have a seismic reflection processing system tailored to the unique problems encountered in high-resolution seismic work.
24-Channel Seismic Field Record showing
many Coherent Events
A seismic reflection section is, in principle, a series of seismic traces recorded by a geophone at the same location as the shot.  Each trace must be time-corrected to allow for the source-geophone offset, the correction depending on the layer velocities.  If the correction is accurate, a given reflection is moved up the trace to the position it would have were the source and receiver coincident.  Using the field procedure described previously, 12 individual traces sorted from 24-channel data, of various source-receiver offsets, will have a common midpoint.  These 12 traces, after correction, are summed to produce one common depth point, or 12-fold CDP trace.  The resulting summed traces are then