| These are terms commonly in use for seismic refraction work.
Arrival times
The time at which the direct or refracted energy first arrives at a geophone.
Approximate Velocity Depth Migration
A procedure for converting the time-depth section to depth units using refractor velocities and all but one layer thicknesses. The one unknown layer thickness is that immediately above the target refracting layer. This method is not highly sensitive to the proper selection of the XY value.
Average Velocity Depth Migration
A procedure for converting the time-depth section to depth units using a velocity which represents the average of all units above the refracting layer. This method is sensitive to the proper selection of the XY value.
Critical Distance
The distance from the shot within which no refracted energy can return to the surface. Only direct or reflected energy can be seen within the critical distance. It is related to the critical angle, which is the incident angle which results in refraction along the interface.
Depth Migration
The process of converting the time-depth section to units of depth. Two methods of depth migration are available in the GRM: Average Velocity and Approximate Velocity methods. Comparison of the results from each method can indicate if hidden layer conditions exist. If the two methods do not approximately agree, then there is cause for concern, and the possibility of a hidden layer should be considered.
Dipping, planar layer
A
smooth (no topography) layer, that lies at some angle to the surface. It is also assumed that there is no variation perpendicular to the survey line.
First arrivals
See Arrival times.
First break picks
See Arrival times. Forward, Mid or Reverse shots
See Shot Direction.
Forward Modelling
The process of constructing artificial results in order to learn something about a real geophysical situation. Interpretation may involve adjusting the artificial (usually mathematical) model until results compare favourably to field data.
Geophone array
See Seismic Spread
Geophone Spacing
The spacing between geophones in a shot.
GRM
The Generalized Reciprocal Method, one of several methods for interpreting shallow seismic refraction data. It provides a more complete solution than other methods, but also requires a more complete data set.
Group interval
See Geophone Spacing
Hidden layer
A layer of material that is not visible on the seismic section, either because geophone spacing is not adequate, or because it has a seismic velocity that is less than that of the material above it. Layer Assignment
The process of determining the layer from which an arrival was refracted. Ideally, all arrivals refracted from the same layer will fall on a line with a constant slope. In practice, however, irregular layer interfaces and velocity variation tend to scatter the first break arrival times. In this case, patterns observed between shots with different offset spacing can be used to guide layer assignment.
Phantoming
The phantoming process involves shifting a set of arrival times vertically in time, so that they link up with arrivals from an adjacent shot. The purpose of phantoming is to create a complete set of forward and reverse arrivals along the line, all referenced to the same shot position.
Raw trace data: The signal recorded by a geophone as a result of one shot. It consists of a set of parameters defining the data (a “header”) and the digitized voltage for a fixed amount of time following the shot time.
Reciprocal Time
The time it takes for energy to travel from the forward shot point to the reverse shot point. This is NOT the time from one shot to the last geophone.
Seismic Line
A combination of seismic spreads collected end to end (often overlapping by a few geophones). Some interpretation programs organize all shots and spreads together in one interpretation file.
Seismic Shot
Some programs organize arrival time information based on shots. Each shot is made relative to a spread, and so 12 or 24 arrival times will be associated with each shot.
Seismic Spread
A seismic spread is a combination of 12 or 24 geophones in position on the ground. Various shots may be made at different positions relative to the spread.
Shot Direction
The shot direction refers to the position of the shot relative to the spread, and relative to the order in which spreads are collected along the seismic line. The shot direction can be either Forward, Mid or Reverse.
Shot Offset
The relative distance between the shot position and the first or last geophone. "Zero offset" refers to shots in which the shot was made at geophone position 1, while "Long offset" refers to shots made some distance from geophone 1, along the seismic line, but not within the range of the spread.
Shot Position
The actual line position of the shot. (ie. the position relative to the start of the seismic line.)
Time Distance plot
A graph showing first arrival time versus distance from the shot (or distance along the line). Time increases up on the Y-axis, and distance increases right on the X-axis.
Time-Depth Analysis
A cross section showing layer positions in units of travel time (as opposed to units of depth). Directly analogous to the time sections created from seismic reflection recording. Time-depth analysis curves are usually presented for a series of XY value, so as to select the optimum XY value.
T-X plot
See Time Distance plot
Velocity Analysis Curves
Curves, calculated using a GRM based equation, which are used to determine both the velocity of the refracting layer and the optimum XY. The inverse of the slope of the velocity analysis curve is the refractor velocity. Changes in the slope of the curve indicates changes in the velocity of the bedrock. The curves are usually calculated for a range of XY value, so as to select the optimum XY value.
Vertical Travel Time
The time it takes seismic energy to travel vertically from the surface to an interface. See also time-depth.
XY Value
A measure of distance, along the ground surface, between the arrival of forward and reverse refracted energy emitted from a single location on the refracting interface. The XY value is central to the GRM procedure, and accurate depth sections can only be determined from proper XY selection. The optimum XY value is determined by inspection of the Velocity Analysis Curves and the Time-Depth Section. |
