Horizontal Interface(s)

Single - Distance versus Travel-Time Equation is a straight line.

Velocity of upper layer - is the inverse of the slope of the direct wave arrivals

Velocity of lower layer - is the inverse of the slope of the direct wave arrivals

Depth to interface - is determined from the intercept time (x=0) for the head wave arrivals

Multiple - additional horizontal interfaces can be determined using the same techniques (e.g., velocity from the inverse slope of the head wave arrivals and the intercept time). Intercept time for the deeper interfaces must be adjusted for the time spent traveling through the overlying layer(s).

The traverse where the energy source is down dip will have a larger incept time. Apparent velocity determined from the slope of the travel time curve is too high.

The traverse with an up dip energy source will have a smaller incept time. Apparent velocity determined from the inverse slope of the travel time curve is too low.

The critical angle and the dip of the interface are determined from the apparent velocities and the velocity of the upper layer. The true velocity of the lower layer is then computed from Snell's law.

The depth to the interface is determined from the intercept time, the velocity of the upper layer, and the critical angle.

Low-Velocity zones - cannot be detected by the refraction method because there is no critical refraction. The inferred depth to an interface below the low-velocity layer will be deeper than the actual depth.

Thin layers - may be missed by the refraction method because the head waves produced at the interface are never first arrivals. The inferred depth to an interface below the thin layer will be shallower than the actual depth.

Horizontal Variations in Velocity - produce a change in slope of the distance versus time curve that may be mistaken for head wave arrivals if velocity increases away from the seismic source. However, if the velocity decreases away from the seismic source, the slope of the distance versus time curve increases (velocity decreases) which can NOT be a head wave arrival.

Interface Discontinuities (diffractions) - produce an offset in the travel time curve. The slopes of the two offset straight line segments are the same. The amount of offset of the interface can be determined from the difference in intercept times.

The delay time is the difference between the actual travel time and the time it takes to travel horizontally from source to receiver at the velocity of the lower medium.

Depth to the interface at each geophone can be determined from the forward travel time, the reverse travel time, the reciprocal time, and the velocities of the upper and lower mediums.

The velocity of the lower medium is determined by fitting the difference in forward and reverse travel times versus distance to a straight line. The velocity is equal to 2 times the inverse of the slope of the line.

Field Methods

Geophone Setup - offset - distance from source to first receiver. Spread - distance between adjacent geophones. As a general rule the furthest geophone should be 3 to 4 times as far away from the source as the deepest interface.

Corrections to data - If the surface has significant topography then the raw data must be corrected for elevation to some horizontal datum (e.g., sea level). The correction is required because the seismic waves are travelling different distances down to the interface depending on the location of the source and the receiver.

Applications - typical shallow use of refraction method is depth to bedrock or depth to the water table.