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Seismic Refraction Technique

General Seismic Refraction Techniques
Seismic Refraction defines the near-subsurface in both velocity and structure. However, because these two factors are intrinsically related, independent geologic knowledge is required to reduce the ambiquity effect and to produce a reliable near-subsufrace image.

Seismic refraction involves placing a line of sensors (geophones) on the surface and measuring the relative arrival time of a seismic wave at the sensors. The seismic source can be any well-timed sonic disturbance such as hammer blows or explosive charges. The relative arrivals are used to define the subsurface. The critical ingredients for successful refraction profiling include:

$refraction setup$

Additional considerations are important in a well designed refraction survey. These include the velocity-structure ambiquity, low-velocity zones (Fermat's principal), and the importance of geologic input.

Accurate placing of sensors
The geophone locations are routinely surveyed (leveled) relative to the shot points to precisions of 1/2 feet. A 1/2 foot precision can produce timing precisions of 1/2 millisecond with the typical near surface velocities of 1500 feet per second. The horizontal placement of geophones is also controlled to precisions of one foot. Controlling geophone locations in plan and level enables a precise geologic model and interpretation.

Timing of relative arrivals to precisions of milliseconds
Recording instrumentation used by Zonge Geosciences, Inc. allows accurate timing of first arrivals to better than 0.5 milliseconds.

shot record

Modeling or Calculating the bedrock depths and velocities
The first step in the analysis is to plot the arrival data in a travel-time curve. The seismograms are picked to obtain source-receiver travel times. These travel times along with source-receiver distances are used to construct a time-distance plot for each shotpoint. The velocities inferred from the travel time curves are apparent velocities, and not necessarily true velocities. True velocities are determined from arrival times from shotpoints at both ends of the sensor line during the modeling procedure. In addition, small variations of individual data points from a true "straight-line" velocity on the time distance curve can indicate either undulations of the subsurface structure or lateral velocity changes. All information obtained from the time-distance plots is used as a basis for further modeling. An example of a typical tomographic model (refraction tomogram) is shown below:

tomogram

In some instances shot coupling is not optimum, or sufficient cultural noise is present to make picking of arrival times inaccurate. As a result, some of the stations or shot arrivals may not be used in the analysis.

Sources for seismic refraction can generate either compressional (P) or shear (S) waves. Obtaining velocity information from both wave types allows the estimation of material properties such as Poisson's ratio. For s-wave refraction, geophones sensitive to horizontal ground motion are used and a source which generates ground motion perpendicular to the line of sensors and parallel to the ground surface is used. The geophones are planted with their sensitive axis oriented parallel to the source-motion direction. A common s-wave source is a thick plank placed on the ground with some overlying mass - such as a vehicle - used to couple the plank to the ground. The plank is oriented perpendicular to the line and struck with a hammer to produce S-waves with particle motion parallel to the axis of the geophones. One unique feature of S-wave refraction is that the source has a polarity: either end of the plank can be struck. By recording S-waves with both polarities, a simple data processing technique (subtraction) can be used to enhance the S-wave signal and diminish the P-wave noise generated by the source.

Additional Considerations
Because the two factors of velocity and structure are intrinsically related in refraction theory, their independent determination from refraction surveying alone is impossible. The ambiguity is that structure can be traded for velocity differences over a broad range of velocity-structure pairs.

Additionally, modeling ambiguity can be introduced due to the existence of low-velocity layers. Because there is no refracted information from a buried layer with a velocity less than that of the overlying material, the low-velocity layer will be hidden in the arrival time data. When this situation occurs, calculated depths to deeper refractors can be offset and in error. Boreholes, downhole logs, and geologic information are critical to limiting the range of these uncertainties.

One additional physical principle applies when considering low-velocity zones - Fermat's principle. Fermat stated that the energy will take the least-time path from one point to the next. This principle is the basis for seismic refraction, but it also means that the first arrival energy will "go around" a low velocity zone. Unless the geometry is favorable (no high velocity path possible) the first arrival information will not reveal a low velocity area. These potential pitfalls, though discouraging, are not insurmountable. When accurate subsurface ties to borings and good estimates of the probable geology are available, these problems are minimized and an accurate subsurface map may be produced.

Seismic Refraction is Cost Effective
Seismic refraction is a cost-effective method for finding the depth to bedrock. Typical applications include unconsolidated alluvium over competent bedrock. Variation of +/- 10% of depth with lateral extent of 50% of the depth are routinely resolved. The water table is another target that is mapped where the velocity distribution of the rocks is favorable.

Difficult refraction problems can often be resolved by the use of shear wave refraction. The water table is often transparent to shear waves and even low-velocity bedrock interfaces can be resolved.

Zonge Geosciences, Inc. (Zonge) has done refraction with various geophone spacings . We use modern instantaneous floating-point digital seismographs for recoding data and and state-of-the-art software for analysis.

We can furnish you with a travel-time curve based on your geologic model. Please furnish us with the rock types, depths (including a range), and any strength or velocity information (blow counts, etc.) that you may have. The probable location of the water table might be the most critical piece of information we would use to estimate the utility of refraction for your project. A well planned survey can be a cost effective means of geologic investigation.

Zonge Geosciences, Inc. will be happy to respond to your questions!! Contact Us