Seismic refraction examples

Here are three short examples of seismic refraction applied to engineering-scale problems.

Velocity as a guide to rock strength

(From P.V.Sharma, 1997, Environmental and engineering geophysics, Cambridge University Press, p175)

Seismic velocity and rock type (empirical, only for one the location mentioned). Degree of separation C Vp (m/s) RQD No joints 0.65-1 > 4500 very good Few joints 0.45-0.65 4000-4500 good Jointed rock 0.3-0.45 3500-4000 moderate Numerous joints 0.15-0.3 3000-3500 bad Strongly jointed rock 0.00-0.15 < 3000 very bad Seismic velocities are dependent upon rock type but also upon factors like porosity, water content, and degrees of fracturing and jointing. In  environments where the rock type is uniform, variations in the  P-wave velocity may sometimes be used to infer how these factors vary with location. As an example, in studying crystalline rocks from Sweden, there is a empirical relationship between the jointing factor (C), seismic velocity (Vp), and the rock quality designation (RQP). These are listed in the table reproduced here from Sharma, 1997.

Rippability chart, courtesy of Caterpilar Inc. (In Sharma, 1997) The P-wave velocities of the surface layers can also be a useful guide to the rippability of the material for excavation. Surface layer materials with velocities less than 2000 m/s may usually be ripped by a bull-dozer. Generally, the lower the velocity of any rock type, the more rippable it is. This is characterized by the table shown to the right.

Mapping Sand and Gravel Deposits

(From P.V.Sharma, 1997, Environmental and engineering geophysics, Cambridge University Press, p180-182)

Delineation of sand and gravel deposits is of special importance in connection with the planning of quarrying operations at potential exploitation sites. These deposits, when located above the water table, are characterized by low velocities in the range 400 -1000 m/s, in contrast to water-saturated sand and moraine which typically have velocities in the range 1300 - 2000 m/s. The following example is from Denmark.

The seismic refraction profile in an area of Quaternary sedimentary deposits Ourdrup Kirke, Denmark is shown below. Interpretation of the travel time curves indicated the first layer (gravel) with velocity varying from 330 - 500 m/s, the second layer (sand) of velocity between 560 and 1000 m/s, and the third layer (water-saturated chalk) of velocity 1650 - 2800 m/s. The lower layer also included a few isolated zones of high velocity (3400 - 4000) m/s as well as zones  of low velocity (1000 - 1250) m/s. The interbed thickness of the gravel deposit varies from 5 m to 14 m and the combined thickness of the gravel and sand is about 25 m. Interpretations of velocities and depths were made using the
plus-minus method.
 

Seismic refraction profile in an area of Quaternary sedimentary deposits, Oudrup Kirke, Denmark. Interpretation of the travel-time curves indicated the presence of three layers corresponding to the gravel, sand and chalk formations. Note the relatively large lateral velocity changes in the chalk. This is a fairly large data set for an engineering project.  Questions:  How many geophones were used for each spread, and what was their spacing? How many spreads were involved in the entire survey? What was the TOTAL number of shots fired into the eastern-most spread? How many of the shots fired into the eastern-most spread were located further east from location 0? How many shots were detected by the geophone at location 0?

Shear wave refraction

This record was recorded using geophones mounted horizontally and transverse to the profile. To generate the shear wave signal a metal bar with fins to anchor it to the ground was placed transverse to the profile, then struck with a hammer on either end. Records from these two shots were subtracted so as to increase signal-to-noise ratio.

Aprominent air wave (A) can be seen, followed by a horizontally polarized shear wave (S). High amplited late arrivals are surface waves or ground roll. S-wave refractions were used to identify the structure of glacial till below the shallow water table. Shear waves are useful here because the water table refraction is nearly invisible because the water does not contribute to shear wave velocity. When P-waves were recorded at this site, the refraction at the top of the water table was the dominant feature because of the significant velocity contrast at the top of the water table. Glacial till structure was therefore easier to interpret from shear wave data than from P-wave data.
 

This is a shear wave refraction record from Baden, Ontario. P-waves are barely visible but their location is marked with the P. Shear refractions were not readily visible on P-wave records. From Greenhouse and Gudjurgis, 1995, Reference notes for an EEGS short course on applications of surface geophysics to environmental investigations.