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In Situ Shear Wave Velocity Measurements in Rocks - Shear Wave Velocity Measurement Methods

2.0 Shear Wave Velocity Measurements

            Many methods have been developed for evaluating the Vs of rock both in situ as well as in the laboratory (e.g. Arroyo et al., 2010; ASTM D4825 among others). Some of the commonly used methods for in situ testing of Vs include:

-          Seismic Refraction Survey

-          Seismic Reflection Survey

-          Surface Wave Methods

-          Crosshole Method

-          Downhole Method

-          Suspension Logging

It should be noted that reflection/refraction surveys and surface wave methods are non-invasive test methods. Crosshole, downhole, suspension logging all require at least one borehole drilled into the material. While non-invasive testing eliminates the need for penetration into the subsurface, it also does not allow for physical sampling of the material. General recommendations for geophysical testing in rock have been provided by the International Society for Rock Mechanics (ISRM) and have been discussed in Takahashi (2004) and Takahashi et al. (2006). ASTM D6429 provides a guide for selecting surface geophysical methods for applications in geologic, geotechnical, and environmental subsurface investigations. All of these methods have strengths/limitations and assumptions which should be considered carefully. 

 

2.1 Refraction Surveys

The refraction survey (ASTM D5777) technique is often used for measuring material stiffness and identifying significant layer interfaces (Stokoe et al. 2004). The energy source for this method is actively generated. Sensors at the surface, typically geophones, detect refracted waves from a high velocity layer at depth. This method requires that a low velocity layer at the surface be underlain by a high velocity layer at depth (Takahashi, 2004).

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Figure 1: Seismic Reflection Survey (Stokoe et al., 2004)

 

2.2 Reflection Survey

            Seismic reflection surveys (ASTM D7128) also use an active wave generation source at the surface along with an array of receivers. Generated waves are reflected off of interfaces at depth and detected by the sensors at the surface. This method is used to identify significant material interfaces at depth (Stokoe et al., 2004). Reflection surveys can also be used to identify depth to bedrock and subsurface cavities (Takahashi, 2004).

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Figure 2: Seismic Reflection Survey (Stokoe et al., 2004)

 

2.3 Surface Wave Methods

Surface wave methods have become popular techniques for rapid realization of Vs profiles in soil and rock. There are several different techniques including Spectral Analysis of Surface Waves (SASW) (Stokoe et al., 1994), Multichannel Analysis of Surface Waves (Park et al., 1998), and Microtremor Analysis Method (Okada, 2003) among others. Unlike the other techniques discussed here, surface wave methods may use either actively generated or passive (background noise) waves and do not directly measure the velocity of body waves. Surface wave methods utilize the frequency-dependent properties of Rayleigh surface waves in order to develop a Vs versus depth relationship (Stokoe and Santamarina, 2000). Rayleigh wave dispersion also has applications in estimating the depth to bedrock (Tamrakar and Luke, 2013). Figure 3 shows the generalized MASW setup used by Sahadewa et al. (2012). This approach, specifically, uses sixteen 4.5 Hz vertical geophones as sensors in a linear array and a 10-lb. sledge hammer with plastic striker plate for an impulsive source.

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Figure 3: Generalized MASW Setup (Sahadewa et al., 2012)

 

2.4 Crosshole Method

            Crosshole (ASTM D4428) Vs measurements are typically performed between boreholes, one with a mechanically-activated source and at least two others with receivers (Takahashi et al., 2006). Crosshole testing allows for the generation and measurement of both vertically (SV) and horizontally (SH) polarized shear waves (Roblee et al., 1994). Source and receivers can be lowered to different depths in the boreholes for additional measurements or the Vs can be measured on inclined paths to generate a tomographic shear wave velocity image (Santamarina and Fratta, 1998).

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Figure 4: Crosshole Seismic Test (Stokoe and Santamarina, 2000)

 

2.5 Downhole Method

            The downhole (ASTM D7400) method is performed in one borehole. A source at the surface generates shear waves which propagate downwards to receivers in the borehole. In general, the down-hole method is less expensive than crosshole because only one borehole is required (Stokoe et al., 2004). Downhole tests can also be used to measure both SV and SH waves.

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Figure 5: Downhole Seismic Test (Stokoe and Santamarina, 2000)

 

2.6 Suspension Logging

            This technique involves suspending source and receivers in a fluid-filled borehole (Kitsunezaki, 1980). Waves generated by the source travel along the borehole until recorded by the sensors. This method can be used to measure Vs to significant depths, over hundreds of meters (Stokoe et al., 2004).

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Figure 6: Suspension Logging Test in Borehole (Stokoe and Santamarina, 2000)

 

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