This report contains a review of the in situ geophysical testing methods used to determine the shear wave velocity of geologic materials. Brief overviews of several geophysical techniques are provided and accompanied by examples and useful references. Three case studies utilizing shear wave velocity measurements are also discussed. Finally, the in situ Vs values for various rock types reported in the technical references herein are compiled for comparison and reference.
The measurement of seismic wavevelocities has become an integral component of material characterization in geotechnical and geological engineering. The propagation velocity of stress waves is related to the mass density (ρ) and small-strain stiffness of the material (Stokoe et al. 2004). The compression wave, or P-wave, velocity (Vp) is related to the material mass density by the constrained modulus (M) as shown in eq. 1. Similarly, the shear wave, or S-wave, velocity (Vs) is related to the material mass density by the shear modulus (G) as shown in eq. 2. These seismic wave velocities are related to each other through Poisson’s ratio (ν) as shown in eq. 3.
While Vp is traditionally more commonly measured seismic velocity in rocks, it is not optimal for measurement in soils. In a fully saturated soil, the compression stiffness of the pore fluid will dominate due to water being incompressible relative to the soil skeleton. As a result, the value of Vp measured in a soil is heavily influenced by the degree of saturation (Stokoe et al. 2004). This becomes critical when considering that a site investigation may encompass both soil and rock depending on the geologic conditions, desired depth of investigation, and characterization goals. For this reason, it would be helpful, and efficient, to simultaneously measure Vs in both materials.Vs in both soil and rock is valuable due to its relation to small-strain shear modulus, use in correlations, and necessity as an input into seismic site response analyses. The Vs of a soil or rock also provides a general estimate of its stiffness and/or degree of weathering.
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:
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.
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).
Figure 1: Seismic Reflection Survey (Stokoe et al., 2004)
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).
Figure 2: Seismic Reflection Survey (Stokoe et al., 2004)
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.
Figure 3: Generalized MASW Setup (Sahadewa et al., 2012)
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).
Figure 4: Crosshole Seismic Test (Stokoe and Santamarina, 2000)
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.
Figure 5: Downhole Seismic Test (Stokoe and Santamarina, 2000)
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).
Figure 6: Suspension Logging Test in Borehole (Stokoe and Santamarina, 2000)
The Vs of rock materials has many practical applications. Vs has been used in correlations for other parameters or to better understand the behavior of the material. Vs has been used to estimate anisotropy in rocks (Rabbel et al., 1998; Winterstein and Paulsson, 1990), determination of fracture strike (Winterstein and Meadows, 1991), monitoring CO2 injection (Chen and Liu, 2011), underground cavity detection (Bianchi Fasani et al., 2013; Harrison and Hiltunen, 2004; Parker and Hawman, 2012; Robison and Anderson, 2008; Rucker et al., 2005) and many more applications beyond. Shear wave velocity has also been used to better understand discontinuities in rock masses. The effects of other factors such as confining stress, temperature, propagation direction, and faulting have been explored extensively in laboratory settings (Punturo et al., 2005; Agosta et al., 2007). Of significant interest is the effect of rock mass discontinuities on Vs and wave propagation. Winterstein (1992) describes basic cases relating shear wave propagation to fractures in rocks. There has been a significant amount of research into understanding how rock discontinuities such as joints, fractures, and foliation interact with seismic waves (Cha et al., 2009; Ivankina et al., 2005; Misra and Marangos, 2011; Winterstein, 1992).
Several references from the literature discussing in situ Vs measurements in a variety of materials. The reported Vs values have been compiled in Table 1 for comparison.This is an incomplete compilation that identifies possible values of in situ Vsthat may be measured in some materials. The values in the table reveal the significant variability in Vsthat can be measured in a single geologic unit. Factors such as weathering and the presence of discontinuities have a significant impact on the Vsof rocks. As observed in Table 1, rocks such as limestone can exhibit profound variations inVs, largely as a function of weathering in the rock mass. Some materials, depending on site conditions, observed an order of magnitude change in Vs over a relatively short distance.
Table 1: In Situ Shear Wave Velocities of Some Rock Types from the Literature
*Back calculated from reported values of small-strain shear modulus and mass density.
Three case studies providing recent examples of applications for in situ measurement of Vs in rocks are summarized in this section. The case studies include identifying underground cavities and characterizing geologic materials (Bianchi Fasani et al., 2013), monitoring of rock mass properties and conditions ahead of tunnel excavation (Godio and Dall’Ara, 2012), and identification of fracture zones in sedimentary bedrock at a contaminated site (Ellefson et al., 2012).
Throughout parts of Rome, Italy, Pleistocene pyroclastic deposits (ignimbrite) were mined at undocumented locations. This resulted in unmarked subsurface cavities and, as a result, made the ground surface susceptible to sinkholes and collapse, similar to sinkholes developed in areas with bedrock dissolution. The cavities are generally at depths equal to 1.5 to 3 times their diameter and were mined using the “room and pillar” technique. The figure below identifies the test sites where the Bianchi Fasani et al. (2013) study was performed. The goal of their testing was to develop a testing program that could be implemented throughout Rome to generate subsurface cavity hazard maps.
Figure 7: Rock Outcrops and Test Sites (Bianchi Fasani et al., 2013)
In order to do this, the authors planned to integrate geophysical measurements with previous borehole logs, verify the homogeneity of the pyroclastic deposits, assess the ability of electrical resistivity tomography to identify cavities, and determine the ranges of some mechanical properties including the Vs. Using previously collected geologic information of the area, a synthetic geologic model was developed and used to determine the necessary depth of investigation and resolution for electrical resistivity tomography, which was the focus of the field testing performed. Additionally, the Vs of different geologic units at the test sites were investigated using the seismic crosshole method. These measurements were coupled with the surface electrical resistivity survey to pursue the goals of the study. The Vs measurements provided in situ data on materials that had previously only been extensively studied in the laboratory. The Vs measurement results from this study are shown in Figure 8.
Figure 8: Resulting VsMeasurements
Sonic logging allows for the measurement of seismic wave velocities in many situations. In the case study reported by Godio and Dall’Ara (2012), sonic logging was used ahead of an advancing tunnel face in order to monitor and update rock mass mechanical properties and detect zones of weakness that may intersect the tunnel. A schematic of the logging system is shown below. The data was collected during the excavation of a 52.7 km tunnel. The authors discuss the first portion of the tunnel which was primarily advanced through quartzite with some tectonized zones and intersections with a gypsum formation.
Figure 9: Sonic Logging System (Godio and Dall'Ara, 2012)
Quality measurements were conducted despite significant water flow from the horizontal boreholes. The collected data was used to accurately predict the changes from hard to soft rock and identify zones weakened by faulting and joints which could inhibit tunnel excavation. This can be observed in Figure 10. In cases where the S-wave was difficult to resolve, a frequency-wavenumber analysis was used on Stoneley waves, which are vertically-polarized waves traveling along the interface (with particle motion perpendicular to the interface) between two materials, to resolve Vs from recorded data.
Figure 10: P- and S-wave Velocities ahead of Tunnel Face during Excavation
Groundwater and sedimentary bedrock at a site in West Trenton, NJ, USA, was contaminated with chlorinated solvents. The authors of the study aimed to further characterize the geologic conditions in order to more accurately predict contaminant transport through the bedrock. The primary goal was to identify fault zones that would significantly control the flow of the contaminated groundwater. Seismic refraction surveys and MASW tests to determine Vs in the bedrock were the primary geophysical methods used. An example refraction survey is shown in the figure below.
Figure 11: Seismic Refraction Survey Record
Borehole logs revealed fractures present in the bedrock but did not accurately describe their persistence in the deposit. Two dimensional Vs profiles were developed to map the Vs of the bedrock containing the contaminated groundwater. The interpretation of the depth to bedrock from the Vs data is shown below. The low velocity anomalies show the extent of fractured zones which are likely to be critical for estimating and monitoring contaminant transport.
Figure 12: 2D Vs Profile with Bedrock Surface Interpretation and Identification of Fracture Zones
There are many geophysical methods that allow for the measurement of shear wave velocity in rock materials. The importance of Vs to soil and rock characterization makes simultaneous measurement in both materials of interest. Several in situ test methods for Vs in rock, or soil, have been highlighted. These tests have been implemented with measurements for Vp and other geophysical methods for thorough site investigations and assessing the condition of a rock mass.A few examples of applications for Vs measurement in rocks have been explored in this report. Additionally, a table of Vs values for some rock types in the discussed references and others from the literature has been included.