Seismic velocity test

Seismic velocity test This field in situ test is conducted to check on the stratigraphy and fracturing of rock masses. It is useful for extrapolating field and lab measurements to rock mass behavior. 

Cermak, V., Bodri, L., Rybach, L., Buntebarth, G., 1990. Relationship between seismic velocity and heat production comparison of two sets of data and test of vahdity. Earth Planet. Sci. Lett. 99, 48-57. 

Data reduction is done by a process called inversion. It is not possible to uniquely derive the structure of a body from first principles based on seismic data. Instead, a model of the structure must be assumed and then the predictions of the model are compared to the observations. The model is then adjusted until the predictions match the observations. The more precise the predictions, the better the model can be tested by the observations. Properties that can be investigated by helioseismic inversion include the density, pressure, sound speed, angular velocity, temperature, and composition. 

These tests show that the average thickness of the seismic lithosphere (crust plus upper-mantle lid) can be as much as c. 160 km and the minimum S-wave velocity beneath the lid can be as high as c. 4.45kms and still produce synthetic waveforms that match the observed waveforms. A thicker lid or higher S-wave velocities at depth below the lid are not consistent with the regional seismic waveforms. It is the high S-wave velocity lid that is unique to the upper mantle of the shield below that, the S-wave velocity is not significantly different from PREM (Preliminary Reference Earth Model) (Fig. 2). 

Dynamic loading problems in the offshore environment depend on either estimated or measured values of shear modulus. In practice, in situ determination of shear wave velocity on land has been used as the best approximation to the actual values for laboratory tests on samples (Richart, 1975). The techniques for using these seismic methods and data acquisition techniques to determine shear wave velocity for land-based applications have been well developed. The problem in the marine environment has been to develop methods to determine in situ shear wave velocity measurements both at the seabed surface and at known depths in the sediment column, which can be determined in a cost-effective manner. 

To solve the problem of measuring shear wave velocity in the soil column, a seismic cone penetrometer has been developed. The seismic cone contains a triaxial set of geophones (i.e., detectors) incorporated in a conventional in situ piezocone. It is typically pushed into the soil from the seabed or from the bottom of an advancing borehole. The source is typically a hydraulically driven spring hammer located on the seabed. It is ideally coupled to the sediment surface and preferentially generates horizontally polarized shear waves. It is important to decouple the drill rods and tools from the seismic cone prior to testing because compression wave energy may be transmitted. This allows the full characteristics of the soil in terms of shear modulus to be determined. 

Field tests which are most commonly employed in geotechnical earthquake engineering can be grouped into those that measure low-strain properties and those that measure properties at intermediate to high strains. Low-strain field tests t5q)ically induce seismic waves in the soil and seek to measure the velocities at which these waves propagate. Due to low strain amplitudes the measured shear wave velocity (V ) along with soil density (p) is used to compute low-strain shear modulus. 

The overall soil profile of 60 m thickness is divided into layers of 1.5 m thick is considered. Each layer is characterized by the unit weight of soil obtained from SPT data and shear wave velocity obtained from the seismic cross hole test data. A typical input soil data used in SHAKE analysis is presented in Figure 9. The water table is taken to be at a depth of 30 m below the ground level. 

The parameters from these tests may be related to other parameters such as compressive strength, modulus of elasticity, seismic wave velocity, resistance to weathering and degree of weathering... 

In order to define the dynamic characteristics of the foundation soils, in relation to the choice of an elastic site-compatible spectrum, it is advisable to evaluate the profile of the shear wave velocity. This profile should be determined on-site by down-hole geophysical tests. As an alternative, it can be defined with the aid of empirical correlations with the site penetration resistance (SPT, CPT) or with other geotechnical properties. For a more complete definition of the dynamical characteristics, it might be necessary to define shear wave velocity values compatible with the deformations induced in the ground by the passage of seismic waves. 

Fig. 11.40. Evolution of rock properties in a deformation test. Schematic illustrates volume change (porosity), AE rate, permeability, and seismic wave velocity (Popp et al. 2002).

Crosshole, downhole, and uphole shear wave velocity measurements are used to determine the primary and shear wave velocities so as to either determine the elastic soil properties of soil and rock or calibrate seismic survey measurements. With the crosshole technique, the travel time is measured between a source in one borehole and a receiver in a second borehole. This technique can be used to directly measure the velocities of various strata. For downhole and uphole logs, the travel time is measured between the ground surface and a downhole source or receiver. Tests are conducted with the downhole source or receiver at different depths. These measurements should preferably be conducted in cased boreholes. 

Anbazhagan P, Kumar A, Sitharam TG (2013) Seismic site classification and correlation between standard penetration test N value and shear wave velocity for Lucknow City in Indo-Gangetic Basin. Pure Appl Geophys 170 299-318... 

Low-strain field tests induce seismic waves in the soil and measure the velocities at which these waves propagate. The maximum shear modulus can be computed using the measured shear wave... 

Shear wave velocities can be measured in situ by several seismic tests, including cross-hole and down-hole, seismic cone penetrometer, suspension logger, SASW (spectral analysis of surface waves), and MASW (multichannel analysis of surface waves). A review of these test methods is given in Woods (1994) and Kramer (1996). Their accuracy can be sensitive to procedural details, soil conditions, and interpretation techniques. Fig. 5 shows the layouts and principles of three established geophysical tests the continuous surface wave (CSW) method, the down-hole... 

Another laboratory technique that allows the measurement of the shear wave velocity of a soil sample is the bender element testing method. Bender elements consist of two piezo-ceramic plates bonded together in such a way that application of a voltage causes one plate to expand and the other to contract. This generates seismic waves in... 

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