Basic estimation of rock-type and rock mass condition, from

shallow seismic P-wave velocity When first investigating the bedrock for suitability for near-surface tunnelling or other relatively shallow con-struction in rock, the preliminary use of shallow refrac-tion seismic is very typical, where surface access (including noise) do not present major problems. As a minimum, the information gives a V_p– depth profile of inestimable value for further planning of the sub-surface investiga-tion, in particular the optimal siting of boreholes for future core-logging and permeability testing.

Figure 1.4 reproduces four examples of shallow refrac-tion results from Sjøgren 1984, demonstrating the help-ful information about the location, width and depth of zones of lower velocity. Later in this chapter, and in sub-sequent chapters, we will be seeing the many ways of interpreting such velocities in terms of rock quality and degree of fracturing, each tempered by the effect of rock type, density, porosity, depth (or stress level), and of course the possible anisotropy (or directional depend-ence) of the result in relation to an anisotropic jointing frequency, and horizontal stress anisotropy.

The seismic refraction survey provides numerous depth to bedrock and quality of bedrock assessments at a small fraction of the cost and time needed for drilling.

Depths are given at the impact points (hammer or shal-low explosive source) and at the detector points (geo-phones or 3D seismometers), so a close spacing of detectors gives the equivalent of a large number of sound-ings or borsound-ings.

Sjøgren, 1984, gives the example of 5 m detector and 25 m source separations for a 10 m deep bedrock inves-tigation. A 100 m profile gives the equivalent of 250 m of soundings, and a complete distribution of relative quality beneath the profile. With the 10 m source and 50 m detector separations needed for a deeper survey to 50 m depth, the equivalent of 650 m of soundings per 100 m profile is given.

The knowledge and experience of the geophysical team is essential in setting out optimal profiles in relation to the geology and structural geology, in particular in relation to anisotropic, layered media, and in relation to fault and shear zones. ‘Correct’ interpretation of the calculated information cannot be divorced from the geology, since a given velocity (V_por V_sor dynamic Poisson’s ratio) is not unique to any one material but part of a scale or gradation in the specific geological profile at the site, and reflects

Figure 1.4 Seismic refraction results illustrating the wealth of potential information obtained concerning near surface conditions. Sjøgren, 1984.

various ‘environmental’ factors acting on each rock domain, as will be demonstrated in subsequent chapters.

The later geological and rock quality interpretation of core recovered from boreholes drilled close to the seismic profiles is the domain of engineering geologists, who besides identifying rock type, will perform careful logging of RQD, joint or fracture spacing, joint rough-ness and discontinuity mineral filling identification (or testing). The performance of rock quality characteriza-tion of drillcore is also standard practice for civil engin-eering and many mining projects, using the Q-value (Barton et al., 1974, Barton, 2002) and RMR (Bieniawski, 1989) as a minimum. Although these two methods have similarities, and common goals, there are differences, and care is needed in converting Q to RMR and visa versa, e.g. Barton, 1995.

Typical ranges of velocities for relatively competent (lit-tle weathered moderately fractured) rocks are given in Figure 1.5. Much lower velocities, covering most of the lower diagonal space between 1 km/s and 6 km/s are seen with extremes of weathering, jointing and fault related fracturing. The following is an example of the effects of weathering for just one rock type, from Sjøgren, 1984:

A similar range of values from the SSDS Project granites in Hong Kong (Gardener, 1992) gives a useful qualitative impression of variations caused by weather-ing and jointweather-ing in the same rock type.

At the hazardous second Severn Estuary crossing between England and Wales, tidal currents are so strong that 85% of the crossing had continuous rock outcrops between low and high tide. Sonar buoys and bottom drag cable gave the following relatively tight ranges of velocities for five rock types that were confirmed with boreholes, enabling the rocks to be identified across the site.

Figure 1.5 Typical ranges of V_pfor sediments and for little weathered, moderately fractured rocks. Sjøgren, 1984.

Table 1.1 Typical range of V_pfor gneiss (Sjøgren, 1984).

500 m/s Soil (above water table)

1700 m/s Highly weathered biotite gneiss

2800 m/s Weathered biotite gneiss

3500 m/s Jointed biotite/granitic gneiss

4900–5400 m/s Sound biotite gneiss

Table 1.2 Typical range of Vp (km/s) for granite (Gardener, 1992).

Decomposed granite (soil) 1.6–1.8

Fracture zones 2.8–3.5

Jointed granite 3.5–4.5

Intact granite 4.5–6.5

Table 1.3 P-wave velocities at the Second Severn Crossing (Gardener, 1992).

Average velocity Velocity range

Rock description (km/s) (km/s)

Griffiths and King, 1987, also give typical V_pranges for common rock types. These are reproduced in Figure 1.6, as a source of cross-referencing. Fractured, faulted and heavily jointed zones extend the six major ranges for these rocks far to the left on occasion. Note the extremely high velocities of the dense, ultramafic rocks, which lie outside the common range of 1 to 6 km/s.

A comprehensive set of in situ seismic Vpvalues, and some V_svalues for common rock types, is also shown in Table 1.4. The data are given by Press, 1966.

The wide ranges of velocity for sandstone, shale, lime-stone and dolomite are mainly due to the wide ranges of porosity (and density) for these materials. The surpris-ingly high range for gneiss is due to the wide range of mineralogical composition (and density) for this rock.

The marked variation of velocities that are measured in superficial deposits (0.5 to 2.0 km/s in Figure 1.5) are partly caused by location either above or below the water table, as shown by Sjøgren’s 1984 data set. The list given in Table 1.4 shows 0.2 to 2.0 km/s just for the case of sand, mostly for this reason.

The following is perhaps a good example of the influ-ence of particle size in river born sediments.

The last line of Table 1.5 (for cobbles and boulders) differs significantly from the range 1.3 to 1.9 km/s for

‘river boulders’ given by Dhawan et al., 1983, presum-ably due to differences in porosities. In the latter case, the ‘silty sand matrix’ is presumably absent. These last

authors also give data for phyllites, which do not appear on the foregoing figures or tables of V_pdata.

1.6 Some preliminary conversions