Rock Quality, Seismic Velocity, Attenuation and Anisotropy

Nick Barton

• Publisher: CRC Press
• Year: 2006
• Print ISBN:
  9780415394413
• eBook ISBN:
  9781134160129
• Topic: Physical Sciences
• Subtopic: Chemical & Biochemical Engineering

Seismic measurements take many forms, and appear to have a universal role in the Earth Sciences. They are the means for most easily and economically interpreting what lies beneath the visible surface. There are huge economic rewards and losses to be made when interpreting the shallow crust or subsurface more, or less accurately, as the case may be.

Part I

1 Shallow seismic refraction, some basic theory, and the importance of rock type

‘Nature has left us an incomplete and often well-concealed record of her activities, and no ‘as constructed’ drawings!’ (Stapledon and Rissler, 1983)

‘Tenders for the Tay pipeline crossing did not allow time for boreholes to locate bedrock. Seismic refraction took one day to confirm that the trench would not encounter rock. The pipeline was laid in sediments.’ (Gardener, 1992)

‘The time may come when the various relations between geophysical parameters and rock properties can be usefully combined into a single classification system.’ (Darracott and Orr, 1976)

1.1 The challenge of the near-surface in civil engineering

Refraction seismics is by far the oldest method used in exploration seismology, with its origin traced to R. Mallet from 1848. Shallow refraction seismic measurements using first arrival, compressional P-wave velocities close to the surface often give a remarkable picture of near surface conditions due to some fortuitous interactions of physical phenomena. Firstly, weathering and the usual lack of significant stress near the surface has allowed joint systems, shear zones and faults to be exaggerated in both their extent and severity. Secondly, stress levels are low enough to allow joints and discontinuities to be seismically visible due to their measurable apertures.

So-called acoustic closure occurs at greater depths than those usually penetrated by conventional hammer seismic, unless rock strengths are rather low (e.g., New and West, 1980; Hudson et al., 1980). (At this juncture, we need to differentiate between two ‘J.A. Hudson’ authors, one in geophysics, the other in rock engineering, and both very prominent in their chosen fields. We will occasionally refer to ‘rock’ Hudson in Part I, and later in Part II to ‘seismic’ Hudson).

Since micro-fractures and rock joints are sensitive to stress levels, the more closed state of the discontinuities that are perpendicular to the major stress, and the more open state of those that are parallel will give the rock mass anisotropic stiffness. Consequently the rock mass will frequently display anisotropic seismic velocities. By implications, hydraulic conductivities and deformation moduli that show anisotropic distributions will be, at least in part, detectable by seismic measurements. Anisotropy will also be caused by layered inter-beds, foliation and schistocity, and of course by a dominant joint set. Simple examples of (azimuthal) anisotropy, applicable in civil engineering, will be given in Chapter 3, while largerscale examples of anisotropy detection will be described in much greater detail, and from various fields of the earthsciences, in Chapters 13, 14 and 15 in Part II.

Despite the obvious challenges of seismic interpretation in fractured and faulted petroleum reservoirs at many kilometers depth, or of mid-ocean ridge investigations beneath three kilometers of ocean, many geophysicists insist that obtaining high resolution images from ground level to just 50 m depth, is still one of the major challenges of modern geophysics. This happens to be the layer of the subsurface closest to most of our civil engineering endeavours, from tunnels, to dams, to the foundations for high buildings.

Undoubtedly, the ‘0 to 50m’ challenge is mainly due to the extreme variability of the near-surface, resulting from the contrasting geological materials and weathering grades that are often present. There is also a velocity gradient that is extreme compared to anything found at greater crustal depths, where consolidation effects smooth out some of the differences. The first 5 m of unconsolidated dry beach sand may see velocity increase from 150 m/s to 300 m/s, (Bachrach et al., 2000), giving a gradient of 30 s-1, which may be an order of magnitude higher than the gradient over the next 50 to 100 m, where weathered and jointed rock may typically be found.

There are an infinite number of challenges in the nearsurface. Some of the worse may be karst phenomena in limestones, or the ‘inverse’ problems of core-stone anomalies in the case of sparsely jointed but deeply weathered granites and gneisses. These features have caused tunnelling surprises in numerous countries, with nearly as numerous arbitrations as a result. Although completely weathered Grade V is an expected feature beneath the Grade VI soil in tropical terrains, Grade V saprolite sometimes confusingly swaps places with the usually deeper, and almost unjointed Grade I or II. (Saprolite is a weak, water sensitive, weathered in-place, sometimes beautifully structured and coloured relic of the rock).

If this reversal of weathering grades appears in a tunnel arch beneath massive, high velocity core-stones, or if there is a generally very undulating rock surface, with frequent tunnel penetrations into weathered materials, there can be major delays. A tunnel collapse is difficult to avoid when water is present, unless preparations have been made, as a result of the more frequent exploratory drilling demanded when seismic anomalies such as these are suspected.

Pre-injection ahead of the tunnel face, and heavier tunnel support, would be the very basic requirements in a drill-and-blasted tunnel. (This is one of the purposes of the ‘Q-system’ of rock mass characterization and tunnel support selection). In the case of a TBM (tunnel boring machine) excavation, a change to a closed mode in the case of a hybrid machine with earth-pressurebalance (EPB) would be needed, especially if the weathered depressions in the bedrock contained water, as is usually the case.

Best advice of all, as a direct result of a seismic refraction survey, would be to drive a deeper tunnel from the start. It is easy to imagine subway station construction under such heterogeneous conditions. It could be extremely time-consuming, and even dangerous. The cost of deeper access to the stations, via longer escalators, would be a small price to pay for much reduced tunnelling and station costs.

Sjøgren, 2000 suggested the following list of essential information expected from near-surface seismic surveys, performed for civil engineering geotechnical investigations:

• The velocities of the overburden layers, including the upper, less consolidated rock layers.
• The thickness of the various overburden velocity layers, and the total depth to the main refractor.
• A detailed determination of the velocity distribution in the main refractor.
• An estimate of the uncertainty of the velocity and depth determinations.
• An analysis of the (velocity-) depth structure.
• An assessment of velocities in vertical and lateral directions in relation to the geology.
• Seismic results in relation to results from other investigations, if available.
• Conclusions and recommendations resulting from the investigation that are of importance to the project.

Although reflection methods have eventually dominated the field of exploration seismics due to the various needs involved with deeper exploration, there is ‘universal’ use of shallow refraction seismic in sub-surface investigations for civil engineering projects around the world, due to its apparent simplicity and low cost. Furthermore, refraction seismics can be used to remove (from the more deeply focussed reflection data), the ‘adverse’ effect of the first meters or tens of meters of the heterogeneous weathered layer, where differences in the original rock quality may cause tens of meters of sub-surface ‘topography’ in the case of on-land exploration.

1.2 Some basic aspects concerning elastic body waves

It is usually assumed that the strains associated with the passage of a seismic wave are of minute, sub-micron magnitude, and except in the neighbourhood of the source, the strains are generally assumed to be elastic. Based on this assumption, the velocities of propagation of seismic waves are determined by the appropriate elastic moduli and densities of the materials passed through. The general form of the classic equations linking these three quantities is V = (E/γ)^½. Compressional bodywaves (primary or P-waves) propagate by alternating compression and dilation (Figure 1.1 a) in the direction of the waves.

Figure 1.1 Elastic deformations and particle motions associated with the propagation of body waves: a) P-wave, b) S-wave. Based on Bott, 1982.

The oscillating uniaxial strain involved in the case of a confined body, means that the axial modulus (ψ) controls the velocity of propagation, thus:

Shear bodywave waves, termed secondary, transverse or S-waves propagate by a sinusoidal pure shear strain (Figure 1.1 b) in a direction perpendicular to the direction of the waves. The shear modulus (μ), which is given by the ratio of shear stress (τ) divided by the shear strain (tan θ), will therefore control the (lower) velocity of propagation, thus:

The third important elastic modulus influencing the conversion between dynamic properties is the bulk modulus (K), defined as the ratio of the volumetric stress (P) and the volumetric stra...

Table of contents

• Cover Page
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Introduction
• Nomenclature
• Part I
• Part II
• Appendix A – The Qrock parameter ratings
• Appendix B – A worked example
• References
• Colour Plates