S-wave velocities

Lee and Collett (2001) measured the compressional (P-wave) and shear (S-wave) velocities of natural hydrates in sediments (33% average total porosity) at the Mallik 2L-38 well. The P-wave velocity of nongas-hydrate-bearing sediment with 33% porosity was found to be about 2.2 km/s. The compressional velocity of gas-hydrate-bearing sediments with 30% gas hydrate concentration (water-filled porosity of 23%) was found to be about 2.7 km/s, and 3.3 km/s at 60% concentration (water-filled porosity of 13%), that is, about a 20% or 50% increase to nongas-hydrate-bearing sediment. The shear velocity was found to increase from 0.81 to 1.23 km/s. 

The Emici-Roccamonfina zone has a crustal thickness of about 30 km. The uppermost mantle is characterised by a thin layer of material with relatively low S-wave velocity (Vs = 3.95 km/sec), which passes into a thick lid that has higher S-wave velocities (Vs = 4.40-4.65 km/sec). This upper mantle structure is unique in the circum-Tyrrhenian area (Panza et al. 2004 Chap. 10). 

Fig. 10.3. Sketch of S-wave velocities (km/s) beneath some Italian volcanic areas. Simplified after Panza et al. (2003, 2004).

Most seismological constraints on mantle composition are derived by comparison of values of seismic wave velocities inferred for particular regions within the Earth to the values measured in the laboratory for particular minerals or mineral assemblages, with such comparisons being made under comparable regimes of pressure (P) and temperature (T). The primary parameters of interest, then, are the compressional (or P-) wave velocities (Vp) and the shear (or S-) wave velocities (Ej). These wave velocities are simply related to the density (p) and to the two isotropic elastic moduli, the adiabatic bulk modulus (Ks)... 

Fig. 6. Cross-sections through the S-wave velocity perturbation models. (See caption for Figure 5 for a description of the cross-sections.)...

Fig. 2) from this analysis has a 42 km thick crust and a 120 km thick high-velocity upper-mantle lid, giving a total thickness of 162 km for the seismic lithosphere. The P and S velocities beneath the Moho are 8.09 km s and 4.62kms , respectively and the compressional and shear velocity gradients in the lid are 0.0008 s and 0.0013 s , respectively. Below the lid, the S-wave velocity drops to at least 4.45 km s at 250 km depth, but no decrease in the P-wave velocity is required by the data. Below 160 km depth, the P-wave gradient increases to 0.0015 s and increases again to 0.0035 s between 250 km depth and the 410 km discontinuity. 

Fig. 3. (a) Sensitivity test of the higher-mode waveforms to the depth to the base of the upper-mantle lid for the SLR seismogram of the 18 July 1986 earthquake (Fig. 1, event 2). The continuous line is the observed waveform, the dotted line is the synthetic for the southern Africa velocity model of Qiu et al. (1996), and the dashed line is the synthetic for the same velocity model but with the lid base increased to the depth indicated at the left of each seismogram, (b) Same as (a) but for the SLR seismogram of the 10 March 1989 earthquake (Fig. 1, event 5). (c) Same as (a) but for the SUR seismogram of the 24 July 1991 earthquake to the minimum S-wave velocity of the low-velocity zone (LVZ). 

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). 

We use the absolute P-wave velocity and the ratio between P- and S-wave velocities (Pp/Ps) to infer the composition of the lower crust beneath the Marcy Anorthosite (New York State,... 

Figure 2 Depth versus P- and S-wave velocity and density for the PREM model (after Dziewonski and Anderson, 1981 and Masters and Shearer, 1995).

Over the past two decades this work has been significantly extended so that now it is widely accepted that Archaean subcontinental lithosphere is thicker, older, colder, more chemically depleted, less dense, and has higher seismic P- and S-wave velocities than its Phanerozoic counterpart. More recently it has been argued that Proterozoic lithosphere is intermediate in composition between Archaean and Phanerozoic (see Table 3.3 for a summary). 

Physical properties provide a lithological and geotechnical description of the sediment. Questions concerning the composition of a depositional regime, slope stability or nature of seismic reflectors are of particular interest within this context. Parameters like P- and S-wave velocity and attenuation, elastic moduli, wet bulk density and porosity contribute to their solution. 

In what follows the theoretical background of the most common physical properties and their measuring tools are described. Examples for the wet bulk density and porosity can be found in Section 2.2. For the acoustic and elastic parameters first the main aspects of Biot-Stoll s viscoelastic model which computes P- and S-wave velocities and attenuations for given sediment parameters (Biot 1956a, b, Stoll 1974, 1977, 1989) are summarized. Subsequently, analysis methods are described to derive these parameters from transmission seismograms recorded on sediment cores, to compute additional properties like elastic moduli and to derive the permeability as a related parameter by an inversion scheme (Sect. 2.4). 

Acoustic and elastic properties are directly concerned with seismic wave propagation in marine sediments. They encompass P- and S-wave velocity and attenuation and elastic moduli of the sediment frame and wet sediment. The most important parameter which controls size and resolution of sedimentary structures by seismic studies is the frequency content of the source signal. If the dominant frequency and bandwidth are high, fine-scale structures associated with pore space and grain size distribution affect the elastic wave propagation. This is subject of ultrasonic transmission measurements on sediment cores (Sects. 2.4 and 2.5). At lower frequencies larger scale features like interfaces with different physical properties above and below and bed-forms like mud waves, erosion zones and ehatmel levee systems are the dominant structures imaged... 

S-wave velocity and attenuation and elastic moduli - with physical and sedimentological parameters like mean grain size, porosity, density and permeability. 

As the acoustic properties of water-saturated sediments are strongly controlled by the amount and distribution of pore space, cross plots of P-wave velocity and attenuation coefficient versus porosity clearly indicate the different bulk and elastic properties of terrigenous and biogenic sediments and can thus be used for an acoustic classification of the lithology. Additional S-wave velocities (and attenuation coefficients) and elastic moduli estimated by least-square inversion specify the amount of bulk and shear moduli which contribute to the P-wave velocity (Breitzke 2000). 

Fig. 2.20 (a) S-wave velocities and (b) attenuation coefficients (at 400 kHz) derived from least square inversion based on Biot-Stoll s theory versus porosities for the four sediment cores 40KL, 47KL, GeoB2821-l, and PS2567-2. NFO and FNO as in Figure 2.19. Modified after Breitzke (2000). 

Fig. 2.21 (a) S-wave velocity versus P-wave velocity and (b) shear modulus versus bulk modulus derived from least... 

Bulk parameters are based on a volume-oriented model which only depends on the total amount of pore fluid and sediment grains. Acoustic/elastic parameters like P- and S-wave velocity and attenuation are based on a micro-structure oriented model which considers both the amount and distribution of pore fluid and sediment grains. 

Once the experiment heating is completed a complete characterization of the pillar will be carried out. This will included visual observations, core sampling from the pillar, micro-crack density, and p-wave and s-wave velocity variation. 

The bulk modulus Ki of the rock matrix in terms of the grain density, Pj, the P- and S-wave velocities of the dry skeleton (v, vj, and the porosity allow us to constrain the double porosity model by use of the well log data and the equation. 

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