Velocity‐porosity relationships for water‐saturated siliciclastic sediments

Abstract

Abstract. Seeking a global empirical relationship between compressional wave velocity and porosity for siliciclastic sediments, we have brought together an extensive suite of both new and published log-and core-based data. We undertook a detailed statistical analysis of Ocean Drilling Program data from Amazon Fan to examine variables affecting compressional velocity in shallow, unconsolidated sediments. We identify three dominant variables (porosity, shale fraction, and consolidation history) and present two empirically determined boundary curves (one for normally consolidated sediments and a second for highly consolidated environments (e.g., accretionary prisms)). These two empirical relationships predict the compressional velocity of siliciclastic sedimentary rocks with water-filled pores as a function of porosity and clay content for the full range of observed porosities. Velocities of siliciclastic sedimentary rocks decrease rapidly with both increasing porosity and increasing clay content. At fractional porosities higher than about 0.4, fluid dominates the elastic properties, and velocity exhibits a subtle dependence on porosity. Remarkably, the Amazon Fan data show that clay content has no direct influence on velocity at high porosities. Both clay content and sorting do indirectly affect velocity, through their control of porosity. Burial affects velocity not only by compaction-related porosity decrease but also by pressure-induced increase of intergrain coupling. Because of the sensitivity of velocity to consolidation history, particularly at intermediate fractional porosities of about 0.30-0.40, no single velocity-porosity relationship can apply to all high-porosity sediments. The two proposed relationships fit the majority of published and new data. They are applicable, however, only for normally pressured, in situ conditions and water-filled pores.

Seismic velocities within the sedimentary succession of the Canada Basin and southern Alpha-Mendeleev Ridge, Arctic Ocean: evidence for accelerated porosity reduction?

“…Unlithified, brine-saturated sands exhibit higher seismic velocity than do clays for a given state of consolidation (Erickson & Jarrard 1998), and this phenomenon is evident in the velocity ratio versus depth trends shown on Fig. 12.…”

“…Erickson & Jarrard (1998) found that for this case the dominant factors affecting seismic velocity are clay content and porosity, and that clay content is a negligible factor for porosities greater than the critical porosity (i.e. the transitional limit above which the sediments are essentially in suspension).…”

“…The succession is also thinner beneath the shelf, so chemical compaction may not be as significant a factor as in subregion 1. However, matrix velocity is an intrinsically difficult measurement to define in shales (Hansen 1996;Erickson & Jarrard 1998;Tudge & Tobin 2013) and it is not strictly equivalent to final velocity, which is a field-scale extrapolation based on an assumption of exponential behaviour. The small variations in the final velocity estimates may be of no physical significance, but the consistency of the estimates and their compatibility with the matrix velocities from Issler (1992) demonstrate that the velocity-depth models do generate plausible extrapolations.…”

“…Sand-clay mixtures exhibit intermediate velocities between the bounds for pure sand and clay. Erickson & Jarrard (1998) used an extensive compilation of core and borehole measurements to determine that the lithological effect is significant at porosities less than about 40 per cent, which provides an approximate measure of critical porosity. Critical velocity in their data set is about 2.1 km s −1 .…”

The spatial extent of the Deep Western Boundary Current into the Bounty Trough: new evidence from parasound sub-bottom profiling

“…However, many analyses of relation between P-wave velocity and density of clastic sediments have shown that P-wave velocity can directly be derived from density via empiric relations (e.g. Erickson and Jarrard 1998;Hamilton 1978;Nafe and Drake 1957). This implies that the velocity shows exactly the same trend as the density and that the density is an adequate proxy for the impedance.…”