### ASA 124th Meeting New Orleans 1992 October

## 1pAO9. Permeability and porosity of clayey sediments in seismo-acoustics.

**William R. Bryant
**

**
**
*Texas A&M Univ., College Station, TX 77843
*

*
NRL, Stennis Space Center, MS 39529
*

*
*
**Michael D. Richardson
**

**
**
*Naval Res. Lab., Stennis Space Center, MS 39529
*

*
*
Permeability of fine-grained marine sediments is the most controversial
and difficult variable to define for seismo-acoustics. Theoretical formulations
of permeability are extremely complex for fine-grained sediments and usually
bear small resemblances to real material. Values of permeability from
consolidation tests and direct analysis of clayey marine sediments greatly
differ from results determined by the Kozeny--Carman equation. These
differences are not accountable by deviations from Darcy's law, electrokinetic
coupling, or viscosity but are related to changes in microfabric. Porosities
relationship to permeability presents natures largest range between cause and
affect. In fine-grained marine sediments permeability changes at a rate from 2
to the 12th power of porosity. A 50% change in porosity, from a mud to
mudstone, can result in a change in permeability of 10[sup -3] to 10[sup -12]
cm/s, which is analogous of going from the speed of light to 60 mph. The
effects of permeability on P-wave velocities of high porosity (85% to 70%)
sediments, particularly Pacific red clays, appears minimal. A measured change
of permeability from 7x10[sup -4] to 1.2x10[sup -7] cm/s has less of an effect
on the velocity (average 1469 m/s) than does a density change from 1.25 to 1.36
g/cc. P-wave velocities of red clays, spanning an age of 45 m.y. with
permeabilities of above, are essentially constant over a depth of 60 m while
S-wave velocities of the same material increase from 5 to 180 m/s. The best
relationship between permeability and porosity of marine clays for
seismo-acoustic determinations is k=6.63x10[sup -9] (beta)[sup 8.1], where
k=permeability (cm/s) and (beta)=decimal porosity.