- journal_title：Geophysics
- Contributor：Michael A. Zimmer ; Manika Prasad ; Gary Mavko ; Amos Nur
- Publisher：Society of Exploration Geophysicists
- Date：2007-
- Format：text/html
- Language：en
- Identifier：10.1190/1.2364849
- journal_abbrev：Geophysics
- issn：0016-8033
- volume：72
- issue：1
- firstpage：E15
- section：BOREHOLE GEOPHYSICS AND ROCK PROPERTIES
Unaccounted-for porosity variation in unconsolidated sediments can cloud the interpretation of the sediment's seismic velocities for factors such as fluid content and pressure. However, an understanding of the effects of porosity variation on the velocities can permit the remote characterization of porosity with seismic methods. We present the results of a series of measurements designed to isolate the effects of sorting- and compaction-induced porosity variation on the seismic velocities and their pressure dependences in clean, unconsolidated sands. We prepared a set of texturally similar sand and glass-bead samples with controlled grain-size distributions to cover an initial porosity range from 0.26 to 0.44. We measured the compressional- and shear-wave velocities and porosity of dry samples over a series of hydrostatic pressure cycles from e-formula" id="inline-formula-1">embed/mml-math-1.gif" />. Over this rangeof porosities, the velocities of the dry samples at a given pressure vary by e-formula" id="inline-formula-2">
embed/mml-math-2.gif" />. However, the water-saturated compressional-wave velocities, modeled with Gassmann fluid substitution, demonstrate a consistent increase with decreasing porosity. In both the dry and water-saturated cases, the porosity trend at a given pressure is approximately described by the isostress (harmonic) average between the moduli of the highest-porosity sample at that pressure and the moduli of quartz, the predominant mineral component of the samples. Empirical power-law fit coefficients describing the pressure dependences of the dry bulk, shear, and constrained (P-wave) moduli from each sample also demonstrate no significant, systematic relationship with the porosity. The porosity dependence of the water-saturated bulk and constrained moduli is primarily contained in the empirical coefficient representing the modulus at zero pressure.