Seismic velocities and electrical resistivity of recent volcanics and their dependence on porosity, temperature, and water saturation

Variation of P‐wave velocities and electrical resistivities of several suites of water‐saturated recent volcanics was investigated. Both P‐velocities and resistivities exhibited strong dependence on porosity. Resistivity was also dependent upon degree of water saturation and temperature. P‐wave velocities, while showing a strong dependence on porosity, appear to be independent of water saturation and temperature. Volcanics, in general, exhibit higher resistivities compared to other igneous rocks and sediments. Electric resistivity of fine‐grained basalts is anomalously low, probably due to higher content of disseminated iron. Pyroclastics and volcanic breccia, on the other hand, exhibit higher resistivities in relation to fine‐grained basalts.

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Laboratory study of the influence of changing the injection rate on P-wave velocities and water saturation in a Limestone

ASEG Extended Abstracts ◽

10.1071/aseg2012ab083 ◽

2012 ◽

Vol 2012 (1) ◽

pp. 1-4 ◽

Cited By ~ 2

Author(s):

Sofia Lopes ◽

Maxim Lebedev

Keyword(s):

Laboratory Study ◽

Water Saturation ◽

Injection Rate ◽

P Wave ◽

Wave Velocities ◽

P Wave Velocities

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Seismic mapping and modeling of near‐surface sediments in polar areas

Geophysics ◽

10.1190/1.1567226 ◽

2003 ◽

Vol 68 (2) ◽

pp. 566-573 ◽

Cited By ~ 31

Author(s):

Tor Arne Johansen ◽

Per Digranes ◽

Mark van Schaack ◽

Ida Lønne

Keyword(s):

Water Saturation ◽

Geological Mapping ◽

P Wave ◽

Unconsolidated Sediments ◽

Polar Regions ◽

Seismic Velocities ◽

S Wave ◽

Delta Front ◽

Near Surface ◽

Water Saturated

A knowledge of permafrost conditions is important for planning the foundation of buildings and engineering activities at high latitudes and for geological mapping of sediment thicknesses and architecture. The freezing of sediments is known to greatly affect their seismic velocities. In polar regions the actual velocities of the upper sediments may therefore potentially reveal water saturation and extent of freezing. We apply various strategies for modeling seismic velocities and reflectivity properties of unconsolidated granular materials as a function of water saturation and freezing conditions. The modeling results are used to interpret a set of high‐resolution seismic data collected from a glaciomarine delta at Spitsbergen, the Norwegian Arctic, where the upper subsurface sediments are assumed to be in transition from unfrozen to frozen along a transect landward from the delta front. To our knowledge, this is the first attempt to study pore‐fluid freezing from such data. Our study indicates that the P‐ and S‐wave velocities may increase as much as 80–90% when fully, or almost fully, water‐saturated unconsolidated sediments freeze. Since a small amount of frozen water in the voids of a porous rock can lead to large velocity increases, the freezing of sediments reduces seismic resolution; thus, the optimum resolution is obtained at locations where the sediments appear unfrozen. The reflectivity from boundaries separating sediments of slightly different porosity may depend more strongly on the actual saturation rather than changes in granular characteristics. For fully water‐saturated sediments, the P‐wave reflectivity decreases sharply with freezing, while the reflectivity becomes less affected as the water saturation is lowered. Thus, a combination of velocity and reflectivity information may reveal saturation and freezing conditions.

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Seismic velocities of unconsolidated sands: Part 2 — Influence of sorting- and compaction-induced porosity variation

Geophysics ◽

10.1190/1.2364849 ◽

2007 ◽

Vol 72 (1) ◽

pp. E15-E25 ◽

Cited By ~ 16

Author(s):

Michael A. Zimmer ◽

Manika Prasad ◽

Gary Mavko ◽

Amos Nur

Keyword(s):

Compressional Wave ◽

P Wave ◽

Unconsolidated Sediments ◽

Seismic Velocities ◽

Porosity Variation ◽

Unconsolidated Sands ◽

Wave Velocities ◽

Grain Size Distributions ◽

Gassmann Fluid Substitution ◽

Water Saturated

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 [Formula: see text]. Over this rangeof porosities, the velocities of the dry samples at a given pressure vary by [Formula: see text]. 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.

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