Novel laboratory equipment has been modified to allow both torsional and flexural oscillation measurements at sub-microstrain amplitudes, thereby providing seismic-frequency constraints on both the shear and compressional wave properties of cylindrical rock specimens within the linear regime. The new flexural mode capability has been tested on experimental assemblies containing fused silica control specimens. Close consistency between the experimental data and the results of numerical modelling with both finite-difference and finite-element methods demonstrates the viability of the new technique. The capability to perform such measurements under conditions of independently controlled confining and pore-fluid pressure, with emerging strategies for distinguishing between local (squirt) and global (specimen-wide) fluid flow, will have particular application to the study of frequency-dependent seismic properties expected of cracked and fluid-saturated rocks of the Earth's upper crust.
SUMMARY Seismic velocity anisotropy measurements are made of a fractured metamorphic formation from the 2.5‐km‐deep International Continental Scientific Drilling Programme (ICDP) borehole in Outokumpu, Finland. Three component walk‐away vertical seismic profile (VSP) measurements are made along two source‐line azimuths at three receiver depths (1000, 1750 and 2500 m) and incidence angle‐dependent qP‐ and qS‐wave velocities are extracted with a τ–p method. The highest qP‐wave anisotropy, 13.6 per cent (vfast= 6160 m s−1, vslow= 5370 m s−1), is measured between 1000‐ and 1750‐m depth, with anisotropy of up to 9.4 per cent (vfast= 6090 m s−1, vslow= 5540 m s−1) measured between 1750 and 2500 m depth. The top ∼1300 m of the region is composed of a homogeneous, strongly intrinsically anisotropic biotite‐rich schist, and is sampled by the shallowest walk‐away profile. Anisotropy of up to 11.1 per cent (vfast= 5950 m s−1, vslow= 5320 m s−1) is measured by the walk‐away VSP between 50 and 1000 m depth, along with shear wave splitting averaging 5 per cent (180 m s−1). Laboratory‐derived intrinsic anisotropy of the schist cannot by itself explain the degree or orientation of the anisotropy measured in the walk‐away VSPs, however, a model which modifies the intrinsic stiffnesses by the inclusion of a single set of dipping, aligned cracks allows the observed in situ velocities to be reproduced. Forward modelling of the qP‐wave walk‐away VSP measurements from 50 to 1000 m depth is undertaken using an effective medium model to develop a 3‐D velocity model of this region. An orthorhombic medium is used to represent the intrinsic anisotropy of the biotite‐rich schist, and a single set of aligned cracks is added to give a bulk elastic stiffness. The resulting model predicts the schist to have an overall anisotropy of 16.8 per cent, with qP‐wave velocities of up to 6315 m s−1. The accuracy of the model is assessed through its fit to the walk‐away VSP measurements as well as a comparison to known geology of the region.
In the field, the seismic waves used for active-source imaging typically contain frequencies from 10 to about 100 Hz, with corresponding wavelengths of tens of meters. This contrasts greatly with the ultrasonic (∼ 1-MHz) wave-speed measurements carried out in the laboratory, with millimeter wavelengths. The purpose of the laboratory measurements is, of course, to provide insight into seismic wave speeds in situ. However, with the presence of a pore fluid, velocity measurements are sensitive to the frequency at which velocity data are collected. A study focuses on such fluid-flow-related dispersion by performing a broadband measurement in the laboratory from millihertz (mHz) to megahertz (MHz) frequencies on a natural quartzite and on a synthetic sintered glass-bead sample. Thermal cracks that have small aspect ratios of about 10−4 to 10−3 were introduced in both samples, which are of low porosities (1% to 2%) even after thermal cracking. A seismic-frequency forced-oscillation method is combined with a high-frequency ultrasonic technique, providing access to a wide frequency range. Under water-saturated conditions, the observed seismic wave speeds display substantial variations between seismic and ultrasonic frequencies in the cracked quartzite. A systematic increase in shear modulus, attributed to the suppression of fluid flow, has been monitored on the cracked glass-bead specimen with both argon and water saturation at ultrasonic frequency.
The effect of pore fluids on acoustic wave dispersion in rocks with low aspect ratio crack porosity is important for the interpretation of laboratory and field observations in hard rock mineral exploration environments. Here we make laboratory measurements of shear modulus dispersion at frequencies 0.01–1 Hz and at 1 MHz with different saturating fluids (dry, argon, and water saturated) in two thermally cracked quartzite samples with ~2% total porosity. Measurements are made across a range of effective pressures (10–150 MPa), with the resulting very low permeabilities of the samples varying from 1–300 × 10−21 m2. Moduli across the 0.01–1 Hz band were typically independent of frequency. The shear moduli measured at sub‐Hz frequencies are unaffected by fluid saturation, as expected for the saturated isobaric (Gassmann) regime. In marked contrast, water saturation of the cracked rocks results in very large increases in the shear moduli measured at 1 MHz and low effective pressures, indicative of saturated isolated conditions. Thus, at an effective pressure of 20 MPa, the shear moduli for the two water‐saturated quartzites increase by 74% and 98% from 1 Hz to 1 MHz. The contrast in elastic moduli between dry and water‐saturated conditions is well represented by the theoretical model developed by Walsh and others. The observed dispersion highlights the need for care in seismological application of results obtained at MHz frequencies from laboratory ultrasonic measurements.
Purple Crow Lidar (PCL) measurements of the vibrational Raman-shifted backscatter from water vapour and nitrogen molecules allows height profiles of the water-vapour mixing ratio to be measured from 500 m up into the lower stratosphere. In addition, the Raman nitrogen measurements allow the determination of temperature profiles from about 10 to 40 km altitude. However, external calibration of these measurements is necessary to compensate for instrumental effects, uncertainties in our knowledge of the relevant molecular cross sections, and atmospheric transmission. A comparison of the PCL-derived water-vapour concentration and temperature profiles with routine radiosonde measurements from Detroit and Buffalo on 37 and 141 nights, respectively, was undertaken to provide this calibration. The calibration is then applied to the measurements and monthly mean-temperature and water-vapour profiles are determined.PACS Nos.: 42.68.Wt, 42.79.Qx
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