Summary Laboratory measurements of seismic attenuation were performed on borosilicate samples using the forced oscillation method. First, we studied the effect of partial water-air saturation. Significant and frequency-dependent attenuation was only observed at nearly full water saturation and at low pore fluid pressures (≤0.5 MPa). A peak in attenuation, quantified by the inverse quality factor 1/Q, of 0.02 was observed at about 1.5 Hz. A similar experiment was performed for water-nitrogen saturation and, again, at low fluid pressures and near full water saturation, a peak in attenuation of 0.015 was observed at about 3 Hz. Increasing the confining stress and fluid pressure equally, and thus keeping the effective stress unchanged, resulted in negligible attenuation. An attenuation mechanism that can explain these results, possibly better than wave-induced fluid flow (WIFF) at the mesoscopic scale, is the dissolution and exsolution of microscopic gas bubbles in response to wave-induced fluid pressure variations.
The combination of laboratory and numerical experiments is a powerful tool to study rock physical processes. While in laboratory experiments it is very difficult (or impossible) to control all the physical processes, in numerical experiments they can be controlled exactly. Numerically, it is even possible to study different physical processes separately from each other, which otherwise coexist in nature. Here, our objective is to understand the fluid-related physical process responsible for intrinsic attenuation in saturated rocks at seismic frequencies. For that, we measured in the laboratory local transient fluid pressure along a partially saturated rock sample. Furthermore, we compared the laboratory results with values calculated numerically using a 3D poroelastic numerical model to approximate the partially saturated rock sample.
We have designed and set up a new pressure vessel for relatively big samples to measure seismic wave attenuation in rocks at frequencies between 0.01 and 100 Hz and to verify the occurrence of fluid-flow induced by stress field change. A dynamic stress is applied at the top of the rock cylinder by a piezoelectric motor that can generate either a stress step of several kPa in few milliseconds or a monofrequency force. The machine measures force through a load cell, and the bulk axial shortening with a strain sensor. The sample is a cylinder 250 mm long and 76 mm diameter. Also, five pressure sensors are buried to measure pore pressure changes in time related to stress field change. The sample is sealed in a pressure vessel that can reach confining pressures of 25 MPa. We present datasets collected at room pressure and temperature. Two attenuation data curves measured on reference samples demonstrate the accuracy of the apparatus. Seismic wave attenuation measurements conducted for frequencies between 0.1 and 50 Hz and strain less than 5e-6 on partially saturated Berea sandstone are presented. Additionally, some time-evolution pore-pressure curves due to stress field changes are exhibited.
The scattering of elastic waves (scattering) causes velocity dispersion that increases uncertainties of seismic analyses and could be misinterpreted as the result of other phenomena. One of these phenomena is the wave-induced fluid flow in saturated rocks, which comprise most of the rocks in the crust of our planet. Therefore, understanding scattering sources and distinguishing among different sources of velocity dispersion is critical to improve subsurface imaging to locate resources and understand subsurface processes. Here, we present and discuss measurements and numerical modeling of ultrasonic wave velocities in homogeneous and heterogeneous carbonate rocks with porosities between 3 and 26%. Ultrasonic velocities were measured at frequencies between 0.3 and 1 MHz, and numerical wave propagation simulations on the CT-scanned samples were performed using an elastic approximation and a finite difference method. The homogeneous sample and the corresponding numerical simulations exhibit negligible velocity dispersion. On the other hand, heterogeneous samples exhibit large dispersion, and the corresponding numerical simulations reproduce well the observed dispersion. We conclude that scattering has a first-order effect on the velocities of the elastic waves and should be considered when applying rock physics models in heterogenous carbonates similar to those studied here. e illustrate a method to characterize frequency-dependent ultrasonic velocities (i.e., dispersion) and show that finite-difference modeling can reproduce the laboratory-observed dispersion, including the typical frequency shift produced by scatterers and dispersive media.
ABSTRACT We measured the extensional‐mode attenuation and Young's modulus in a porous sample made of sintered borosilicate glass at microseismic to seismic frequencies (0.05–50 Hz) using the forced oscillation method. Partial saturation was achieved by water imbibition, varying the water saturation from an initial dry state up to ∼99%, and by gas exsolution from an initially fully water‐saturated state down to ∼99%. During forced oscillations of the sample effective stresses up to 10 MPa were applied. We observe frequency‐dependent attenuation, with a peak at 1–5 Hz, for ∼99% water saturation achieved both by imbibition and by gas exsolution. The magnitude of this attenuation peak is consistently reduced with increasing fluid pressure and is largely insensitive to changes in effective stress. Similar observations have recently been attributed to wave‐induced gas exsolution–dissolution. At full water saturation, the left‐hand side of an attenuation curve, with a peak beyond the highest measured frequency, is observed at 3 MPa effective stress, while at 10 MPa effective stress the measured attenuation is negligible. This observation is consistent with wave‐induced fluid flow associated with mesoscopic compressibility contrasts in the sample's frame. These variations in compressibility could be due to fractures and/or compaction bands that formed between separate sets of forced‐oscillation experiments in response to the applied stresses. The agreement of the measured frequency‐dependent attenuation and Young's modulus with the Kramers–Kronig relations and additional data analyses indicate the good quality of the measurements. Our observations point to the complex interplay between structural and fluid heterogeneities on the measured seismic attenuation and they illustrate how these heterogeneities can facilitate the dominance of one attenuation mechanism over another.
Abstract Gypsum ( CaSO 4 ·2 H 2 O ), alunite ( KAl 3 ( SO 4 ) 2 ( OH ) 6 ), and rare phosphate–sulphate sanjuanite Al 2 ( PO 4 )( SO 4 )( OH ) 9( H 2 O ) and rossiantonite ( Al 3 ( PO 4 )( SO 4 ) 2( OH ) 2 ( H 2 O ) 14 ) have recently been identified as secondary mineral deposits in different quartz‐sandstone caves in the Gran Sabana region, Venezuela. Due to the extended time scale required for speleogenesis in the hard and barely soluble quartz‐sandstone lithology, these caves are considered to be as old as 20 to 30 My. The study of these peculiar secondary mineral deposits potentially reveals important insights for understanding the interaction between deep, superficial and atmospheric processes over thousands to perhaps millions of years. In this study, chemical and petrographic analyses of potential host rock sources, sulphur and oxygen isotope ratios, and meteorological, hydrological and geographical data are used to investigate the origin of sulphates and phospho–sulphates. The results suggest that the deposition of sulphates in these caves is not linked to the quartz‐sandstone host rock. Rather, these mineral deposits originate from an external atmospheric sulphate source, with potential contributions of marine non‐sea salt sulphates, terrestrial dimethyl sulphide and microbially reduced H 2 S from the forests or peatbogs within the watershed. Air currents within the caves are the most plausible means of transport for aerosols, driving the accumulation of sulphates and other secondary minerals in specific locations. Moreover, the studied sulphate minerals often co‐occur with silica speleothems of biological origin. Although this association would suggest a possible biogenic origin for the sulphates as well, direct evidence proving that microbes are involved in their formation is absent. Nonetheless, this study demonstrates that these quartz‐sandstone caves accumulate and preserve allogenic sulphates, playing a yet unrecognized role in the sulphur cycle of tropical environments.
The study of acoustic emissions (AEs) is of paramount importance to understand rock deformation processes. AE recorded during laboratory experiments mimics, in a controlled geometry and environment, natural and induced seismicity. However, these experiments are destructive, time consuming and require a significant amount of resources. Lately, significant progresses have been made in numerical simulations of rock failure processes, providing detailed insights into AE. We utilized the 2-D combined finite-discrete element method to simulate the deformation of Stanstead Granite under varying confining pressure (Pc) and demonstrated that the increase of confining pressure, Pc, (i) shifts failures from tensile towards shear dominated and (ii) enhance the macroscopic ductility. We quantitatively describe the AE activity associated with the fracturing process by assessing the spatial fractal dimension (D-value), the temporal distribution (AE rate) and the slope of the frequency–magnitude distribution (b-value). Based on the evaluation of D-value and AE rate, we defined two distinct deformation phases: Phase I and Phase II. The influence of Pc on the spatial distribution of AE varies according to the deformation phase: for increasing Pc, D-value decreases and increases during Phases I and II, respectively. In addition, b-value decreases with increasing Pc during the entire experiment. Our numerical results show for the first time that variations of D- and b-values as a function of in situ stress can be simulated using the combined finite-discrete element approach. We demonstrate that the examination of seismicity should be carried out carefully, taking into consideration the deformation phase and in situ stress conditions.
In the past few decades, great attention has been focused on uncovering the physics of seismic wave attenuation in fluid-saturated rocks. However, the relationship among many variables affecting attenuation is still not completely clear. For instance, although the role of strain in enhancing friction dissipation is relatively well known for dry rocks, it remains unclear how and how much it affects attenuation in fluid-saturated rocks. We experimentally measured attenuation in the extensional mode in Berea sandstone at strains between [Formula: see text] and [Formula: see text], and at frequencies in the seismic bandwidth (1–100 Hz). These strains were similar to those typically observed in seismic exploration ([Formula: see text]). We also measured the transient fluid pressure caused when a stepwise stress was applied resulting in such strains. For the studied strain range, our results indicated that: (1) the overall attenuation in dry Berea sandstone increased linearly with strain, (2) the frequency-dependent component of attenuation, which was associated with fluid saturation, was approximately insensitive to strain, and (3) the overall attenuation can be considered as a sum of a frequency-independent and a frequency-dependent components.
