Abstract
Coda wave interferometry (CWI) is a high-resolution technique that aims at tracking small changes in a diffusive medium from the time correlation of seismic waveforms. CWI has been widely used in recent years to monitor the fine-scale evolution of fault zones and more recently of deep reservoirs. However, to provide a quantitative interpretation of the reservoir, direct modeling of physical effects like the influence of temperature on seismic wave scattering is required to investigate temperature effects from measurements of velocity changes. Here, we propose to quantify the impact of thermo-elastic deformation on CWI measurements by comparing experimental results obtained from a previous study on Westerly Granite to a numerical approach based on two combined codes (SPECFEM2D and Code_Aster) for modeling wave propagation in complex media during thermo-elastic deformation. We obtain two major results. First, we show that multiple reflections on the boundaries of our simplified numerical sample reproduce well the wave scattering properties of the experimental granitic sample characterized by a complex mineral assembly and a large set of microcracks. We based our comparison on the wave diffusion model that describes both the experimental and numerical samples (similarity in energy density function and mean free path). We also show that both samples share a similar thermo-elastic behavior, but only after the second heating and cooling cycle. Second, the stretching technique used for CWI measurements on both samples reveals reversible time shifts correlated with the thermo-elastic deformation of the sample. However, the influence of thermo-elastic deformation is different between our numerical proxy and the experimental sample. We discuss the role of irreversible deformation (e.g., microcracking) for the observed discrepancy by introducing temperature dependence of elastic moduli in the model. These results suggest that there are open perspectives to monitor thermal strain in geothermal reservoirs using CWI.
Highlights
At the laboratory scale, seismic methods, both active and passive, provide a range of monitoring techniques to analyze the influence of temperature on a rock sample
This study consists of a comparison of Coda wave interferometry (CWI) results obtained from two recently developed approaches and aims at monitoring changes within a dispersive Westerly Granite sample heated up to 450 °C
Griffiths et al (2018) highlighted strong irreversible changes during the first cycle and reversible changes in second and third cycles. This laboratory experiment is duplicated in a numerical scheme developed by Azzola et al (2018) that aims to explain part of the CWI signal measured in the experiments
Summary
Seismic methods, both active (ultrasonic wave velocity measurements) and passive (acoustic emission monitoring), provide a range of monitoring techniques to analyze the influence of temperature on a rock sample. Crystals present in a granitic rock sample, for example, play a specific role in the thermal microcracking, and act as many distinct scatterers, contributing to the diffusion of the wavefield propagating within the sample This method is appropriate to follow both reversible phenomena such as elastic deformation, and irreversible phenomena such as thermo-elastic effects, which deeply impact the propagation of elastic waves and can be studied using late arrivals in the coda. Grêt et al (2006) used CWI on an Elberton Granite sample heated to 90 °C to measure the influence of temperature changes on ultrasonic wave velocity They interpreted the arrival time perturbations measured in the coda of strongly scattered waves recorded while heating as an apparent decrease in wave velocity. While large and mostly permanent changes in the waveforms were measured during the first cycle, interpreted as an apparent reduction in velocity with temperature, the reductions in wave velocity during the following cycles were lower in amplitude (Griffiths et al 2018)
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