DAS synthetic dataset

Deliverable D1.3 of the ACT DigiMon project is a synthetic microseismic distributed acoustic sensins (DAS) dataset. There are a number of possible uses for such a dataset; for example supporting the development and testing of DAS processing algorithms, testing the efficacy of different array geometries in detecting and characterising events, or simulating a field experiment to better understand observed processes. Given the large number of possible uses it was decided that rather than simply delivering a collection of files of synthetic seismic events, it would be more valuable to deliver a modelling framework from which synthetic data can be generated as the need arises, combined with a small example dataset of a few events to demonstrate the capabilities. DAS systems record seismic wavefields and ground motion due to their sensitivity to strain along the axis of the fibre. To understand the response of DAS it is necessary to understand (1) the seismic source, (2) the path effects and (3) the site and instrument effects. In this report we discuss the modelling of the first two contributions of the DAS response; the source and path effects. We simulate the resulting particle motion and strain at the fibre location, resulting from realistic microseismic sources in geological models representative of the North Sea. The third contribution; site and instrument effects, is contained in the transfer function, which describes the mathematical relationship between the wavefield properties at the cable location to the recorded DAS output. The form of the transfer function is a key unanswered question which will be addressed in Task 1.2 of the DigiMon project

Project report and algorithms for optimizing acquisition layout and frequency

D2.7. Project report and algorithms for optimizing acquisition layout and frequency. We evaluate the capability of 3D finite difference codes to model Distributed Acoustic Sensors (DAS) at reservoir scale for monitoring of CO2 sequestration. This work builds on previous DigiMon deliverables: 1.3 - DAS synthetic dataset (Baird et al, 2020b) and 2.1 - Framework for forward modelling of the DigiMon data (Vandeweijer et al, 2021). The goals of this work include 1) evaluation of the computational load and trade-offs needed to model Distributed Acoustic Sensing (DAS) signals from a 3D (~14x14x3 km) model of a CO2 sequestration reservoir; 2) sensitivity of various DAS deployment models (borehole versus surface); 3) comparison of DAS (linear and helical) with respect to geophones for both vertical and surface installations; and 4) measurements of possible induced seismicity with DAS

Framework for forward modelling of the DigiMon data

Deliverable D2.1 adds to the main goal of WP2 of the ACT DigiMon project, which is to develop the integrated DigiMon system. The key target for WP2 is to optimally integrate various system components into a reliable and usable system. This deliverable (D2.1) describes the key forward modelling tools of the DigiMon monitoring system. In particular, the modelling tools required to simulate the data response for the individual DigiMon system components that is; Distributed Acoustic Sensing (DAS), conventional seismic, 4D gravity data, and seafloor deformation.Framework for forward modelling of the DigiMon datapublishedVersio

An Energy Based Discontinuous Galerkin Method for Coupled Elasto-Acoustic Wave Equations in Second Order Form

We consider wave propagation in a coupled fluid-solid region, separated by a static but possibly curved interface. The wave propagation is modeled by the acoustic wave equation in terms of a velocity potential in the fluid, and the elastic wave equation for the displacement in the solid. At the fluid solid interface, we impose suitable interface conditions to couple the two equations. We use a recently developed, energy based discontinuous Galerkin method to discretize the governing equations in space. Both energy conserving and upwind numerical fluxes are derived to impose the interface conditions. The highlights of the developed scheme include provable energy stability and high order accuracy. We present numerical experiments to illustrate the accuracy property and robustness of the developed scheme

Application of smoothed point interpolation methods to numerical modelling of saturated and unsaturated porous media

This study aims to develop an efficient computational framework for a rigorous coupled flow and deformation analysis of saturated and unsaturated porous media. The governing equations are derived based on equations of equilibrium, and conservation equations of mass and momentum for each phase. For numerical solution of the governing equations, the edge-based smoothed point interpolation method (ESPIM) is employed due to its numerous advantages over the classical techniques. The ESPIM was originally introduced for problems in single phase media. The extension of the technique to multiphase media is not trivial, and therefore as the first development step, ESPIM is extended for the solution of the coupled hydro-mechanical problems in saturated porous media through a novel approach for evaluation of the coupling matrix. Verification of the proposed ESPIM formulation is performed using several benchmark numerical examples. Subsequently, the method of manufactured solutions (MMS) is introduced, for the first time in geomechanics, for a systematic and more rigorous verification of the computational scheme. The proposed numerical framework is then extended to include material nonlinearity. For this purpose, a non-associative Mohr-Coulomb constitutive model is adopted and an algorithm is developed based on the modified Newton-Raphson technique to address the nonlinearities arisen from the elasto-plastic constitutive model. Stress integration is performed using the substepping method. The computational framework is then further extended to include the problems in unsaturated soil mechanics, taking account of coupling among different phases, and the hydraulic hysteresis observed in the behaviour of unsaturated soils. A framework based on the effective stress principle is followed in the formulation and a hysteretic water retention model is taken into account which includes the evolution of water retention curve (WRC) with changes of void ratio. An elasto-plastic constitutive model is employed within the context of bounding surface plasticity theory for predicting the nonlinear behaviour of soil skeleton in saturated and unsaturated porous media. The model is validated by comparing the numerical predictions with experimental or numerical data from the literature for fully and partially saturated soils. The results demonstrate the capability of the proposed numerical framework to predict essential characteristics of variably saturated soils

Exploring infrasound wavefields to characterize volcanic eruptions

Thesis (Ph.D.) University of Alaska Fairbanks, 2020Infrasound has become an increasingly popular way to monitor and characterize volcanic eruptions, especially when combined with multidisciplinary observations. Regardless of how close the infrasound instruments are to the eruption, the effects from propagation must be considered prior to characterizing and quantifying the source. In this dissertation, we focus on modeling the effects of the atmosphere and topography on the recorded infrasound waveforms in order to better interpret the acoustic source and its implications on the volcanic eruption as a whole. Alaska has 54 historically active volcanoes, one third of which have no local monitoring equipment. Therefore, remote sensing (including that of infrasound arrays) is relied upon for the detection, location, and characterization of volcanic eruptions. At long ranges, the wind and temperature structure of the atmosphere affects infrasound propagation, however, changes in these conditions are variable both in time and space. We apply an atmospheric reconstruction model to characterize the atmosphere and use infrasound propagation modeling techniques for a few recent eruptions in Alaska. We couple these atmospheric propagation results with array processing techniques to provide insight into detection capability and eruption dynamics for both transient and long-duration eruptions in Alaska. Furthermore, we explore the future implementation of this long-range infrasound propagation modeling as an additional monitoring tool for volcano observatories in real time. The quantication of volcanic emissions, including volume flow rate and erupted mass, is possible through acoustic waveform inversion techniques that account for the effects of propagation over topography. Previous volcanic studies have generally assumed a simple acoustic source (monopole), however, more complex source reconstructions can be estimated using a combination of monopole and dipole sources (multipole). We deployed an acoustic network around Yasur volcano, Vanuatu, which has eruptions every 1-4 minutes, including acoustic sensors along a tethered aerostat, allowing us to better constrain the acoustic source in three dimensions. We find that the monopole source is a good approximation when topography is accounted for, but that directionality cannot be fully discounted. Inversions for the dipole components produce estimates consistent with observed ballistic directionality, though these inversions are somewhat unstable given the station conguration. Future work to explore acoustic waveform inversion stability, uncertainty, and robustness should be performed in order to better estimate and quantify the explosion source. Volcanic explosions can produce large, ash-rich plumes that pose great hazard to aviation. We use a single co-located seismic and infrasound sensor pair to characterize 21 explosions at Mount Cleveland, Alaska over a four-year study period. While the seismic explosion signals were similar, the acoustic signals varied between explosions, with some explosions exhibiting single main compressional phase while other explosions had multiple compressions in a row. A notable observation is that the seismo-acoustic time lag varied between explosions, implying a change in the path between the source and receiver. We explore the influence of atmospheric effects, nonlinear propagation, and source depth within the conduit on this variable seismo-acoustic time lag. While changes in the atmospheric conditions can explain some of the observed variation, substantial residual time lags remain for many explosions. Additionally, nonlinear propagation does not result in a measurable difference for the acoustic onset. Therefore, using methods such as seismic particle motion analysis and cross correlation of waveforms between events, we conclude that varying source depth within the conduit likely plays a key role in the observed variation in the seismo-acoustic time lags at Mount Cleveland.National Science Foundation Grants EAR-1331084 (AMI and DF), EAR-1620576 (RSM), and EAR-1847736 (RSM), Alaska Volcano Observatory, New Zealand Strategic Science InvestmentChapter 1: General introduction -- Chapter 2: Application of an updated atmospheric model to explore volcano infrasound propagation and detection in Alaska -- Chapter 3: Three-dimensional acoustic multipole waveform inversion at Yasur Volcano, Vanuatu -- Chapter 4: Seismo-acoustic characterization of Mount Cleveland Volcano explosions -- Chapter 5: general Conclusion