David F. Aldridge

Observation and modeling of source effects in coda wave interferometry at Pavlof volcano

Sorting out source and path effects for seismic waves at volcanoes is critical for the proper interpretation of underlying volcanic processes. Source or path effects imply that seismic waves interact strongly with the volcanic subsurface, either through partial resonance in a conduit (Garces et al., 2000; Sturton and Neuberg, 2006) or by random scattering in the heterogeneous volcanic edifice (Wegler and Luhr, 2001). As a result, both source and path effects can cause seismic waves to repeatedly sample parts of the volcano, leading to enhanced sensitivity to small changes in material properties at those locations. The challenge for volcano seismologists is to detect and reliably interpret these subtle changes for the purpose of monitoring eruptions.

Development of a high-fidelity simulation capability for battlefield acoustics.

Findings are presented from the first year of a joint project between the U.S. Army Engineer Research and Development Center, the U.S. Army Research Laboratory, and the Sandia National Laboratories. The purpose of the project is to develop a finite-difference, time-domain (FDTD) capability for simulating the acoustic signals received by battlefield acoustic sensors. Many important effects, such as scattering from trees and buildings, interactions with dynamic atmospheric wind and temperature fields, and nonstationary target properties, can be accommodated by the simulation. Such a capability has much potential for mitigating the need for costly field data collection and furthering the development of robust identification and tracking algorithms. The FDTD code is based on a carefully derived set of first-order differential equations that is more general and accurate than most current sound propagation formulations. For application to three-dimensional problems of practical interest in battlefield acoustics, the code must be run on massively parallel computers. Some example computations involving sound propagation in a moving atmosphere and propagation in the presence of trees and barriers are presented.

A "Gaussian Explosion" Seismic Energy Source.

A point seismic energy source, with spatial support given by a three-dimensional (3D) Dirac delta function, is commonly utilized in seismic wave propagation theory and numerical modeling. However, there is increasing interest in spatially-extended sources, perhaps because they constitute more realistic representations of physical seismic sources. Aldridge (2002) summarizes an extensive body of literature dating back to Jeffreys (1931) and Sharpe (1942) treating the classic “pressurized spherical cavity” source. More recently, Blair (2007, 2010) has re-examined and improved Heelan’s (1953) elastic radiation solution for a finite-length explosive column. A common aspect of these sources is that they are mathematically characterized as time-varying boundary conditions applied to the surface of a medium. In this investigation, we adopt the alternative point of view that a spatially-extended source may be considered a body source of seismic waves, and is thus represented by inhomogeneous terms in the governing system of partial differential equations. In effect, the source energy is instantaneously “deposited” within a portion of the medium supporting wave propagation. This may be a reasonable approximation in certain seismic source scenarios, as with large chemical or nuclear explosions.

Comparison of Poroelastic And Elastic Full-Waveform AVO Responses

Full-waveform seismic reflection responses of an isolated porous sandstone layer are simulated with three-dimensional (3D) isotropic poroelastic and isotropic elastic finite-difference (FD) numerical algorithms. When the pore-filling fluid is brine water with realistic viscosity, there is about a ~10% difference in synthetic seismograms observed in an AVO recording geometry. These preliminary results suggest that equivalent elastic medium modeling is adequate for general interpretive purposes, but more refined investigations (such as AVO waveform analysis) should account for poroelastic wave propagation effects.

Grid Search Algorithm For 3D Seismic Source Location

The spatial and temporal origin of a seismic energy source are estimated by minimizing (in the weighted least squares sense) the misfit between observed and predicted arrival times at a set of receiver stations. A search is conducted for the best source position within a 3D gridded volume of trial locations. Rapid calculation of predicted traveltimes is achieved by evaluating closed-form formulae appropriate for a homogeneous or 1D layered velocity model. The method is applicable to microseismic event location for mapping hydraulic fracturing in a petroleum reservoir.

3D Acoustic and Elastic Modeling with Marmousi2.

The Marmousi model is a synthetic 2D earth model developed from geologic aspects of the Cuanza basin of offshore Angola. It contains several structural features relevant in marine seismic exploration for petroleum: a water layer with a horizontal seabed, a sequence of dipping growth faults that offset and truncate sedimentary beds, anticlines, two salt sills, a near-horizontal erosional unconformity, and a deep petroleum reservoir. The model is commonly utilized to generate synthetic data for evaluating seismic reflection imaging algorithms. Recently, Martin et al. (2006) created a sophisticated elastic upgrade to the original acoustic Marmousi model, dubbed Marmousi2. Lithology-based formulae appropriate for shale, sandstone, marl, and salt are used to assign shear (S) wave speed and mass density values to the various layers, given the compressional (P) wave speed. Several gas-, oil-, and brine-saturated units are inserted into the layering. The new model is still 2D, although it is extended both laterally and vertically, and water depth is increased to 450 m to enable simulation of deep water seismic exploration experiments. Figure 1 displays the 2D Marmousi2 S-wave velocity distribution; the P-wave velocity and density models are structurally similar.

High-fidelity simulation capability for virtual testing of seismic and acoustic sensors

This paper describes development and application of a high-fidelity, seismic/acoustic simulation capability for battlefield sensors. The purpose is to provide simulated sensor data so realistic that they cannot be distinguished by experts from actual field data. This emerging capability provides rapid, low-cost trade studies of unattended ground sensor network configurations, data processing and fusion strategies, and signatures emitted by prototype vehicles. There are three essential components to the modeling: (1) detailed mechanical signature models for vehicles and walkers, (2) high-resolution characterization of the subsurface and atmospheric environments, and (3) state-of-the-art seismic/acoustic models for propagating moving-vehicle signatures through realistic, complex environments. With regard to the first of these components, dynamic models of wheeled and tracked vehicles have been developed to generate ground force inputs to seismic propagation models. Vehicle models range from simple, 2D representations to highly detailed, 3D representations of entire linked-track suspension systems. Similarly detailed models of acoustic emissions from vehicle engines are under development. The propagation calculations for both the seismics and acoustics are based on finite-difference, time-domain (FDTD) methodologies capable of handling complex environmental features such as heterogeneous geologies, urban structures, surface vegetation, and dynamic atmospheric turbulence. Any number of dynamic sources and virtual sensors may be incorporated into the FDTD model. The computational demands of 3D FDTD simulation over tactical distances require massively parallel computers. Several example calculations of seismic/acoustic wave propagation through complex atmospheric and terrain environments are shown.

Seismic-acoustic finite-difference wave propagation algorithm

An efficient numerical algorithm for treating earth models composed of fluid and solid portions is obtained via straightforward modifications to a 3D time-domain finite-difference algorithm for simulating isotropic elastic wave propagation.