Stephen A. Ketcham

Effects of free-surface topography on moving-seismic-source modeling

A curved-grid velocity-stress formulation for viscoelastic wave modeling is used with an arbitrary number of relaxation mechanisms to model a desired Q -behavior. These equations are discretized by high-order staggered finite differences (FDs) in the interior of the medium, and we gradually reduce the FD order to two at the stress-free surface, where we implement our boundary conditions for an arbitrary topographic surface. A moving source is simulated along the surface of a relatively general and locally steep surface topography and, for comparison, along a plane surface. The topography consists of a significant hill surrounded by a valley. Similar two-layered geologic models are used with both topographic surfaces, with the upper layer being a lossy sedimentary layer having a relatively strong contrast with the lower, higher-velocity half-space. Local topographic highs create varying amplitude amplifications at different times during motion of the source. A pronounced wavefield accumulation is evident a...

Quick and accurate Q parameterization in viscoelastic wave modeling

We introduce a procedure for including the attenuation factor Q in a consistent manner in seismic modeling and show 3D examples. The Q fitting over a chosen frequency band involves two algorithms: The first creates starting values of relaxation times, and the second does nonlinear inversion using the results of the first as initial values. The resulting Q function gives a good approximation to a constant Q over the chosen frequency band. The algorithm is combined with a finite-difference (F-D) code that includes topographies in 3D seismic media. The velocity-stress formulation for viscoelastic wave modeling is used with an arbitrary number of relaxation mechanisms to model a desired Q behavior. These equations are discretized by high-order F-Ds in the interior of the medium, and we gradually reduce the F-D order to two at the stress-free surface, where we implement our free-surface boundary conditions. The seismic F-D algorithm is applied to a marine seismic experiment, with and without viscoelasticity, to emphasize the importance of including physical attenuation and dispersion in seismic modeling. Their inclusion, even for marine surveys, is clearly important for lossy ocean bottoms. Our procedure for more accurate modeling of physical dispersion and attenuation may increase future motivation to include viscoelasticity in seismic inversion.

State-Space Model and Kalman Filter Gain Identification by a Superspace Method

This paper describes a superspace method to identify a state-space model and an associated Kalman filter gain from input-output data. Superstate vectors are simply vectors containing input-output measurements, and used directly for the identification. The superstate space is unusual in that the state portion of the Kalman filter becomes completely independent of both the system dynamics and the input and output noise statistics. The system dynamics is entirely carried by the measurement portion of the superstate Kalman filter model. When model reduction is applied, the system dynamics returns to the state portion of the state-space model.

The effect of a topographic depression on guided seismic surface waves

Summary We consider the complex effects of topographical features and shallow geological structure on guided seismic surface waves using a 3-D finite-difference propagation model. In particular we show results from a model of a soil layer over a limestone half-space, and show that a shallow erosional depression dramatically alters the modal character of the wavetrain. The depression reflects low-frequency, longwavelength, leaky-mode energy while the higher frequency Rayleigh wave energy passes across the depression relatively undisturbed. In addition we describe a method of generating calibrated force excitations within the curved grids of models with topography. The finite-difference model is validated using analytical and wavenumber-integration solutions.

Signal fading curves from computed urban acoustic wave fields

Future US Army ground sensors in urban terrain will process acoustic signals to detect, classify, and locate sources of interest. Optimal processing will require understanding of the effects of the urban infrastructure on sound propagation. These include multi-path phenomena that must be accounted for in sensor placement and performance algorithms. This work applies Fourier analysis to urban acoustic wave-field data from three-dimensional high-performance computations to generate statistical measures of signal fading caused by scattering. The work calculates these measures from ratios of Fourier transforms of wave-field signals with and without scattering to isolate the structure-induced scattering.

A Non-causal Inverse Model for Source Signal Recovery in Large-Domain Wave Propagation

A non-causal inverse model for source signal recovery is formulated. The inverse model is derived from a causal forward model. For a dynamical system where the causal inverse is unstable, a non-causal inverse model can be used instead. Application of the non-causal inverse technique on a High Performance Computing (HPC) acoustic propagation model of an office and laboratory campus in Hanover, New Hampshire, USA is presented.

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.