Mark L. Moran

Time reversal processing for source location in an urban environmenta)

A simulation study is conducted to demonstrate in principle that time reversal processing can be used to locate sound sources in an outdoor urban area with many buildings. Acoustic pulse propagation in this environment is simulated using a two-dimensional finite difference time domain (FDTD) computation. Using the simulated time traces from only a few sensors and back propagating them with the FDTD model, the sound energy refocuses in the vicinity of the true source location. This time reversal numerical experiment confirms that using information acquired only at non-line-of-sight locations is sufficient to obtain accurate source locations in a complex urban terrain.A simulation study is conducted to demonstrate in principle that time reversal processing can be used to locate sound sources in an outdoor urban area with many buildings. Acoustic pulse propagation in this environment is simulated using a two-dimensional finite difference time domain (FDTD) computation. Using the simulated time traces from only a few sensors and back propagating them with the FDTD model, the sound energy refocuses in the vicinity of the true source location. This time reversal numerical experiment confirms that using information acquired only at non-line-of-sight locations is sufficient to obtain accurate source locations in a complex urban terrain.

Multidimensional GPR array processing using Kirchhoff migration

We compare the ability of several practical ground-penetrating radar (GPR) array processing methods to improve signal-to-noise ratio (SNR), increase depth of signal penetration, and suppress out-of-plane arrivals for data with SNR of roughly 1. The methods include two-dimensional (2-D) monostatic, three-dimensional (3-D) monostatic, and 3-D bistatic Kirchhoff migration. The migration algorithm is modified to include the radiation pattern for interfacial dipoles. Results are discussed for synthetic and field data. The synthetic data model includes spatially coherent noise sources that yield nonstationary signal statistics like those observed in high noise GPR settings. Array results from the model data clearly indicate that resolution and noise suppression performance increases as array dimensionality increases. Using 50-MHz array data collected on a temperate glacier (Gulkana Glacier, AK), we compare 2-D and 3-D monostatic migration results. The data have low SNR and contain reflections from a complex, steeply dipping bed. We demonstrate that the glacier bed can only be accurately localized with the 3-D array. In addition, we show that the 3-D array increases SNR (relative to a 2-D array) by a factor of three.

Seismic source model for moving vehicles

We develop a method for the loading of ground by moving vehicles in large finite-difference time-domain simulations of seismic wave propagation. The objective is to realistically produce two distinct types of ground loading for either wheeled or tracked vehicles in our propagation models: lower frequency loading associated with suspension dynamics and higher frequency impulsive loading associated with tire treads or wheels rolling over individual track blocks. These loading characteristics are important because field measurements show that vehicle ground forcing in both frequency bands produces seismic surface waves that networked sensors can remotely process for security applications. The method utilizes a vehicle-dynamics model to calculate a response to vehicle acceleration and ground features such as bumps; calculates forces transmitted to the ground; distributes these forces to staggered points of a finite-difference model; and simulates seismic wave propagation away from the vehicle. We demonstrate the method using bounce-and-pitch models of wheeled and tracked vehicles. We show that by carefully preprocessing force inputs, we can accurately simulate wave propagation and seismic signatures in finite-difference analyses of vehicles moving continuously over terrain.

Tracked vehicle simulations and seismic wavefield synthesis in seismic sensor systems

A finite-difference time-domain (FDTD) method models moving vehicle ground motion in 3D geologic environments, producing results similar to those from actual field experiments. Simulations provide a low-cost alternative to traditional prototype development schemes, which rely on expensive field tests.

3-D Characterization of Seismic Properties At the Smart Weapons Test Range, YPG

The Smart Weapons Test Range (SWTR) is a new facility constructed specifically for the development and testing of futuristic intelligent battlefield sensor networks within the Yuma Proving Ground (YPG), Arizona. In this paper, results are presented for an extensive high-resolution geophysical characterization study at the SWTR site along with validation using 3-D modeling. In this study, several shallow seismic methods and processing techniques were used to generate a 3-D grid of earth seismic properties, including compressional (P) and shear (S) body-wave speeds (V p and V s ), and their associated body-wave attenuation parameters (Q p and Q s ). These experiments covered a volume of earth measuring 1500 m × 300 m × 25 m deep (11 million cubic meters), centered on the vehicle test track at the SWTR site. The study has resulted in detailed characterizations of key geophysical properties. To our knowledge, results of this kind have not been previously achieved, nor have the methods developed for this effort been reported elsewhere. In addition to supporting materiel developers with important geophysical information at this test range, the data from this study will be used to validate sophisticated 3-D seismic signature models for moving vehicles.

Modeling GPR radiation and reflection characteristics for a complex temperate glacier bed

We demonstrate that ground penetrating radar (GPR) reflection data from a temperate glacier are accurately modeled using a Helmholtz‐Kirchhoff diffraction integration technique that incorporates the radiation characteristics of point dipoles on a half‐space interface. This is accomplished by comparing field data to simulated data. Our 40‐MHz field data are from a 100 × 340 m (x‐ and y‐dimensions, respectively) survey grid containing 51 parallel survey lines. The field data were collected with the dipole oriented perpendicular to the survey line (x‐dipole). The synthetic data used isotropic, x‐dipole, and y‐dipole antennas, and reflections were calculated using a bed topography previously defined by 3D Kirchhoff migration. The comparisons between the real and synthetic waveforms show excellent agreement, including reflection arrival times, amplitude trends, interference patterns, and false layering from out‐of‐plane reflections. The location of reflectors determined from exploding reflector rays explains t...

Seismic waves from light trucks moving over terrain

Seismic sensing is one sensor mode employed in US unattended ground sensor systems (UGS). Seismic sensors possess the advantage of beyond-line-of-sight sensing. They can detect ground vibrations generated by moving vehicles or personnel and they can be used to cue other sensors or possibly to classify or even identify targets. As a complement to field trials, our work has produced a simulation capability to support seismic UGS developments. We model ground vibrations from moving vehicles and generate synthetic seismic wavefield data over terrain of interest to US forces. Using supercomputers to simulate seismic wave propagation in large finite-difference time-domain simulations, our objective is to generate high fidelity data sets that provide new opportunities for understanding and exploiting signal features that may be unrecognizable in limited field trials. The method utilizes a vehicle-dynamics model to calculate the vehicle response to vehicle acceleration and movement over bumpy roads or terrain. It calculates forces transmitted to the ground; distributes these forces to grid points of a finite difference model; and simulates seismic waves propagating away from the vehicle. The current work focuses on light trucks moving toward and through a mountain pass and signature features associated with suspension and wheelbase characteristics. The results from two analyses show seismic waves propagating away from one and two trucks, respectively. We conclude that the wavefield data is realistic and suitable for virtual trials of seismic UGS.

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.

12- to 100-MHz depth and stratigraphic profiles of temperate glaciers

Current practice prescribes the long standing use of frequencies less than 10 MHz for GPR sounding of temperate glaciers to depths greater than about 100 m. We have obtained continuous profiles of ice depth over temperate ablation zones and have reached at least 278 m at 30 MHz on the Matanuska, 195 m at 100-MHz on the Muir and 295 m at 12 MHz on the Gulkana Glaciers of Alaska. Internal stratigraphy occurs in the Matanuska profile. Multiple bottom returns are ascribed to thrust planes and out-of-plane reflections from sloping bottoms. Inability to reach greater depths is ascribed to unfavorable bottom slopes, as verified by drilling on the Matanuska, and by beam steering studies on the Gulkana. Lateral wave clutter contaminates profiles at all frequencies and polarizations where crevasses are present. The strength of 12-MHz Gulkana bottom signals precluded the use of range gain and suggests that 500 m depth might be achieved at 12 - 30 MHz.

GPR radiation pattern effects on 3D Kirchhoff array imaging

A variety of 2-D acoustic migration algorithms are routinely applied to ground penetrating radar (GPR) array data to form images of reflecting surfaces. In a few instances 3-D migration has been performed. We demonstrate the GPR dipole radiation effects on imaging planar reflection surfaces under a 3-D acoustic Kirchhoff imaging condition. This is accomplished by analyzing migration results using synthetic GPR array data generated with a new diffraction integration method that includes interfacial dipole radiation patterns. The impact of the GPR radiation pattern is demonstrated on migration images by considering three antennas types (isotropic, x-, and y-dipoles). For all the results discussed, we use the relative dielectric permittivity for ice, and monostatic transmitter and receiver geometry. We find that an array having a 2.5:1 length-to-width (x:y) ratio and populated with isotropic elements, has a directivity of 4.8. The same array using x-oriented antenna elements has a directivity of 11; with y-oriented elements the directivity is 7. The dipole radiation pattern dominates the array factor. In migration results, when the array factor is aligned with the H-plane dipole pattern, surfaces as steep as 45 degrees are readily imaged. Conversely, when the array factor is aligned with the E-plane pattern, reflectors with dips as low as 20 degrees are difficult to image. These results stress that correct dipole orientation with-in the imaging array is vital for achieving optimal reflector images using migration techniques.