Thomas E. Blum

Multicomponent wavefield characterization with a novel scanning laser interferometer

The in-plane component of the wavefield provides valuable information about media properties from seismology to nondestructive testing. A new compact scanning laser ultrasonic interferometer collects light scattered away from the angle of incidence to provide the absolute ultrasonic displacement for both the out-of-plane and an in-plane components. This new system is tested by measuring the radial and vertical polarization of a Rayleigh wave in an aluminum half-space. The estimated amplitude ratio of the horizontal and vertical displacement agrees well with the theoretical value. The phase difference exhibits a small bias between the two components due to a slightly different frequency response between the two processing channels of the prototype electronic circuitry.

Theory and Laboratory Experiments of Elastic Wave Scattering by Dry Planar Fractures

Remote sensing of fractures with elastic waves is important in fields ranging from seismology to nondestructive testing. In many geophysical applications, fractures control the flow of fluids such as water, hydrocarbons or magma. While previous analytic descriptions of scattering mostly deal with very large or very small fractures (compared to the dominant wavelength), we present an analytic solution for the scattering of elastic waves from a fracture of arbitrary size. Based on the linear slip model for a dry fracture, we derive the scattering amplitude in the frequency domain under the Born approximation for all combinations of incident and scattered wave modes. Our analytic results match laser-based ultrasonic laboratory measurements of a single fracture in clear plastic, allowing us to quantify the compliance of a fracture. Copyright

Analyzing the coda from correlating scattered surface waves

The accuracy of scattered Rayleigh waves estimated using an interferometric method is investigated. Summing the cross correlations of the wave fields measured all around the scatterers yields the Greens function between two excitation points. This accounts for the direct wave and the scattered field (coda). The correlations themselves provide insights into the location of the scatterers, as well as which scatterer is responsible for particular parts of the coda. Furthermore, these measurements confirm a constant-time arrival in the correlations, not part of the Greens function, but which has previously been derived as a result of the generalized optical theorem.

Surface wave isolation with the interferometric Green tensor

SUMMARY Surface and body waves inherently coexist in seismic records. Usually, we are interested in the one, while the other is considered unwanted. To complicate things, body and surface waves often overlap in time and space. Hence, separation of these different wave modes is complicated, and remains an active topic of research. Here we use estimates of the Green tensor obtained via seismic interferometry to provide waveforms with isolated body and surface waves, allowing us to focus our further studies on one or the other. These ideas are illustrated with laboratory and numerical examples.

Advances in Laboratory Modeling of Wave Propagation

Laboratory studies of ultrasonic wave propagation can serve as either scaled modeling of challenges in seismic imaging, or as a way to investigate fundamental advancements in wave propagation. Particularly non-contacting laser ultrasonics provides tremendous opportunities toward both mentioned purposes, because the laser acquisition allows for automated scanning, a small source/receiver footprint and does not suffer mechanical ringing of traditional contacting ultrasonic sensor. Here we present calibration measurements of a new two-component laser receiver, as well a novel way of scaled modeling with structures made in glass.

Characterization of a geothermal system in the Upper Arkansas Valley, CO

Over the past five years, geophysics students and faculty from Colorado School of Mines, Boise State University, and recently Imperial College of London have used a combination of geological and geophysical methods to characterize the subsurface in the Upper Arkansas Valley, Colorado. We present and integrate seismic, selfpotential, gravity and magnetic data, as well as water temperature measurements in local wells to ascertain the overall basin structure and investigate a geothermal system in the Mt. Princeton area. We conclude that a shallow orthogonal fault system in this area appears to be responsible for the local geothermal signature at and near the surface. The extent to which high temperatures exist throughout the deeper basin is still under investigation. Geological background The Upper Arkansas basin is the northernmost extension of the Rio Grande Rift. This rift system starts near Leadville, Colorado and extends southward over 700 kilometers to Socorro, New Mexico (Halley, 1978). The north-trending rift consists of a series of four en echelon basins. From north to south these basins are the Upper Arkansas, San Luis, Espanola, and Albuquerque Basins. The Upper Arkansas Basin is the least understood of the basins as deep drill hole studies are not available, and deep seismic profiles have only become available in the last five years. An overview of the basin and its main features is presented Figure 1. The basin is assumed to be the youngest expression of an Eocene collapse of the Laramide uplift (Tweto, 1979), with the rifting beginning around 25 to 30 Ma. It consists of a north-northwest-trending half-graben, dipping westward, bounded between the Mosquito range on the east and the Sawatch range on the west. It is about 100 km long and 5 to 10 km wide. The basin splits into a northern section close to Leadville, and the main section spanning from Poncha Springs to the north of Buena Vista.

Scatterer Characterization from Correlated Surface Wave Coda

The accuracy of scattered Rayleigh waves estimated using an interferometric method is investigated. Summing the cross correlations of the wave fields measured all around the scatterers yields the Greens function between two excitation points. This accounts for the direct wave and the scattered field (coda). The correlations themselves provide insights into the location of the scatterers as well as which scatterer is responsible for particular parts of the coda.

Elastic Scattering By Planar Fractures

Remote sensing of fractures with elastic waves is important in fields ranging from seismology to non-destructive testing. While previous analytic descriptions of scattering mostly concern very large or very small fractures (compared to the dominant wavelength), we present an analytic solution for the scattering of elastic waves from a fracture of arbitrary size. Based on the linear-slip model for a fracture, we derive the scattered amplitude in the frequency domain under the Born approximation for all combinations of incident and scattered wave modes. Our analytic results match laser-based ultrasonic laboratory measurements of a single fracture in clear plastic, allowing us to quantify the compliance of a fracture.