Kasper van Wijk

Laser Excitation of a Fracture Source for Elastic Waves

Thomas E. Blum et al. — Wed, 25 Jan 2012 13:27:49 PST

We show that elastic waves can be excited at a fracture inside a transparent sample by focusing laser light directly onto this fracture. The associated displacement field, measured by a laser interferometer, has pronounced waves that are diffracted at the fracture tips. We confirm that these are tip diffractions from direct excitation of the fracture by comparing them with tip diffractions from scattered elastic waves excited on the exterior of the sample. Being able to investigate fractures—in this case in an optically transparent material—via direct excitation opens the door to more detailed studies of fracture properties in general.

Departmental Colloquium, Invited Presentation

Kasper van Wijk — Fri, 16 Dec 2011 10:18:48 PST

Earthquakes

Kasper van Wijk — Fri, 16 Dec 2011 10:15:09 PST

Every 30 seconds, somewhere in the world the ground shakes. Most earthquakes are too tiny to be felt, but some earthquakes can be devastating. Why do earthquakes happen? What should you do if the earth shakes? Can we predict when they will happen? The guest scientists for "Earthquakes" are: •Dave Rodgers, Professor of Geosciences and Associate Dean of Science and Engineering, Idaho State University •Kasper van Wijk, Assistant Professor, Department of Geosciences, Boise State University

Seismic Refraction Interferometry with a Semblance Analysis on the Crosscorrelation Gather

Dylan Mikesell et al. — Fri, 16 Dec 2011 09:52:11 PST

Crosscorrelatingwavefields recorded at two receivers to produce data as ifone receiver was a source is commonly referred to asseismic interferometry, or the virtual source method. An artifact inseismic interferometry related to critically refracted waves allowed us toestimate the velocity in the refracting layer. In addition, wedevised a new semblance analysis on the crosscorrelation of reflectionand refraction energy to robustly estimate the depth and velocityof the slow layer, tested with a numerical example andfield data from the Boise Hydrogeophysical Research Site.

The Establishment of a Geophysics Field Camp in Northern Thailand

Lee Liberty et al. — Fri, 16 Dec 2011 09:52:09 PST

As a participant in SEG's Geoscientists Without Borders program, wehave developed a geophysics field camp in northern Thailand totrain students and professionals from throughout Southeast Asia in field-basedgeophysical methods. Over the past two years, faculty, technicians, professionals,and students from 18 institutions and 11 countries have acquired,processed, and interpreted geophysical data at field sites in andaround Chiang Mai, Thailand. Participation from undergraduate students, graduate students,and private and public sector geoscience professionals provides a broadbase of experience, background, and insight. Our training has providedopportunities for cross-cultural collaboration and education, and a greater useof field-based geophysical methods for academic, private sector, and governmentagencies throughout Southeast Asia.

Geophysical Assessment of the Mount Princeton Geothermal Area, Colorado

M. Batzle et al. — Wed, 28 Sep 2011 13:07:38 PDT

The upper Arkansas basin has some of the greatest potential for geothermal development in Colorado. Over several summers, students conducted an investigation using seismic, gravity, and magnetics in order to help identify the large structures and basin fill associated with the Rio Grande Rift. Adjacent to Mount Princeton Hot Springs, near surface geophysics was conducted to help understand the faulting and fractures that act as a conduit for hydrothermal flow. The methods used included high frequency 3D seismic, magnetics, gravity, D.C. resistivity, and self potential. Throughout the Mount Princeton area, passive seismic, vertical seismic profiling, and well logging were also used to better understand the geology and activity of the region. The results indicate for the area northwest of the hot springs, a 40 to 50 m thick unconsolidated layer overlying the fractured granitic basement. The DC profile shows a deep geothermal anomaly of low resistivity which correlates with a positive SP anomaly in the same region. In the Chalk Creek valley, a shear zone extends from the north east to the south west, just south of the Chalk Cliffs. There appears to be an upward migration of the water through this shear zone which then flows along the porous sediments towards the south in this area. The deep seismic data collected in the center of the basin suggests that the depth to the basement is approximately 2000m. Also identified on the deep seismic are two major faults on the west end and two minors at the east end of the cross-section. An alteration zone is identified between the faults and a possible Precambrian northeast trending shear zone. The potential for developing an Enhanced Geothermal System (EGS) near the center of the valley is high. Our geophysical results indicate a deep basin fill near the center of the basin providing high temperatures. The range front (Sawatch) fault is observed to penetrate under the center of the basin and is an attractive target for natural fractures.

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

Thomas E. Blum et al. — Mon, 26 Sep 2011 14:12:11 PDT

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.

Estimating the Rayleigh-Wave Impulse Response Between Seismic Stations with the Cross Terms of the Green Tensor

Kasper van Wijk et al. — Thu, 08 Sep 2011 09:03:27 PDT

The development of ambient noise tomography has provided a powerful tool to investigate the Earth's subsurface with increased resolution. Most commonly, surface-wave tomography is performed on inter-station estimates of the vertical component of Rayleigh waves, stemming from crosscorrelations of ocean-generated noise. Here, we estimate the cross terms of the Rayleigh-wave Green tensor, and show this is less sensitive to signal not in-line with the seismic stations. We illustrate this result with the Batholiths temporary seismic deployment, showing estimates of the Rayleigh wave with a higher signal-to-noise ratio and a consequently better phase-velocity dispersion curve. This approach provides an opportunity for reliable ambient noise crosscorrelations over shorter time windows and more closely spaced stations in the future.

The Virtual Refraction for Noisy Data and Elastic Media

Kasper van Wijk et al. — Mon, 18 Jul 2011 08:52:01 PDT

In this work we explain the origin of spurious energy related to refracted waves, but more importantly, we use this spurious event - which we call the virtual refraction - to quantify subsurface parameters (i.e., wave speeds and depth to interface). Using numerical examples we show the cause of this spurious wave. We illustrate its use in a numerical two-layer refraction experiment, providing an alternative approach to conventional intercept-time seismic refraction analysis (Palmer, 1986). Finally, we discuss possible advantages of this technique to conventional refraction methods; particularly, the ease of picking the virtual refraction velocity (even in the presence of noise) and its self-contained nature to invert for the subsurface parameters.

Multicomponent Wavefield Characterization with a Novel Scanning Laser Interferometer

Thomas E. Blum et al. — Tue, 21 Jun 2011 15:43:26 PDT

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.

Application of the Virtual Refraction to Near-Surface Characterization at the Boise Hydrogeophysical Research Site

Josh Nichols et al. — Thu, 26 May 2011 13:14:58 PDT

Seismic interferometry is a relatively new technique to estimate the Green's function between receivers. Spurious energy, not part of the true Green's function, is produced because assumptions are commonly violated when applying seismic interferometry to field data. Instead of attempting to suppress all spurious energy, we show how spurious energy associated with refractions contains information about the subsurface in field data collected at the Boise Hydrogeophysical Research Site. By forming a virtual shot record we suppress uncorrelated noise and produce a virtual refraction that intercepts zero offset at zero time. These two features make the virtual refraction easy to pick, providing an estimate of refractor velocity. To obtain the physical parameters of the layer above the refractor we analyse the cross-correlation of wavefields recorded at two receivers for all sources. A stationary-phase point associated with the correlation between the reflected wave and refracted wave from the interface identifies the critical offset. By combining information from the virtual shot record, the correlation gather and the real shot record we determine the seismic velocities of the unsaturated and saturated sands, as well as the variable relative depth to the water-table. Finally, we discuss how this method can be extended to more complex geologic models.

The Virtual Refraction: Useful Spurious Energy in Seismic Interferometry

Dylan Mikesell et al. — Wed, 27 Oct 2010 08:42:24 PDT

Seismic interferometry is rapidly becoming an established technique to recover the Green’s function between receivers, but practical limitations in the source-energy distribution inevitably lead to spurious energy in the results. Instead of attempting to suppress all such energy, we use a spurious wave associated with the crosscorrelation of refracted energy at both receivers to infer estimates of subsurface parameters. We named this spurious event the virtual refraction. Illustrated by a numerical two-layer example, we found that the slope of the virtual refraction defines the velocity of the faster medium and that the stationary-phase point in the correlation gather provides the critical offset. With the associated critical time derived from the real shot record, this approach includes all of the necessary information to estimate wave speeds and interface depth without the need of inferences from other wave types.

Observation and Modeling of Source Effects in Coda Wave Interferometry at Pavlof Volcano

Matthew M. Haney et al. — Wed, 27 Oct 2010 08:42:22 PDT

We examine seismic records of repeating explosions from Pavlof volcano, Alaska, during its 2007 eruption. Repetitive explosions are typical of Strombolian-style eruptions and allow measurement of relative time shifts between similar late-arriving phases using the technique called coda wave interferometry (Snieder et al., 2002). The measurements enable the detection of small changes in the volcanic interior of Pavlof. We are able to resolve an increase in the relative traveltime change of late-arriving seismic waves on the order of 0.3% over the course of two weeks. Based on the spectra of the explosions, their location inside the magma conduit, previous studies of Pavlof volcano, and 3D seismic modeling, we argue the most likely scenario is one in which the velocity and/or the geometry of the conduit changes. This demonstrates the sensitivity of coda wave interferometry to source effects, in addition to path effects, at volcanoes.

Seismic Wave Attenuation in Carbonates

L. Adam et al. — Wed, 27 Oct 2010 08:42:20 PDT

The effect of pore fluids on seismic wave attenuation in carbonate rocks is important for interpreting remote sensing observations of carbonate reservoirs undergoing enhanced oil recovery. Here we measure the elastic moduli and attenuation in the laboratory for five carbonate samples with 20% to 30% porosity and permeability between 0.03 and 58.1 mdarcy. Contrary to most observations in sandstones, bulk compressibility losses dominate over shear wave losses for dry samples and samples fully saturated with either liquid butane or brine. This observation holds for four out of five samples at seismic (10–1000 Hz) and ultrasonic frequencies (0.8 MHz) and reservoir pressures. Attenuation modeled from the modulus data using Cole-Cole relations agrees in that the bulk losses are greater than the shear losses. On average, attenuation increases by 250% when brine substitutes a light hydrocarbon in these carbonate rocks. For some of our samples, attenuation is frequency-dependent, but in the typical exploration frequency range (10–100 Hz), attenuation is practically constant for the measured samples.

1D Energy Transport in a Strongly Scattering Laboratory Model

Kasper van Wijk et al. — Mon, 21 Sep 2009 11:46:03 PDT

Radiative transfer (RT) theory is often invoked to describe energy propagation in strongly scattering media. Fitting RT to measured wave field intensities is rather different at late times, when the transport is diffusive, than at intermediate times (around one extinction mean free time), when ballistic and diffusive behavior coexist. While there are many examples of late-time RT fits, we describe ultrasonic multiple scattering measurements with RT over the entire range of times—from ballistic to diffusive. In addition to allowing us to retrieve the scattering and absorption mean free paths independently, our results also support theoretical predictions in 1D that suggest an intermediate regime of diffusive (nonlocalized) behavior.

Physical Modeling and Analysis of P-Wave Attenuation Anisotropy

Yaping Zhu et al. — Wed, 14 Jan 2009 13:18:49 PST

Anisotropic attenuation can provide sensitive attributes for fracture detection and lithology discrimination. This paper analyzes measurements of the P-wave attenuation coefficient in a transversely isotropic sample made of phenolic material. Using the spectral-ratio method, we estimate the group (effective) attenuation coefficient of P-waves transmitted through the sample for a wide range of propagation angles (from 0° to 90°) with the symmetry axis. Correction for the difference between the group and phase angles and for the angular velocity variation help us to obtain the normalized phase attenuation coefficient A governed by the Thomsen-style attenuation-anisotropy parameters ε_Q and δ_Q. Whereas the symmetry axis of the angle-dependent coefficient practically coincides with that of the velocity function, the magnitude of the attenuation anisotropy far exceeds that of the velocity anisotropy. The quality factor Q increases more than tenfold from the symmetry axis (slow direction) to the isotropy plane (fast direction). Inversion of the coefficient using the Christoffel equation yields large negative values of the parameters ε_Q and δ_Q. . The robustness of our results critically depends on several factors, such as the availability of an accurate anisotropic velocity model and adequacy of the homogeneous concept of wave propagation, as well as the choice of the frequency band. The methodology discussed here can be extended to field measurements of anisotropic attenuation needed for AVO (amplitude-variation-with-offset) analysis, amplitude-preserving migration, and seismic fracture detection.

Cancellation of Spurious Arrivals in Green’s Function Extraction

Roel Snieder et al. — Wed, 14 Jan 2009 13:18:46 PST

The extraction of the Green's function by cross correlation of waves recorded at two receivers nowadays finds much application. We show that for an arbitrary small scatterer, the cross terms of scattered waves give an unphysical wave with an arrival time that is independent of the source position. This constitutes an apparent inconsistency because theory predicts that such spurious arrivals do not arise, after integration over a complete source aperture. This puzzling inconsistency can be resolved for an arbitrary scatterer by integrating the contribution of all sources in the stationary phase approximation to show that the stationary phase contributions to the source integral cancel the spurious arrival by virtue of the generalized optical theorem. This work constitutes an alternative derivation of this theorem. When the source aperture is incomplete, the spurious arrival is not canceled and could be misinterpreted to be part of the Green's function. We give an example of how spurious arrivals provide information about the medium complementary to that given by the direct and scattered waves; the spurious waves can thus potentially be used to better constrain the medium.

Modified Kubelka-Munk Equations for Localized Waves Inside a Layered Medium

Matthew M. Haney et al. — Wed, 14 Jan 2009 13:18:43 PST

We present a pair of coupled partial differential equations to describe the evolution of the average total intensity and intensity flux of a wave field inside a randomly layered medium. These equations represent a modification of the Kubelka-Munk equations, or radiative transfer. Our modification accounts for wave interference (e.g., localization), which is neglected in radiative transfer. We numerically solve the modified Kubelka-Munk equations and compare the results to radiative transfer as well as to simulations of the wave equation with randomly located thin layers.