Shelby Peterie

Near-surface scattering phenomena and implications for tunnel detection

AbstractTunnel locations are accurately interpreted from diffraction sections of focused mode converted P- to S-wave diffractions from a perpendicular tunnel and P-wave diffractions from a nonperpendicular (oblique) tunnel. Near-surface tunnels are ideal candidates for diffraction imaging due to their small size relative to the seismic wavelength and large acoustic impedance contrast at the tunnel interface. Diffraction imaging algorithms generally assume that the velocities of the primary wave and the diffracted wave are approximately equal, and that the diffraction apex is recorded directly above the scatterpoint. Scattering phenomena from shallow tunnels with kinematic properties that violate these assumptions were observed in one field data set and one synthetic data set. We developed the traveltime equations for mode-converted and oblique diffractions and demonstrated a diffraction imaging algorithm designed for the roll-along style of acquisition. Potential processing and interpretation pitfalls spe...

Diffraction Imaging Versus Reflection Processing For Shallow Void Detection

Summary Voids were imaged with greater detail using a diffraction imaging techniq ue than with traditional seismic reflection processing. Subsurface voids in extensively mined areas pose a threat to human life and property. Seismic reflection techniques applied to the void detection problem have had limited success. Data acquired in July of 1985 over a former coal mine were originally processed with standard reflection processing techniques. These data were reprocessed using a diffraction enhancement algorithm to image corners of voids using diffracted waves. Results from diffraction imaging are consistent with the reflection processing results and other information from the site, and image additional voids that were not initially detected using the reflection approach.

Multi-channel Analysis of Surface Waves (MASW) of Models With High Shear-wave Velocity Contrast

We use the multi-channel analysis of surface waves (MASW) method to analyze synthetic seismic data calculated using models with high shear-wave velocity (Vs) contrast. The MASW dispersion-curve images of the Rayleigh wave are obtained using various sets of source-offset and spread-size configurations from the synthetic seismic data and compared with the theoretically calculated fundamentaland highermode dispersion-curves. Such tests showed that most of the dispersion-curve images are dominated by higher-mode energy at the low frequencies, especially when analyzing data from long receiver offsets and thus significantly divert from numerically expected dispersion-curve trends, which can lead to significant Vs overestimation. Further analysis showed that using data with relatively short spread lengths and source offsets can image the desired fundamental-mode of the Rayleigh wave that matches the numerically expected dispersioncurve pattern. As a result, it was concluded that it might be possible to avoid higher-mode contamination at low frequencies at sites with high (Vs) contrast by appropriate selection of spread size and seismic source offset.

Estimation of Near-surface Quality Factors By Inversion of Rayleigh-wave Attenuation Coefficients

Quality factors of near-surface materials are as important as velocities of the materials in many applications. High-frequency (≥ 2 Hz) surface-wave data are generally inverted to determine near-surface shear (S)-wave velocities, in which only phase information of surface-wave data is utilized. Amplitude information of high-frequency surface-wave data can be used to determine quality factors of near-surface materials. Given S-wave velocity, compressional (P)-wave velocity, and Rayleigh-wave phase velocities, it is feasible to solve for S-wave quality factor QS and P-wave quality factor QP (for some specific velocity models) in a layered earth model down to 30 meters below the ground surface in many settings by inverting high-frequency Rayleigh-wave attenuation coefficients. Because an inversion system of this problem is unstable, we introduce a regularization parameter to limit a model length. Based on the linear nature of the inversion system, we search for a smooth model that is a trade-off solution between data (attenuation coefficient) misfit and model (quality factor) length. Several real-world examples demonstrate that the optimal regularization parameter can be found by the L-curve method and so can a smooth model.

Revisiting levees in southern Texas using Love-wave multichannel analysis of surface waves with the high-resolution linear Radon transform

AbstractShear-wave velocities were estimated at a levee site by inverting Love waves using the multichannel analysis of surface waves (MASW) method augmented with the high-resolution linear Radon transform (HRLRT). The selected site was one of five levee sites in southern Texas chosen for the evaluation of several seismic data-analysis techniques readily available in 2004. The methods included P- and S-wave refraction tomography, Rayleigh- and Love-wave surface-wave analysis using MASW, and P- and S-wave cross-levee tomography. The results from the 2004 analysis revealed that although the P-wave methods provided reasonable and stable results, the S-wave methods produced surprisingly inconsistent shear-wave velocity VS estimates and trends compared with previous studies and borehole investigations. In addition, the Rayleigh-wave MASW method was nearly useless within the levee due to the sparsity of high frequencies in fundamental-mode surface waves and complexities associated with inverting higher modes. T...

Near-surface shear-wave velocity measurements in unlithified sediment

S-wave velocity can be directly correlated to material stiffness and lithology making it a valuable physical property that has found uses in construction, engineering, and environmental projects. This study compares different methods for measuring S-wave velocities, investigating and identifying the differences among the methods’ results, and prioritizing the different methods for optimal S-wave use at the U. S. Army’s Yuma Proving Grounds (YPG). Multichannel Analysis of Surface Waves (MASW) and S-wave tomography were used to generate S-wave velocity profiles. Each method has advantages and disadvantages. A strong signal-to-noise ratio at the study site gives the MASW method promising resolution. S-wave first arrivals are picked on impulsive sledgehammer data which were then used for the tomography process. Three-component downhole seismic data were collected in-line with a locking geophone, providing ground truth to compare the data and to draw conclusions about the validity of each data set. Results from these S-wave measurement techniques are compared with borehole seismic data and with lithology data from continuous samples to help ascertain the accuracy, and therefore applicability, of each method. This study helps to select the best methods for obtaining S-wave velocities for media much like those found in unconsolidated sediments at YPG.

Seismic investigations of subsidence hazards

Summary Seismic investigation of subsidence asso ciated with dissolution, piping, and roof rock failure in mines/voids is beginning to reach critical mass such that it is possible to begin assimilating characteristics that can be used to better interpret data and predict which portion of the seismic wavefield will be most sensitive to particular features in a defined geology and for a particular void/collapse. Examples of seismic data with different portions of the wavefield enhanced to allow improved interpretation of subsurface anomalies related to ground instability are provided and discussed. With more than a dozen examples with characteristics considered ‘normal or expected’ for a particular setting, improved confidence becomes a reasonable outcome from incorporation of various components of the wavefield when investigating a particular target.

Feasibility of high resolution seismic reflection to improve accuracy of hydrogeologic models in a culturally noisy part of Ventura County, CA, USA

A high-resolution seismic reflection investigation mapped reflectors and identified characteristics potentially influencing the interpretation of the hydrogeology underlying a portion of the Oxnard Plain in Ventura County, California. Design and implementation of this study was heavily influenced by high levels of cultural noise from vehicles, power lines, roads, manufacturing facilities, and underground utilities/vaults. Acquisition and processing flows were tailored to this noisy environment and relatively shallow target interval. Layering within both upper and lower aquifer systems was delineated at a vertical resolution potential of around 2.5 m at 350 m depth.

BASELINE MICROSEISMIC ACTIVITY IN SUMNER COUNTY, KANSAS

Microseismicity is well known to be observable at nearly any location on the earth. Historical trends of felt earthquakes do not do sufficiently characterize the frequency and distribution of microseismic events. Regional earthquake networks are typically not designed to detect and locate microseismic events, which can have negative magnitudes. Only three earthquakes were recorded in Sumner County since 1977 by regional seismic networks operating in Kansas. A local network was recently deployed over a producing oil field in Sumner County, Kansas, to monitor microseismic activity. From July to September, 2014, fifteen seismometers were deployed in a 1.2 km area. In two months of recording, 125 microseismic events were detected within 15 km of the network ranging in magnitude from -0.6 to 2.4, none of which were reported by the regional network operators. The observed microseismic activity is most likely both natural and anthropogenic related to reservoir activity. Caution is called for when interpreting the source and cyclicity of microseismic events detected by localized temporary networks with minimal to no background data. Without having an understanding of the localized seismic activity the source and recursion of present activity cannot be assumed. Even with a regional network in place, it is critical to establish baseline microseismicity through many cycles and periods of subsurface activity.

DISPERSION-CURVE IMAGING CONSIDERATIONS WHEN USING MULTICHANNEL ANALYSIS OF SURFACE WAVE (MASW) METHOD

The multichannel analysis of surface wave (MASW) method can be an efficient tool for mapping the near-surface shear-wave velocity (Vs). Data acquisition, dispersion-curve imaging and estimations, inversion, and 2D visualization are distinct MASW components. Dispersion-curves can be estimated on images that can be obtained by various transforms including converting seismic data from the time-space domain (i.e., t-x domain) into frequency–wave-number (i.e., f-k domain) by applying the Fourier transform to both time and space, Phase-velocity–frequency domain (e.g., Cf-f or f-v domain), slowness–frequency domain (i.e, p-f domain), phase-velocity–wavelength domain, etc. It has been our observation that while the mathematical link between such transforms is well known, the relationship between the corresponding images can be visually clarified for better comprehension. In this work we show the visual relationship between the f-k and the Cf-f domain images. We also demonstrate the visual effects of using fewer geophones (i.e., data along the space axis) with different spread sizes on the phase-velocity–frequency dispersion-curve imaging using surface-wave data with dominant single- and multi-mode surface-wave forms of expressions. These examples could help better understand and make more efficient use of the MASW data acquisition, analysis, and interpretation of final results.