A Note on the Goochland County, Virginia, Earthquake of March 15, 1991

This is the publisher's version, also available electronically from "http://srl.geoscienceworld.org".The Goochland county, Virginia, earthquake of March 15, 1991 (mb (Lg) = 3.8), was the largest earthquake recorded in the central Virginia seismic zone (CVSZ) since the Cunningham, Virginia, earthquake of 1984 (mb(Lg) = 4.2). The 1991 event was felt over 23,000 km2 with a maximum MM V epicentral intensity. The preferred depth of focus was 12.5 km as determined graphically from the T2 vs X2 plot, whereas the computer program HYPOELLIPSE gave a more model-sensitive depth of 15 km. The average focal depth for shocks in this zone is 8.6 km, and the 90% quantile depth is 13.3 km. This earthquake is important because it is the largest shock that has occurred near the base of the CVSZ since network recording begun in 1978. P wave first motions and (SV/P)Z amplitude ratios define a focal mechanism exhibiting primarily strike-slip faulting with a north-south or east-west strike. The P axis trends northwest, similar to deeper focus earthquakes (>8 km) in the CVSZ. P wave spectral analyses indicate a corner frequency at 8 Hz and a low stress drop level of under 100 bars

GPR imaging of dual-porosity rocks: Insights to fluid flow

This is the published version. Reuse is subject to Society of Exploration Geophysicists terms of use and conditions.Dual-porosity rocks, such as those that are fractured and/or karstic, are highly heterogeneous and present a great challenge to predicting fluid-flow properties

Investigating multi-polarization GPR wave transmission through thin layers: Implications for vertical fracture characterization

This is the published version. Copyright 2006 American Geophysical Union. All Rights Reserved.We investigate the controls governing the response of ground-penetrating radar (GPR) wave transmission through thin layers in order to explore the use of variable polarization GPR signals for remote characterization of fracture aperture and fluid-fill. We employ an experimental setting that provides controlled observations of the effects of thin-layer properties to the transmitted GPR wavefield. GPR signals of variable polarization, variable angle of incidence, and variable frequency are transmitted through an air- and water-filled layer of variable thickness. We observe that at high angles of incidence, variable polarization GPR signals display characteristic and quantifiable phase and amplitude responses that are related to thin-layer properties. The GPR data are in agreement to analytical solutions of plane-wave oblique-incidence transmission through layered media. We conclude that multi-polarization GPR observations can be exploited to determine fracture properties. This work has implications in the remote determination of fractured formation anisotropic properties, such as fluid-flow

Comparing flux-averaged and resident concentration in a fractured bedrock using ground penetrating radar

This is the published version. Copyright 2010 by the American Geophysical Union. All rights reserved.Saline tracer transport experiments were performed to compare flux-averaged and resident concentration in a single subhorizontal fracture in sandstone bedrock. Tracer migration over a 14 m distance was monitored at an extraction well and imaged within the rock as it passed below a ground penetrating radar (GPR) positioned at the surface. Reflected radar amplitude was calibrated to tracer concentration by circulating saline fluid of known concentration through the fracture. Saline breakthrough curves measured at the well and within the rock were comparable but showed differences in both magnitude and shape. Transport differences were explored using flux-averaged and resident concentration first-passage-time models combined with streamline advective tracking. Application of the appropriate transport model to the two breakthrough curves produced identical estimates of dispersivity and similar estimates of effective fracture aperture. The tracer-derived fracture aperture also agreed reasonably well with hydraulic aperture derived from cross-hole pump tests. The availability of both flux and resident concentrations helped constrain the interpretation of the flow and transport behavior in the fracture. Flow appeared to be highly channelized with less than half the hydraulically swept area of the fracture contributing to efficient tracer transport

A High Precision Aeromagnetic Survey Near the Glen Hummel Field in Texas; Identification of Cultural and Sedimentary Anomaly Sources

This is the publisher's version, also available electronically from "http://library.seg.org".We present a case study of a high resolution aeromagnetic survey near the Glen Hummel oil field (Figure 1). The survey was originally flown to support a study of aeromagnetic methods for environmental hazard detection, but it provides an opportunity to investigate various problems that arise when analyzing low level total field magnetic anomalies in an area of petroleum production

Ultra-shallow seismic imaging of the top of the saturated zone

This is the published version. Copyright 2010 by the American Geophysical Union. All Rights Reserved.We collected ultra-shallow seismic-reflection data to image the near-surface stratigraphy of a Kansas River point bar. We were successful in identifying a discontinuous clay layer and the top of the saturated zone at depths of 0.95 and 1.4 m. Seismic walkaway data collected using various .22-caliber ammunition show that decreased source energy is necessary to generate higher frequencies and prevent clipping of critical near-offset traces needed to identify ultra-shallow reflections. The seismic reflections exhibited average normal moveout velocities of 180–195 m/s with dominant frequencies of 200–450 Hz. Coincident subsurface features were also imaged using 200-MHz ground-penetrating radar. This study presents the shallowest seismic reflection from the top of the saturated zone reported in the literature to date and further demonstrates the potential of using seismic-reflection methods for ultra-shallow imaging of the subsurface as a stand-alone tool or in conjunction with other high-resolution geophysical techniques

Shallow seismic AVO variations related to partial water saturation during a pumping test

This is the published version. Copyright 2007 by the American Geophysical Union. All Rights Reserved.High-resolution shallow seismic reflection experiments were conducted during and after a pumping test of an agricultural irrigation well to image the cone of depression. Although variations in the reflection time from the top of the saturated zone were not observed, amplitude-versus-offset (AVO) analysis revealed changes in reflection amplitude responses that correlate temporally and spatially to expected changes to the partially saturated zone induced by the pumping and recovery of the aquifer. The AVO responses exhibit dependence on aquifer drawdown and recovery cycles and the distance from the pumping well. We propose that near-surface soil heterogeneity and relatively rapid changes in the water table elevation during irrigation cycles caused a thickening of the partially saturated zone above the water table, which resulted in detectable changes in seismic reflection amplitudes. This study offers insights about the response of shallow seismic reflections to changes in subsurface water saturation and the potential application of seismic techniques to hydrogeophysical problems

An improved vacuum formulation for 2D finite-difference modeling of Rayleigh waves including surface topography and internal discontinuities

This is the published version. Reuse is subject to Society of Exploration Geophysicists terms of use and conditions.Rayleigh waves are generated along the free surface and their propagation can be strongly influenced by surface topography. Modeling of Rayleigh waves in the near surface in the presence of topography is fundamental to the study of surface waves in environmental and engineering geophysics. For simulation of Rayleigh waves, the traction-free boundary condition needs to be satisfied on the free surface. A vacuum formulation naturally incorporates surface topography in finite-difference (FD) modeling by treating the surface grid nodes as the internal grid nodes. However, the conventional vacuum formulation does not completely fulfill the free-surface boundary condition and becomes unstable for modeling using high-order FD operators. We developed a stable vacuum formulation that fully satisfies the free-surface boundary condition by choosing an appropriate combination of the staggered-grid form and a parameter-averaging scheme. The elastic parameters on the topographic free surface are updated with exactly the same treatment as internal grid nodes. The improved vacuum formulation can accurately and stably simulate Rayleigh waves along the topographic surface for homogeneous and heterogeneous elastic models with high Poisson’s ratios (>0.4). This method requires fewer grid points per wavelength than the stress-image-based methods. Internal discontinuities in a model can be handled without modification of the algorithm. Only minor changes are required to implement the improved vacuum formulation in existing 2D FD modeling codes

Field investigation of Love-waves in near-surface seismology

This is the publisher's version, also available electronically from "http://library.seg.org".We examine subsurface conditions and survey parameters suitable for successful exploitation of Love waves in near-surface investigations. Love-wave generation requires the existence of a low shear-velocity surface layer. We examined the minimum thickness of the near-surface layer necessary to generate and record usable Love-wave data sets in the frequency range of 5–50Hz . We acquired field data on a hillside with flat-lying limestone and shale layers that allowed for the direct testing of varying overburden thicknesses as well as varying acquisition geometry. The resulting seismic records and dispersion images were analyzed, and the Love-wave dispersion relation for two layers was examined analytically. We concluded through theoretical and field data analysis that a minimum thickness of 1m of low-velocity material is needed to record usable data in the frequency range of interest in near-surface Love-wave surveys. The results of this study indicate that existing guidelines for Rayleigh-wave data acquisition, such as receiver interval and line length, are also applicable to Love-wave data acquisition

Investigation of Geological Anomalies at Pile Foundation Location in Urban Karst Areas Using Single Borehole Radar

Karst geological anomalies at pile locations significantly affect the bearing capacity and construction safety of the piles, posing a significant challenge for urbanization. Borehole geophysical methods are required to extend the detection range and identify karst voids that are at pole locations and near drilled boreholes. In this paper, we developed a near offset and small diameter single borehole ground penetration radar (GPR) prototype. A signal processing method combining complex signal analysis by Hilbert transform (HT) and medium filtering was suggested to differentiate the weak backscattered wave from borehole background noise. A controlled horizontal borehole experiment was used to demonstrate the applicability of the prototype and the advantages of the signal analysis method prior to application in a real project. The controlled test presented three typical wave events corresponding to a soil–rock interface, rock fractures, and karst voids. Field tests were conducted at a freeway bridge extension project in an urban karst area. Multiple karst voids, sinkholes, rock fractures, and integrated bedrock were identified by analysis of four typical detection scenarios. The remediation of the karst voids and a rotary bored piling with real-time steel casing construction strategy were designed based on the investigation results. The construction feedback demonstrates that single borehole radar detection is effective for the investigation of anomalies at pile locations in urban karst areas