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

The Delaware Series LLC: Sophisticated and Flexible Business Planning

The authors conclude that the Delaware series supplies a beneficial, efficient use of a combined contractual Delaware entity form when pooled with sensible, informed planning by sophisticated business attorneys. Such benefits are particularly noticeable in investment vehicles where managers embark to minimize risk by diversifying the fund’s assets or receive funding with specific covenants attached that limit the acceptable uses of the funds. The series is not, however, for general practitioners who have the occasional client wishing for the latest benefit Delaware has to offer its investors. To provide context, Parts II-IV of this article provide a brief overview of how the series structure evolved and introduces the reader to the Delaware Series. Next, Part V of this article sheds light on the Delaware series LLC and provides practitioners with a useful guide to facilitate forming this specialized business entity. Part VI answers some of the commonly asked questions about a Delaware series. Finally, Part VII of this article addresses the common mistakes made by practitioners when forming a Delaware LLC series

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

Application of the multiaxial perfectly matched layer (M-PML) to near-surface seismic modeling with Rayleigh waves

This is the published version. Reuse is subject to Society of Exploration Geophysicists terms of use and conditions.Perfectly matched layer (PML) absorbing boundaries are widely used to suppress spurious edge reflections in seismic modeling. When modeling Rayleigh waves with the existence of the free surface, the classical PML algorithm becomes unstable when the Poisson’s ratio of the medium is high. Numerical errors can accumulate exponentially and terminate the simulation due to computational overflows. Numerical tests show that the divergence speed of the classical PML has a nonlinear relationship with the Poisson’s ratio. Generally, the higher the Poisson’s ratio, the faster the classical PML diverges. The multiaxial PML (M-PML) attenuates the waves in PMLs using different damping profiles that are proportional to each other in orthogonal directions. The proportion coefficients of the damping profiles usually vary with the specific model settings. If they are set appropriately, the M-PML algorithm is stable for high Poisson’s ratio earth models. Through numerical tests of 40 models with Poisson’s ratios that varied from 0.10 to 0.49, we found that a constant proportion coefficient of 1.0 for the x- and z-directional damping profiles is sufficient to stabilize the M-PML for all 2D isotropic elastic cases. Wavefield simulations indicate that the instability of the classical PML is strongly related to the wave phenomena near the free surface. When applying the multiaxial technique only in the corners of the PML near the free surface, the original M-PML technique can be simplified without losing its stability. The simplified M-PML works efficiently for homogeneous and heterogeneous earth models with high Poisson’s ratios. The analysis in this paper is based on 2D finite difference modeling in the time domain that can easily be extended into the 3D domain with other numerical methods