Joseph B. Dunbar

Time-lapse seismic study of levees in southern Texas

The primary objective of this work was to measure changes in compressional(Vp) and shear-wave (Vs) velocities in an earthen levee during a ponding experiment designed to simulate flood conditions on the Rio Grande in south Texas. Preliminary testing at five levee sites, all within a 10 km radius and each with unique physical, EM, and core characteristics, was completed and a single low-conductivity, highly fractured site was selected for investigation. Several different types of seismic data were recorded. Seismic data analysis techniques appraised included Pand S-wave refraction tomography and Rayleigh surface-wave analysis using multichannel analysis of surface waves (MASW). P-wave methods provided reasonable results, but no change was observed in velocity even after full pool had been maintained against the levee side for two days. The S-wave velocity change was rapid and isolated to one area within the confines of the pool. The reason for the latter effect cannot be uniquely determined; however, it may possibly be the result of an isolated variable expansion of the clay core, a likely explanation considering the preceding years of drought. Alternatively, these changes could be related to mechanical compaction variability and variable material distribution within the levees. Figure 1. Location of the San Juan Quadrangle, Texas, USA.

Time-lapse seismic study of levees in southern New Mexico

The primary objective of this work was to measure changes in compressional(Vp) and shear-wave (Vs) velocities in an earthen levee during a ponding experiment designed to simulate flood conditions on the Rio Grande in south New Mexico. Although similar to such experiment, performed an year earlier on the Rio Grande in south Texas, the levee seismic response results are different. This work was similar to previous Preliminary testing at three levee sites, all within a 1 km radius and each with unique physical, EM, and core characteristics, was completed and a single low-conductivity, highly fractured site was selected for investigation. Several different types of seismic data were recorded. Seismic data analysis techniques appraised included Prefraction tomography and Rayleigh surfacewave analysis using multichannel analysis of surface waves (MASW). P-wave velocity change (decrease) was rapid and isolated to one section within the pool confines, which already had anomalously high velocity most likely related to burrowing animals modification of the levee structure. S-wave velocity change was gradual and could be observed along the whole width of the pond within and below the levee. The results within the levee sand core were consistent with the observations of sand S-wave velocity changed due to saturation.

Applications of the JARS method to study levee sites in southern Texas and southern New Mexico

We apply the joint analysis of refractions with surface waves (JARS) method to several sites and compare its results to traditional refraction-tomography methods in efforts of finding a more realistic solution to the inverse refractiontraveltime problem. The JARS method uses a reference model, derived from surface-wave shear-wave velocity estimates, as a constraint. In all of the cases JARS estimates appear more realistic than those from the conventional refraction-tomography methods. As a result, we consider, the JARS algorithm as the preferred method for finding solutions to the inverse refraction-tomography problems.

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...

Remote Sensing and Monitoring of Earthen Flood-Control Structures

Abstract : The purpose of this study was to identify and review technologies that are applicable in locating weaknesses and poor performance within flood-control structures from extreme loading events. The focus of this study was to assess current technologies and state-of-practice techniques involving remote sensing, testing, and real-time monitoring of earthen structures. Advancements in satellite and sensor technology combined with high-speed internet and telecommunication capabilities and smart decision-making software permits real-time monitoring of earthen flood-control structures such as dams and levees. Technologies evaluated included both active and passive sensing methods. These technologies included satellite, airborne, and ground-based sensor systems to identify surface and subsurface characteristics of the watershed, as well as point sensors typically embedded in hydraulic structures to monitor the health of the structure. Point sensors typically record water loading, soil pore pressures, soil movements, and other important properties to evaluate global stability of the water control structure. Geophysical-based methods are typically used in mapping, monitoring, and detection of subsurface stratigraphy, seepage, and any changes in subsurface conditions through time within flood-control structures and their foundations.

Geotechnical Evaluation of the Brownsville Levee Cracking and Partial Slope Failure

Abstract : The U.S. International Boundary and Water Commission (IBWC) discovered cracks and a partial slope failure on a newly refurbished levee section and adjacent floodplain along the Rio Grande River in Brownsville, TX. The partial failure followed a significant drop inwater level in early April 2014. A geotechnical investigation was performed by the U.S. Army Engineer Research and Development Center (ERDC) to determine the causes for the partial levee failure and provide remediation alternatives. A series of events, combined with the local geologic conditions, led to the partial slope failure. Events included the 2012 levee construction, fluctuation and rapid drawdown conditions in the Rio Grande, and a higher elevation of Lake Brown (an oxbow of the Rio Grande) relative to the river. Progressive or creep-type failure mode was identified as the probable mechanism to explain the deformation observed in the field, and this was confirmed by seepage and stability analyses. Based on this evaluation, recommendations for remediation include: (1) implementation of a vegetation control program, (2) short-term monitoring, (3) evaluation of other locations along the river with similar river geometry and groundwater conditions, (4) efforts to minimize sudden drawdown, (5) additional analyses using the design hydrograph, and (6) incorporating cost/benefit analyses for the different alternatives.

The legacy of the 1948 underseepage and crevasse maps, Lower Mississippi River levees

Abstract : Seepage and crevasse maps showing levee performance from the 1937 and 1945 floods in the LMV were compiled as part of the comprehensive study by USACE (1956) into levee underseepage and its control. This map folio was limited in distribution, never officially published as a technical memorandum/report, and was thought to have been lost. The map folio was discovered as part of ongoing research into sand boils in the LMV and consists of 40 15-min scale maps extending from Cairo, IL, to New Orleans, LA. The complete set of maps was scanned and is being presented herein for wider dissemination. This map folio identifies sand boil locations, historic crevasses, severity of seepage, and extent of seepage control measures that were present prior to 1945. A historic summary is presented for background information and importance to ongoing studies of seepage control and internal erosion of levees.

Geologic controls of sand boil formation at Buck Chute, Mississippi

Abstract : Sand boil formation due to underseepage is a potential failure mechanism for levees in the Lower Mississippi River Valley. Sand boils were identified in the Buck Chute study area in the 1990s during high-water events and during the 2009 Flood. The site is unique due to the presence of point bar and abandoned channel deposits. To understand the role of these alluvial deposits on sand boil formation at the site, a geologic investigation of the subsurface was conducted. Using shallow geophysics, cone penetrometer tests (CPT), borings, and a geographic information system (GIS), researchers concluded that the thin blanket associated with point bar deposits, abandoned channel deposits causing a blocked seepage path, and head differential changes caused by the Muddy Bayou Control Structure were the controls of sand boil formation at Buck Chute.

PRACTICAL FOCUSING OF SURFACE-WAVE INVERSION TO IMAGE LEVEES IN SOUTHERN NEW MEXICO

Existing algorithms for inversion of dispersion-curves estimated from analyzing the seismic surface-wave data provide a reasonable representation of the overall shear-wave velocity structure. However, in some cases these results may not meet the resolution requirement of the survey objectives targeting the upper portion of the subsurface section. Near-surface geophysical surveys often look for greater detail in the very near surface, such as the case for levees, while still being interested in the overall velocity structure at greater depth. The proposed method for applying surface-wave inversion revolves around applying a greater degree of focusing on the very near-surface component of the inversion model through the practical use of existing algorithms. Specifically by eliminating the low frequency portion of the dispersion-curve data and using a shallower model. The proposed inversion technique is demonstrated using the multi-channel analysis of surface waves (MASW) inversion algorithm. As a result of using this near-surface focusing approach, surface-wave velocity estimates of the levees possess greater detail and resolution. Results presented here demonstrate the potential of the proposed approach to more accurately image the very shallow subsurface top portion of other near surface MASW sites.

Repeatability Observations from a 2D Time‐Lapse Seismic Survey

Timelapse seismic surveys are particularly useful for detecting changes in material characteristics. Distinguishing changes in subsurface conditions from noise, source and receiver coupling variability, source wavelet change, and other sources of error related to the dynamics of surface conditions in timelapse seismic images is challenging, at best. The objective of this study is to identify factors that affect repeatability of high-resolution near surface time-lapse seismic data when using a projectile source in unconsolidated sediments and investigate minimizing the effects of these factors. Data for this study were acquired as part of a 2D time-lapse seismic reflection survey in the floodplain of the Rio Grande near Las Cruces, New Mexico. The source was a 30.06 fired into pre-drilled holes in sandy soil; the holes were used for the baseline and eight monitor surveys. Changes in wavelet characteristics clearly indicate that the source signature varied nonlinearly as a function of the number of shots fired downhole. Results indicate that changes in the wavelet become increasingly nonlinear at increasing distance from the source and spectrally degrade with continuous use despite attempts to maintain the same firing conditions. This change through use is caused in part by change in source signature due to degradation of the holes during initial shots. It appears necessary to fire multiple shots downhole prior to acquisition to compact unconsolidated sediments and condition holes to increase repeatability of the data. The number of conditioning shots is likely related to soil conditions (saturation, compaction, clay to sand content, etc.).