NS31B-01
Integrated Geophysical and Geotechnical Investigation of the Failed Portion of a Road in Basement Complex Terrain, Southwest Nigeria.
Several efforts by the local authority to fix the bad portion of the Ijebu Ode – Erunwon road, southwest Nigeria have over the years yielded no meaningful result as the road often gets deteriorated shortly after repair. Motorists and other road users have abandoned this road that is meant to link a commercial center with several farming communities, thus making the cost of transportation of farm produce very high. Geophysical methods of investigation integrated with geotechnical studies of the road, especially the failed portion were undertaken with the object of finding causes of the road failure as well as determine the most appropriate design method. Very Low Frequency Electromagnetic (VLF-EM) and electrical resistivity methods were employed to determine sections of the road with anomalous electrical responses that could be interpreted in terms of structures, lithology, water saturation etc. Geotechnical studies such as California Bearing ratio (CBR), grain size analysis, liquid limit, linear shrinkage, plastic limit, etc of samples obtained from sections identified as having high electrical anomaly were also carried out to determine geotechnical properties. Plots of inverted VLF-EM real and filtered real data identified bad sections of the road show peaks with positive filtered real amplitudes greater than 30% and lower amplitudes less than 30% and were interpreted to indicate major and minor linear features of faults and fractured zones, which in addition also display very high conductivity values. Vertical Electrical Soundings (VES) probe of the failed portions gave three to four layered earth interpreted to contain clayey-sand and loose saturated sand units at the upper portion, highly saturated weathered/fractured basement and fresh basement at the lower portion. Soaked and unsoaked CBR result range from 70.3 – 83.9%, and 12.9 – 31.6% respectively, indicating percentage reduction in strength with wetness of 55.7% - 83.8%. Liquid limit, linear shrinkage and plastic limit index gave values in the range of 24.0 – 48.5%, 2.1 – 12.9%, 7.5 – 27.4% respectively. The plot of the elevation values from Global Positioning System (GPS) readings gave a 3D topographical model of the area and shows the failed portion to be topographically depressed, lower in elevation than the surrounding stable portion. This study implies that integrated geophysical and geotechnical investigation offer very useful approach for characterizing near surface earth and thus can help in preparation before engineering structures are found on same.
NS31B-02
Spectral Correlation of Thermal and Magnetotelluric Responses in a 2D Geothermal System
A methodology of thermal response observations at regional scale in geothermal systems was implemented using magnetotelluric(MT) data that was analyzed by spectral correlation of EM anomalies. Local favorability indices were obtained enhancing the anomalies of thermal flow and their corresponding magnetotelluric responses related to a common source. A C++ code was developed to compute magnetotelluric and thermal responses using finite differences of a geothermal field model. The problem of thermal convection was solved numerically using the approach of Boussinesq and temperature and thermal flow profiles are obtained, also is solved to the equations of electromagnetic induction 2D that govern the wave equation for the H-polarization case in a two-dimensional model of the system. This methodology is useful to find thermal anomalies in conductive or resistive structures of a geothermal system, which is directly associated with the litology of the model such as magmatic chamber, basement and hydrothermal reservoir.
NS31B-03
The Effects of Topography on Time Domain Controlled-Source Electromagnetic Data as it Applies to Impact Crater Sites
Controlled-source electromagnetic geophysical methods provide a noninvasive means of characterizing subsurface structure. In order to properly model the geologic subsurface with a controlled-source time domain electromagnetic (TDEM) system in an extreme topographic environment we must first see the effects of topography on the forward model data. I run simulations using the Texas A&M University (TAMU) finite element (FEM) code in which I include true 3D topography. From these models we see the limits of how much topography we can include before our forward model can no longer give us accurate data output. The simulations are based on a model of a geologic half space with no cultural noise and focus on topography changes associated with impact crater sites, such as crater rims and central uplift. Several topographical variations of the model are run but the main constant is that there is only a small conductivity change on the range of 10-1 s/m between the host medium and the geologic body within. Asking the following questions will guide us through determining the limits of our code: What is the maximum step we can have before we see fringe effects in our data? At what location relative to the body does the topography cause the most effect? After we know the limits of the code we can develop new methods to increase the limits that will allow us to better image the subsurface using TDEM in extreme topography.
NS31B-04
Electrical Resistivity Study of a Pleistocene Riverbed in Saltville, VA
A shallow capacitively coupled resistivity survey was performed in Saltville, VA, in an area of suspected buried Pleistocene river deposits. Previous excavations in the sediments beneath the Saltville valley floor had been performed to recover late Pleistocene megafaunal remains and possible Clovis-age human artifacts. These digs encountered a zone, one to two meters deep, of gravel-sized rock fragments, including some boulders up to 75 cm. in diameter. These large clasts are rounded, show some imbrication (shingle-like overlapping indicative of current flow), and have been interpreted as river channel deposits. Carbon 14 dates from the megafaunal bones within and just above the gravel bed yielded dates of 14,500 years BP. Resistivity signals in a number of locations were consistent with cobbles and boulders deposited in a river channel. These signals are generally bowl- shaped areas with large circular (2-d scans) anomalies near the center, and smaller circular anomalies tapering out towards both sides. The bowl-shaped anomalies are within 3 meters of the surface. With several lines imaged in this survey a rough path of the riverbed, along with a number of branchings is traceable in the survey area. An exploratory hole confirmed the presence of a layer of rounded cobbles and boulders 1.3 meters deep beneath one of the survey lines.
NS31B-05
First results of a high resolution reflection seismic survey of the Central Northern Venezuelan Shelf
In September - November 2007 the first high resolution marine seismic campaign on the North-Central coast of Venezuela was carried out between Cabo Codera and Golfo Triste. The principal aim of this work was to characterize the active San Sebastian Fault (SSF) and to analyze Cenozoic sedimentation on the Venezuela shelf focusing on: i) effects of active tectonics and ii) coastal landslides/flashflood deposits related to 1999 Vargas catastrophic event or to similar phenomena. Data were acquired onboard R/V GUAIQUERI II from the Oceanographic Institute of the Oriente University. The seismic source was a “CENTIPEDE” sparker (RCGM) operated between 300 and 600 J, 1.3 kHz main frequency. We used a single-channel streamer with 10 hydrophones. In total, 49 seismic profiles were collected, with a cumulative length of 1000 km approximately. In these seismic profiles we identified and separated the deposits into three main units. Unit (U1) comprises low energy reflectors mainly dipping in southward direction (i.e. toward the coast bounded by the San Sebastian Fault). This unit also includes a number of isolated acoustic anomalies, which we tentatively interpret as coral reefs. Its top is defined as Basal Erosional Discontinuity (BED) onto which Unit 2 (U2) deposits are onlapping. U2 is acoustically well-stratified, with strong reflectors. Gradual variations in thickness and a wavy configuration allow us to interpret U2 as probably Quaternary current-related deposits. Last Unit (U3) was defined on the Venezuela shelf and corresponds to prograding sequences probably related to the terrigenous input of the Tuy River. Impact of eustatic fluctuations on these deposits are discussed. The data were also used to construct a simplified bathymetry of the studied area. The lateral transition from the western Cariaco-Tuy pull-apart basin to the (single) SSF was clearly imaged (mostly folds and gravity faults). The survey also displayed prograding sediments bodies in La Tortuga Shelf. The main SSF fault is not clearly expressed in all profiles as a clear rupture of the sea floor sediments but rather as highly deformed zones. Our data set, also evidences –preliminary, due to relatively wide spacing fo the seismic grid- the interest of the studied areas for the investigation of regional natural hazards (climatic, seismo-tectonic), and of the impact of sea level changes in the southeastern Caribbean. This project was funded by FONACIT project PI-2003000090 and French-Venezuelan ECOS-Nord scientific exchange program.
NS31B-06 [WITHDRAWN]
Seismic Imaging and Hydrogeologic Characterization of the Potomac Formation in Northern New Castle County, Delaware
A land streamer system, an alternative to conventional seismic acquisition equipment for collecting large
amounts of seismic reflection data in urbanized and semi urbanized areas, is being used to conduct a near
surface high-resolution seismic experiment in Northern New Castle County, Delaware. The main goal of this
project is to provide continuous data of the subsurface in order to improve our understanding on the connectivity
of sand bodies and water flow pathways distribution in ancient fluvial deposits, such as those of the Potomac
Formation, that were deposited along passive margin, alluvial plain settings. Such understanding is necessary to
create accurate models for groundwater flow and to identify groundwater contaminant pathways.
The Potomac Formation was deposited during the Albian to early Cenomanian. In northern Delaware, these
sediments are entirely fluvial deposits that are thought to onlap Paleozoic basement, and are truncated by an
unconformity. McKenna et al. (2004) recognized five facies for this unit in Delaware: amalgamated sands, thick
individual sands, thin sands, interlaminated sands, and mottled silts and clays, and described the sands of the
unit as being laterally discontinuous, resulting in a "labyrinth style heterogeneity".
Benson's (2006) well-log correlations show the depth of the basement ranging from 115 m to 400 m in the study
area of this project. A noise test and a 1.2 km long high-resolution seismic reflection line collected using
conventional seismic reflection methods during the preliminary phase of the project indicate that seismic
methods can be used in this area to image the subsurface as shallow as 18 m and as deep as 315 m, and
suggest that the basement is being imaged. During this project, a 30-km seismic dataset and two continuous
cores will be collected. Sonic logs collected at the cores will be used to create synthetic seismograms to create
depth sections that will be correlated with existing geophysical logs and existing sediment samples to create
cross sections, a model of the geometry of the fluvial system, and facies maps. The core samples will be used to
determine porosity and permeability which will allow better understanding of the heterogeneity of this unit.
This project is important because the methodology to be used will provide a robust 2-D dataset that will allow one
to test/revise the existing facies analysis, and stratigraphic correlations that are based in 1-D well data and are
actually used for ground water modeling in the state of Delaware where the population depends and benefits
from groundwater supply.
NS31B-07
Examining the Impact of Non-Linearity on Rock Physics Relationships from a Generalized Stochastic Perspective
One of the major challenges in rock physics research is understanding how and when relationships determined in one setting can be applied in another. This problem underscores the statistical nature of rock physics problems in practice, i.e., how can we assess whether processes captured by a specific rock physics relationship are stationary over space, time, or measurement scale? We hypothesize that answers to this question depend strongly on the degree of non-linearity of the underlying rock physics problem. Non-linearity causes physical processes to become dependent on the specific arrangement and magnitude of properties in a sample of the subsurface. To investigate this hypothesis, we use a generalized stochastic framework that considers the conditional probability density function (pdf) between measurable geophysical properties, G, and other subsurface properties of interest, H. Observable values of G and H are assumed to be dependent upon the same properties at smaller scales as defined by the random fields g and h . We expand the conditional pdf f(H|G) into a product of statistical distributions having dependencies on: (i) geologic variability controlling the distribution of h, (ii) uncertainty in the local rock physics processes linking g and h, and (iii) errors in the underlying physics describing the upscaling of g and h to obtain G and H, respectively. Using this result, the statistical assumptions built into rock physics relationships can be identified. Furthermore, the sensitivity of rock physics relationships calibrated with a given set of assumptions can be tested to assess their generality under varying degrees of model and conceptual error. As an example, we consider a crosshole radar tomography problem where two different approaches for dealing with geophysical scale and resolution issues are examined. The first method is the random field averaging approach of Day-Lewis et al. (2005). The second method is the full-inverse statistical calibration approach of Moysey et al. (2005). In the example, the degree of non-linearity in the underlying radar measurements is controlled by successively increasing the variance of the water content distribution. The degree of non-linearity is quantified with the pdf for errors between the predicted and actual upscaled geophysical properties over an ensemble of realizations. The impact of these errors on estimated rock physics relationships by each method and the resultant error in the prediction of water content is then assessed.
NS31B-08
2D Simulation of Surface GPR Wave Propagation with TDFEM: Preliminary Results
Ground Penetrating Radar (GPR) has a wide range of applications in sedimentological, hydrogeological and archeological studies. The presence of complex subsurface features often poses difficulties in radar data interpretation. Thus, numerical models are required to better understand the physical behavior of the propagating electromagnetic wave (radar wave) and its interactions with surrounding subsurface materials. Of the two most effective numerical techniques for solving electromagnetic wave equations, finite difference (FD) and finite element (FE), the FEM, based on mesh structures, provides greater flexibility for better approximation of complex geologic features e.g. karst features. We have developed a Time Domain Finite Element Method (TD FEM) simulation of surface GPR wave propagation in 2D, using FEM based software, Comsol Multiphysics. The model involves propagation in transverse electric (TE) mode. For complete absorption of radiated energy at the boundaries of the solution domain, we have implemented Perfectly Matched Layer (PML) boundary conditions. Comsol Mutliphysics allows easy implementation of subsurface material properties. Complex geologic features e.g. irregular fractures in karst terrains as well as micron-scale fine structures can be modeled with ease using this program.
NS31B-09
Geoelectrical signatures of microbial activity: A laboratory tank investigation
Experiments in a 2D static tank (dimensions 145 cm x 85 cm x 6 cm) were performed to assess the effectiveness of various geo-electrical methods (induced polarization [IP], resistivity [R], self potential [SP], electrodic potential [El]) to monitor microbial activity. The tank was filled with coarse sand and saturated with water from the river Lagan (Belfast, UK) diluted with tap water (20:80 ratio). The river water was used to ‘seed' microbial activity in the tank. We created 2 separate zones of activity by applying a layer of Hydrogen Release Compound (HRC) and a layer of Oxygen Release Compound (ORC) at the bottom and the middle of the tank respectively. This design aimed to simulate conditions at the oxic – anoxic interface commonly found in contaminated groundwaters. We recorded IP and R signals using a network of 32 electrodes on one side of the tank; the same electrodes were used for El measurements; we recorded the SP signals with 9 Petiau electrodes located at the top of the tank. El measurements provide us with detailed information on the spatial and temporal Eh evolution in the tank, as a result of microbial activity (indirect evidence), whereas the SP measurements are related directly to the microbial activity (direct evidence); IP and R measurements were aimed to provide additional insight on microbial induced fluid conductivity / porosity / mineralization processes expected to occur under our experimental conditions. The combination of El and SP measurements is shown to be a powerful approach for remote monitoring of microbial activity in the subsurface; the methods supplement each other to more accurately describe the microbial evolution. Furthermore , the interpretation of the subsurface conditions are improved when constrained by the fluid conductivity and mineralization changes inferred by the IP and R measurements. The results of this work suggest that a combination of different geo-electrical methods enhance the subsurface monitoring of microbial activity.
NS31B-10
Non-invasive Investigation of Free Phase Gas Accumulation in a Northern Peatland Using GPR: Vegetation Effects
Northern peatlands are known to produce methane, although the contribution of this source to the atmospheric methane burden is still uncertain. Biogenic methane releases have become an increasingly important issue with regard to assessing the impact of northern peatlands impact on the global carbon budget. Previous workers have suggested that methane production is pronounced in a zone a few meters below the surface and that accumulation of free phase gas is encouraged by the presence of confining layers that act to hinder FPG release. We used ground penetrating radar (GPR) to non-invasively investigate (1) where free-phase gas (FPG) methane may be accumulating vertically within the peat column of a northern peatland, and (2) the dependence of methane production on vegetation type. Common mid-point (CMP) measurements were applied to investigate the likely vertical spatial distribution/ concentration of trapped methane within the peat profile within different vegetation units. The CMP data were modeled using the Complex Refractive Index Model (CRIM) to obtain a one dimensional model of interval layer velocities presumably representing the vertical distribution of gas content. CMP surveys were conducted at several locations in Caribou Bog peatland (Orono, Maine) in order to assess how peat thickness and surface vegetation communities may be impact FPG accumulation in the sub-surface. These locations include an open pool system, a low shrub heath area and a densely forested zone. As well as having variable surface vegetation communities, the total peat thickness is distinctly different at each of these sites. Preliminary results suggest that the CMP models are distinctly different between these sites and suggest a dependence of FPG accumulation on vegetation type.
NS31B-11
Geo-electrical profiling over peatlands underlain by a granitic aquifer
Peat thickness can play an important role in the hydrogeological balance of catchments underlain by crystalline bedrock. The accurate estimation of its depth provides a more scientifically justifiable basis for locating hydrogeological investigation/monitoring points. Geophysical methods are commonly employed because they can rapidly provide wider continuous spatial coverage compared to drilling /coring. We performed surface resistivity and induced polarization (IP) measurements to estimate the depth of peat layer overlying granitic aquifer in a densely forested upland catchment in County Galway, Ireland. Seven geo-electrical tomography profiles (120 – 200 m) provided detailed information about the peat thickness and the overall depth to bedrock. Profile results, verified by selective drilling showed the peat to overlay an intermediate zone (transition zone) extending from the base of the peat base to the top of intact bedrock. IP measurements corroborated this interpretation of the resistivity data. The investigation highlights the benefits of geophysical methods as a reconnaissance tool in crystalline catchments continuing by peat.
NS31B-12
Hyporheic Exchange in a Small Mountain Catchment (Valles Caldera, NM): Observations Using Temperature Sensors
Over 100 temperature/light sensors have been deployed in a small (about 3.7 km2) mountain catchment in the Valles Caldera National Preserve of New Mexico to observe water exchange between the stream and the hyporheic zone. These probes were installed in November of 2006 and have been recording continuously since at 3-hour intervals. Three pieces of information can be gleaned from this dataset. (1) Flow permanence can be detected based on the difference between streambed temperature and air temperature. The two forks of the stream have different character throughout the year; the east fork is dry for much of the year, and its flow seems to be controlled primarily by snowmelt. The west fork flows for much of the year. (2) The onset and end of seasonal snow cover can be determined at each streambed point as the times when temperature fluctuation ceases and resumes. This can be verified using light intensity data, although a dry and wet streambed beneath a snowpack cannot be differentiated. (3) Gaining reaches of the river can be easily detected based on their daily temperature fluctuation; residence time in the subsurface decreases fluctuation amplitude. Inferences on the amount of upwelling from the hyporheic zone and the length of the flowpath can be made based on temperature fluctuation.