F. Dale Morgan

Dielectric constant determination using ground-penetrating radar reflection coefficients

A method to determine ground-penetrating radar (GPR) velocities, which utilize Brewster angles, is presented. The method determines the relative dielectric constant ratio at interface boundaries where the radar wave is traveling from a low-velocity to a high-velocity medium. Using Brewster angle analysis is currently the only means to determine the velocity of the medium below the deepest detectable reflector. Data are presented for water-saturated clean sand with a known velocity of 0.52 m/ns, which overlays a sandy silt with a known velocity of 0.13 m/ns. Brewster angle analysis of a common midpoint (CMP) survey gives a relative dielectric constant ratio of 33/4.77. The Brewster angle relative dielectric constant ratio is in good agreement with the relative dielectric constant ratio calculated from the known velocities.

Pore-scale modeling of electrical and fluid transport in Berea sandstone

The purpose of this paper is to test how well numerical calculations can predict transport properties of porous permeable rock, given its 3D digital microtomography (μCT) image. For this study, a Berea 500 sandstone sample is used, whose μCT images have been obtained with resolution of 2.8 μm . Porosity, electrical conductivity, permeability, and surface area are calculated from the μCT image and compared with laboratory-measured values. For transport properties (electrical conductivity, permeability), a finite-difference scheme is adopted. The calculated and measured properties compare quite well. Electrical transport in Berea 500 sandstone is complicated by the presence of surface conduction in the electric double layer at the grain-electrolyte boundary. A three-phase conductivity model is proposed to compute surface conduction on the rock μCT image. Effects of image resolution and computation sample size on the accuracy of numerical predictions are also investigated. Reducing resolution (i.e., increasi...

Adaptive Moving Window Method for 3D P-Velocity Tomography and Its Application in China

A new tomography method, the adaptive moving window, is introduced and applied to construct the velocity structure of the crust and upper mantle of China and surrounding areas. More than 345,000 high-quality compressional body-wave phase data extracted from the Annual Bulletin of Chinese Earthquakes spanning from 1990 to 1998 are used. The area of interest is represented horizontally by 2338 points with 1° intervals. Each point is assigned a window (a cell or a region centered at each point) whose size is varied depending upon the ray path density. A five-layer 1D model from surface down to uppermost mantle is then determined at each point by performing a Monte Carlo random search where earthquake locations are held constant. Combining and smoothing the obtained 1D models, an equivalent 3D model is achieved. The predicted travel times through the 3D model match very well with the observed ones from local to regional distances. The model has a good correlation with tectonic features and is generally consistent with the existing models constructed by other researchers. Our model gives detailed information about structure and is feasible for application to high-quality earthquake location problems. Manuscript received 24 June 2003.

Application of electrical resistivity tomography to image Harrison Caves in Barbados, West Indies

In this presentation we demonstrate a new electrical resistivity tomography technique to image an underground cavern system. The success of this technique not only depends on the inversion procedure but also on the data acquisition methodology. We compare inversion results which use two such methodologies as inputs, e.g. standard pseudo-section data acquisition and data acquisition with a spatially varying source dipole. We illustrate that the variable source dipole tomographic data acquisition geometry can lead to better resolution and placement of sub-surface structure, especially for structures which contain extended lateral anomalies. The technique is tested on a numerically generated synthetic model and successfully applied to map a limestone cave system in Barbados, West Indies.

INDUCED POLARIZATION WITH ELECTROMAGNETIC COUPLING: 3D SPECTRAL IMAGING THEORY: EMSP PROJECT NO. 73836

The principal objective of the project was to develop a non-invasive imaging technique, based on spectral induced polarization (SIP), to characterize in-situ distribution of organic and inorganic contaminants. This was to be an advance over a similar technique offered by the DC resistivity method. The motivation for the choice of IP over resistivity is rooted in the fact that resistivity response is governed by volume distributions of electrical parameters and therefore is relatively insensitive to small changes contributed by the presence of contaminants. IP response on the other hand is governed by the electrochemical properties of the rock-grain pore-fluid interface, which can be significantly altered by the incoming contaminant (ions) over long residence times. Small concentrations of contaminants are the rule rather than the exception thus, the detection threshold for IP, which is more sensitive to small concentrations, is much lower than for resistivity (IP field threshold for PCE/TCE is about 1mg/g). Additionally, the observation that IP depends on the chemistry of the contaminants provided the motivation that a spectral IP response could lead to a database of identifying signatures by which contaminants can be discriminated.

3-D Spectral IP Imaging: Non-Invasive Characterization of Contaminant Plumes

The overall objective of this project is to develop the scientific basis for characterizing contaminant plumes in the earths subsurface using field measurements of induced polarization (IP) effects. Three specific objectives towards this end are: (1) Understanding IP at the laboratory level through measurements of complex resistivity as a function of frequency in rock and soil samples with varying pore geometries, pore fluid conductivities and saturations, and contaminant chemistries and concentrations. (2) Developing effective data acquisition techniques for measuring the critical IP responses (time domain or frequency domain) in the field. (3) Developing modeling and inversion algorithms that permit the interpretation of field IP data in terms of subsurface geology and contaminant plume properties.