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Sam is a high-school student who translates Geophysics jargon into normal speech. Click AskSam buttons throughout the webpage to view a translation!Sam is a high-school student who translates Geophysics jargon into normal speech. Throughout this website, you can click on AskSam buttons, like the one on the left, to view a translation!

Environmental Geophysics
Sam is a high-school student who translates Geophysics jargon into normal speech. Click AskSam buttons throughout the webpage to view a translation!

Environmental Geophysics is the use of geophysical methods to image and understand the properties and processes in the top ~100 meters of the earth. This is the region of the earth that has a direct and daily impact on our lives (and on which we have a direct and daily impact!) yet we know surprising little about this near-surface region. Our work in environmental geophysics involves laboratory studies, theoretical modeling, and field work. We use these three different approaches to investigate the links between the geophysical parameters that we can measure and the physical, chemical and biological properties and processes of interest. Many of our research projects can be described as “hydrogeophysics” – using geophysics to address problems in hydrogeology.

 

Ground Penetrating Radar(GPR) and Borehole Radar

GPR and borehole radar are near-surface geophysical techniques that can provide high resolution images of the top few 10’s of meters of the earth. The radar data are acquired by sending electromagnetic waves through the earth and recording the timing and magnitude of energy that arrives at the receiver antennas. A radar image is actually an image of the dielectric properties of the subsurface, as it is the dielectric constant that controls the velocity and the path of electromagnetic waves

Laboratory studies - While radar methods give us an image of dielectric properties, what we want from the data is information about the geological material such as lithology, water content, porosity, permeability. There is also interest in using radar data for the direct detection of organic contaminants. Laboratory studies on rocks and soils allow us to study the relationship between the measured dielectric properties and the materials properties of interest.

Some related publications:
Li, C., P. Tercier, and R. Knight, The effect of adsorbed oil on the dielectric properties of sand and clay, Water Resources Research, 37, 1783-1793, 2001. PDF File
Chan, C. Y. and R.J. Knight, Laboratory measurements of electromagnetic wave velocity in layered sands, Water Resources Research, 37, 1099-1105, 2001. PDF File
Knoll, M.D., Knight, R.J. and Brown, E., Can accurate estimates of permeability be obtained from measurements of dielectric properties? Proceedings, Symposium for the Application of Geophysics to Environmental and Engineering Problems, Orlando, FL, (1995).
Knight, R.J. and Nur, A., The dielectric constant of sandstones, 50 kHz to 4MHz, Geophysics, 52, 644-654, (1987). PDF File

Theoretical/Numerical Studies – Understanding the link between the dielectric properties measured using radar methods and the hydrologic properties, such as water content, is essential if radar is to be used as a hydrologic characterization tool. Once we have an understanding of the relationship between dielectric properties and material properties at the lab-scale, we need to develop the relationships at the field-scale. Stephen Moysey has built a geostatistical framework for upscaling rock physics relationships from the lab to the field (for more information) . Related to this, we are also pursuing ways of integrating radar data with other geophysical and hydrologic data sources to provide hydrologic models that minimize parameter uncertainty.

Some related publications:
Chan, C.Y., and Knight, R., Determining water content and saturation from dielectric measurements in layered materials, Water Resources Research, 35, 85-93, (1999). PDF File

Field studies - GPR images of the subsurface contain much information about spatial heterogeneity – an important aspect of accurately modeling the properties of the subsurface. Stephen Moysey is exploring ways to use neural networks to classify regions in a GPR image so as to build a large-scale model of the subsurface (for more information). This is described in the following publication:
Moysey S., J. Caers, R.J. Knight, R.M. Allen-King, Stochastic estimation of facies using ground penetrating radar data, Stochastic Environmental Research and Risk Assessment, 17, 306–318, 2003.

Some related publications:
Moysey, S., R.J. Knight, and H.M. Jol, Texture based classification of ground-penetrating radar images, Geophysics, 71(6), K111-K118, 2006. PDF File
Moysey S., K. Singha, and R. Knight, A framework for inferring field-scale rock physics relationships through numerial simulation, Geophysical Research Letters, 32, DOI 10.1029/2004GRL022152, 2005.

Forward modeling allows us to predict the way in which variations in subsurface material properties (and the related variation in dielectric properties) is captured in a radar image. James Irving developed 2-D finite-difference time-domain (FDTD) forward modeling codes of GPR for MATLAB as a part of his PH.D. research. Forward modeling code (zip file)

Once we have this model, we have shown how geostatistical methods can be used to quantify the spatial variability seen in a GPR image. Current research is focused on the Hanford site in eastern Washington, where hydrogeologists need an accurate model of the subsurface to better understand the controls on contaminant transport.

Some related publications:
Rea, J., and Knight, R., Geostatistical analysis of ground penetrating radar data: A means of describing spatial variation in the subsurface, Water Resources Research, 34, 329-339, (1998). PDF File
Tercier, P., Knight, R., and Jol, H. , A comparison of the correlation structure in GPR images of deltaic and barrier spit depositional environments, Geophysics, 65, 1142-1153, 2000. PDF File

Additional research, addressed by James Irving as a part of his Ph.D.:

(i) efficient modeling of antenna transmission and reception for crosshole GPR (for more information, PDF File)

(ii) investigation of the effects of antenna length on crosshole GPR tomography (for more information, PDF File)

(iii) strategies for improving crosshole GPR tomography at close borehole spacings

(iv) GPR diffraction velocity analysis (with Paul Sava in SEP) (for more information)


Nuclear Magnetic Resonance

NMR is a measurement technique that can provide an incredible amount of information about the pore-scale properties of geological materials. NMR measurements can be made in the laboratory or in the field using logging devices. There is also a new surface NMR system, designed to sample the top 100-150m of the earth, for groundwater applications.

Laboratory studies – Proton-NMR directly detects hydrogen atoms so is sensitive to the presence and molecular environment of water and hydrocarbons. We are using this fact to study the use of NMR for the detection of organic contaminants.

Some related publications:
Bryar, T.R. and R.J. Knight, Laboratory studies of the effect of sorbed oil on proton nuclear magnetic resonance, Geophysics, 68, 942-948, 2003. PDF File
Daughney, C., Bryar, T., and Knight, R., Detecting sorbed hydrocarbons in a porous medium using proton nuclear magnetic resonance, Environmental Science & Technology, 34, 332-337 (2000). PDF File
Hedberg, S. A., Knight, R.J., MacKay, A.L. and Whittall, K.P., The use of nuclear magnetic resonance for studying and detecting hydrocarbon contaminants in porous rocks, Water Resources Research, 29, 1163-1170, (1993). PDF File

One of the challenges of using NMR data is the accurate inversion and interpretation of the data. Kristina Keating is currently exploring the effect of iron on NMR data from the near-surface region of the earth (for more information).

Some related publications:
Keating, K., and R. Knight, A laboratory study to determine the effect of iron-oxides on proton NMR measurements, Geophysics, 72(1), E27-E32, 2007. PDF File


Electrical Resistivity Imaging

Electrical resistivity in a geophysical property that is very sensitive to subsurface water content and salinity. Electrical resistivity imaging is a geophysical methods that uses many electrodes to obtain an image of subsurface electrical resistivity; this can then be used, for example, to monitor the infiltration of water or the movement of a fresh water-salt water interface. Adam Pidlisecky has developed a algorithm to invert 3-D resitivity data as a part of his Ph.D. A link to the code will be posted here in the future.

Some related publications:
Pidlisecky, A., E. Haber, and R. Knight, RESINVM3D: A MATLAB 3-D Resistivity Inversion Package, Geophysics, 72(2), H1-H10, 2007. PDF File


“Cone Geophysics”

“Cone geophysics” is a new word for a new area of research where we are developing ways of using cone penetrometers to obtain geophysical data from the top 100 m of the earth. Cone penetrometers are commonly used in engineering studies and are pushed into the earth, making it a measurement that is considered to be “minimally invasive”. We are investigating ways of conducting geophysical surveys, which usually require the drilling of boreholes, using the cones. We are also finding ways of using the data typically collected with cones to help with the inversion and interpretation of near-surface geophysical data. Research on this topic was conducted by Adam Pidlisecky as part of his Ph.D. (for more information).

Some related publications:
Pidlisecky, A., R. Knight, and E. Haber, Cone-based Electrical Resistivity Tomography, Geophysics, 71(4), G157-G167, 2006. PDF File
Knight, R. and Pidlisecky, A., Cone-based geophysical imaging: A proposed solution to a challenging problem, The Leading Edge, 24, 34-38, 2005. PDF File
Jarvis, K., and Knight, R., Near-surface VSP surveys using the seismic cone penetrometer, Geophysics, 65, 1048-1056, 2000. PDF File


 

 

 

 

 

 

 

 

 

 

  Last modified Tuesday, 06-Feb-2007 10:30:02 PST
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