Download
1 / 67
680 likes | 783 Vues
Needs to be more interactive. Environmental and Exploration Geophysics II. Hand out sections and have them interpret/identify example GPR sections. Even with the review of resolution concepts, we finished in one hour. Radar Methods - An Overview. tom.h.wilson wilson@geo.wvu.edu.
E N D
Needs to be more interactive Environmental and Exploration Geophysics II Hand out sections and have them interpret/identify example GPR sections. Even with the review of resolution concepts, we finished in one hour Radar Methods - An Overview tom.h.wilson wilson@geo.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV
The radar band is loosely taken to extend from approximately 0.1cm to 100cm. The microwave region is often used for surface imaging from airborne or satellite platform.
Ground penetrating radar (GPR) systems often operate in the tens of MHz to GHz region of the spectrum. 25MHz = 12m wavelength (40ns) 50MHz = 6m (20ns) 100MHz = 3m (10ns) 1GHz = 0.3m (1ns) Times in nanoseconds represent the time it takes light to travel through 1 wavelength in a vacuum.
Visual wavelength image Shuttle Imaging Radar - SIR A ~ 25cm Sabins, 1996
0.25 meter wavelength, 1.2 GHz dry sand penetration depth of ~ 1.25m Sabins, 1996
Ground Surveys GPR bistatic and monostatic transmitter-receiver configurations. Note similarity to coincident source-receiver and offset source receiver configurations discussed in the context of seismic methods Daniels, J., 1989
Spectral and temporal characteristics of the GPR wavelet. Sensors & Software Inc. - Ekko Updates
As with seismic data, reflection arrival times are 2-way times and depth equals two-way time x average velocity. Velocity in air is approximately velocity of light in a vacuum - c. c = 3 x 108 m/sec = 9.84 x 108 f/s or approximately 1 foot per nanosecond. 1 nanosecond is 10-9th seconds. Thinking in terms of two-way times, it takes 2ns to travel 1 ft.
In general the velocity of the radar wave is defined as where c is the velocity of light in a vacuum (or air), and r is the electric permitivity of the material through which the radar wave travels. Examples of r (see Daniels) are 81 for water 6 for unsaturated sand 20 for saturated sand The presence of water has a significant effect on velocity.
Typical velocities c ~ 1ft/ns in air v ~ 1/2 to 1/3rd ft/ns in unsaturated sand v ~ 1/3rd to 1/5th ft/ns in saturated sand is proportional to conductivity - materials of relatively high conductivity have slower velocity than less conductive materials.
Daniels makes note of an equation which specifies how the electric field will respond to varying conditions of conductivity () and permitivity (). Daniels goes on to note, specifically that the ratio determines whether the material behaves primarily as a conductor or as a dielectric
where In low-loss materials P ~ 0 and the speed of the radar waves is As implied, low loss materials are also low conductivity () materials.
In our discussions of seismic we recognized absorption as an important process affecting the ability of the seismic wave to penetrate beneath the earth’s surface. High attenuation coefficient produces rapid decay of seismic wave amplitude with distance traveled (r). The same process controls the ability of electromagnetic waves to penetrate beneath the earth’s surface. The expression controlling attenuation is a function of several quantities, the most important of which are conductivity and permitivity.
Attenuation of electromagnetic waves is controlled by the propagation factor which has real and imaginary parts. The real part (the attenuation coefficient) illustrates the influence of permitivity and conductivity on absorption. Note in this equation that increases of translate into increased attenuation. Also note that increases of angular frequency (=2f) will increase attenuation.
The role of the attenuation coefficient shows up clearly in the bistatic radar equation. P is power, A is area and G is gain. The in this equation refers to scattering cross section. You’ll note some similarities between this equation and the one we derived in class for seismic waves.
On page 96 of Daniels paper there is some discussion of velocity determination. Reference is made to use of the diffraction response from an object atknown depth. If the depth is known then the velocity is just depth divided by one-half the arrival time at the diffraction apex. If the depth were not known, how else might you determine velocity from the appearance of the diffraction event?
Critical refraction Direct arrival Reflection hyperbola The characteristics of a common midpoint gather of GPR records looks very similar to those for seismic data. Smith and Jol, 1995
Diffraction and normal incidence reflection response Daniels, J., 1989
GPR diffractions Daniels, J., 1989
Thinning layer response and resolution considerations. Daniels, J., 1989
The Fresnel Zone Radius Rf An approximation
Topographic variations must also be compensated for. Daniels, J., 1989
GPR data is often collected by pulling the GPR unit across the surface. Subsurface scans are made at regular intervals, but since the unit is often pulled at varying speeds across the surface, the records are adjusted to portray constant spacing between records. This process s referred to as rubbersheeting. Daniels, J., 1989
Spectral and temporal characteristics of the GPR wavelet. Sensors & Software Inc. - Ekko Updates
Comparison of the 25MHz and 100 MHz records Smith and Jol, 1995, AG
Noise corrupts GPR as well as seismic data. GPR sources are quite different. Daniels, J., 1989
In the acquisition of GPR data one must worry about overhead reflections. Daniels, J., 1989
…. and tree branches! Daniels, J., 1989
The abrupt discontinuity of velocity associated with the house basement produces a nice diffraction. Daniels, J., 1989
GPR unit Sensors & Software Inc. - Ekko Updates
Terrain Conductivity Landfill Magnetic Field Switzerland, glacial sediments Green et al., 1999, LE
Ground Penetrating Radar data on and off the site Green et al., 1999, LE
Time slice map from 3D data volume of radar data. This is a surface of equal travel time. Disruptions in the reflection pattern are associated with the waste pit. Green et al., 1999, LE
Bright red areas define the location of the landfill; the orange objects represent gravel bodies. The brownish-pink lobes are high reflectivity objects of unknown origin. The view at bottom profiles the underside of the landfill and gravel bodies. Green et al., 1999, LE
Some general information about cost and effectiveness of various geophysical methods, Green et al., 1999, LE
