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Method Name: |
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Ground penetrating radar - Borehole tomography |
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Method Type: |
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Electromagnetic Methods
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Assigned Problems: |
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Principle: |
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Determination of the electromagnetic wave velocity distribution and attenuation between two boreholes. GPR responds to changes in electromagnetic properties that are related to characteristics of soil, rock and interstitial fluids. |
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Keywords: |
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Georadar; electromagnetic reflection method (EMR); dielectric permittivity; electrical conductivity; velocity tomograms; attenuation tomograms; 3-D georadar cubes |
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Prerequisites: |
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- Target must be characterized by a contrast in electromagnetic properties (dielectric permittivity, electrical conductivity)
- Uncased or PVC-cased boreholes, air or water-filled
- Penetration of georadar signals in conductive material, such as clays / clay-rich sediments and in materials having conductive pore fluid, may be limited or even impossible.
- Borehole deviation logs should be available (i.e., exact antenna coordinates)
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Resolution: |
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Resolution depends primarily on the frequency of the emitted signals and ranges from a few centimeters (2 GHz antennae), to approx. 1 m (100 MHz antennae) and a few m (25 MHz antennae). The horizontal resolution is additionally constrained by the borehole separation and the ratio of borehole separation and borehole depth.
Maximum depth of investigation is constrained by borehole depth. Good resolution is only provided in areas where ray paths intersect, hence a lack of resolution exists at the top and bottom of the boreholes. |
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Expected Results: |
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- Measured parameter: electromagnetic signals reflected at discontinuities within the Earth (radargram).
- Data analysis: determination of onset times and amplitudes of georadar waves. Tomographic inversion of travel times (velocity tomograms) and amplitudes (attenuation tomograms).
- Interpretation: conversion of velocities and attenuations to dielectric permittivity and electrical resistivity. Association of these electromagnetic parameters with geological units or presence of interstitial fluids.
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Combination with other Methods: |
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- Required additional information: geological information for reliable interpretation
- Related add-on information:surface-based ground-penetrating radar data
- Independent additional information: crosshole seismics; crosshole geoelectrics; surface-based geoelectric data; electromagnetic data
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Operating Expense: |
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- Crew size: 1 key person, 1-2 assistants
- Acquisition speed: depends mainly on the antenna interval: 1 - 2 tomographic planes may be acquired during 1 day
- Processing: requires 3 -4 days per acquisition day
- Equipment rental costs: intermediate
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Parameters to specify: |
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- Dominant antennae frequency
- Source and receiver intervals (usually a few dm)
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QC Documents: |
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- Ray diagrams or resolution matrices
- Field notes (e.g., all activities, effective time schedule, present personnel)
- Optional: Map of buried cables, roads
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Products: |
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- Velocity tomograms
- Attenuation tomograms
- Interpretation
- Optional: dielectric number resp. conductivity tomograms
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