1. Geology 5660/6660 Applied Geophysics 28 Feb 2014 © A.R. Lowry 2014 Last Time: Ground Penetrating Radar (GPR) Radar = electromagnetic radiation (light) in the 50-1000 MHz (radio) frequency band  Governed by wave equation (  similar to seismic!)  Source & receiver are dipole antennae  Signal is a single pulse  Processing & display analogous to seismic section  High frequency  high resolution but also high attenuation  Images changes in electromagnetic impedance Z For Mon 3 Mar: Burger 349-378 (§6.1-6.4)

2. Last Time: Ground Penetrating Radar (GPR) Velocity (usually) is not estimated; emphasis is mostly on the the imaging of structure rather than physical properties. Instead TWTT  depth is approximated from rough ~ V Radar reflections image variations in Dielectric constant  r ( = relative permittivity )  3-40 for most Earth materials;  higher when H 2 O &/or clay present Geology 5660/6660 Applied Geophysics 28 Feb 2014 © A.R. Lowry 2014 For Mon 3 Mar: Burger 349-378 (§6.1-6.4)

3. Applied Geophysics “In the News”: Texas A&M researchers use GPR to image Civil War era fortress structure under Alcatraz… On the BBC.

4. For most applications (i.e., near-surface)  1 ≈  2 ≈ 1 ;  (10 -4 –10 -1 ) «  (10 6 –10 10 !), and hence (i.e., we are imaging velocity variations corresponding to changes in dielectric constant!) For the water table, R ~ 0.1 Recall seismic waves attenuate as where Q is quality factor; Radar waves attenuate similarly as ; where Attenuation is extremely high for shale, silt, clay, and briny water (which is why GPR rarely penetrates > 10 m!). 

5. Skin depth, or depth of penetration, is ~ 1/ . Hence main applications are in archaeology, environmental, engineering site investigation… Also used for cavity detection and other very near-surface applications GPR freqs

6. Frequency-dependence of the attenuation results in dispersion : High frequencies attenuate more rapidly; pulse appears to “broaden” and the phase is delayed: This has “appearance” of a lower velocity medium. GPR freqs

7. (From a very old cemetery in Alabama…) “Black-box” processing is simplistic so see some of the same features observed in low-level (brute stack) seismic processing:

8. Assuming a constant velocity can introduce a factor of 2 to 3 scale error in converting velocity to depth! ( But one could reduce velocity scaling error if  were calculated from, e.g., travel-time amplitude decay)…

9. V1V1 Alternatively can use moveout on Diffractions : h1h1 h2h2 x The equations are the same as they were for seismic, but since GPR is (nearly) always zero offset, x s = x g ! rsrs xgxg

11. Note some data processing steps are similar to seismic but lack some tools (such as refraction velocity analysis). Commonly do static corrections for elevation, filtering, automatic gain control; much less common to migrate.