-Common Shot Point CSP is used to get groundwave velocity. For 200MHz antennas, v=0.39ft/ns. For 100MHz antennas, v=0.38ft/ns. It is in velocity range of unsaturated sand and gravel (0.38-0.53 ft/ns) according to table data in the textbook.
-Identification of water table reflector: 50-60 ns in time. Using eqn: d=vt/2 to convert into water table depth: 10-12ft.
-Comparison data of 200MHz and 100MHz antennas: Although the 200MHz antennas give better resolution of subsurface than the 100MHz antennas (~0.5ft vs 1ft), the 100MHz antennas produce a greater depth of penetration (~19ft vs 14ft)
-Gage height of Colorado River: 8.5-9 ft (USGS)
-The use of GPR for determining water table depth is effective to study area (near river, has shallow water table), but is difficult in fine-grained materials. If water table depth exceeds 15ft, the 200MHz antennas can’t image it. Estimated water table level is 10-12ft higher than gage height.
-Accuracy: water table depth depends on the thickness of capillary fringe, fluctuation of water table. Groundwave velocity is calculated average velocity that has error. Wave velocity varies in depth, but using 1 single velocity to get water table depth converted causes interpretational error. However, errors are taken into account.
Image of GPR units we’ll be using (incomplete). Honestly, I’m shocked at their total costs (~35k). Should be so careful using these.
Topic: Water table depth comparison between coarse-grained and fine-grained’s material areas using GPR.
Hypothesis: There’ll be a major change in water table depth on GPR profile because of different rock characteristics (ie, permeability, porosity) leading to attenuation change->wave form changes.
Acquisition: 1.Need topographic map and GPS for location. 2.Set up survey area, using parallel lines-> make grid map. The study site is between Colorado River and Paria River: alluvium terraces and sand dunes. 3.Using Mala Geoscience ProEx system with 100 Mhz and 200 Mhz unshielded antennas. Stacking at each point to get good results. For 100 Mhz antennas, depth of penetration could be 2-15m (5-50t). For higher frequency 200 Mhz antennas, there is a trade-off since we can get better resolution but less depth of penetration (0-9m or 0-30ft). Our team will be trained to set up the system later (maybe 2 following weeks Jamie said). We’ll switch our parts working with GPR units.
Processing: using software I guess!! (Emma noted it, I forgot it ehh) Make some corrections (elevation, noise, attenuation….). Maybe. Maybe not much. Compare data from 2 different frequency GPR units. Telling the sed structures is great, if not, it’s fine. We’ll mostly use raw data cuz real processing data is @@!
Other interesting things: there is a term “capillary fringes” which makes me wonder if I can tell the real water table on GPR profile or not. It can be slightly off.So we can test if water table is shallow using pipe?! (I’m not sure if it’s possible or not). Beside, comparing water table depth (groundwater level) and river level is interesting. In this rainy season, water flows into river (gaining stream), so groundwater level should be higher than river level.
brief introduction about GPR
It would be better if I could find something about Lees Ferry that related to geophysical stuff I’ve studied so far. I’d like to know how geophysics is applied and contributed to understand geology. However, I found other topics also drew my attention like petrified woods and petrifaction around Lees Ferry, or weathering features of rocks, or how flood or controlled-flood dam’s effects (I know it’s too general so I’m still thinking about it!). By the way, I look forward to class tomorrow to have a final conclusion with Emma 😀
This is definitely a beautiful place to go!!!
Lee’s Ferry is both a historic site and a boating, fishing, hiking site within Glen Canyon National Creation Area. It is named after Mormon leader John D Lee, who followed church orders to establish a ferry crossing for church emigrants going south to Arizona. Not long after that, Lee was executed for his role in the Mountain Meadows Massacre, but it’s said that he was a scapegoat. Besides, there used to be an extravagant investment for gold mining at Lee’s Ferry, but it failed due to sparse gold amount in shale. However, there are a lot of petrified woods and it’s worth seeing them. http://utahtravelcenter.com/pointsofinterest/general/leesferry.htm