Processing was done in Matlab using Crewes software, RD3 GPR library and MATGPR software. The average trace was removed from the original data to make the changes in the data more pronounced. An automatic gain recovery was applied to the data. Since the topography was generally flat, a migration does not seem necessary. However, it is being considered. The Common Shot Point (CSP) concluded a velocity of 393 ft/mus for the 200 MHz and 376 ft/mus for the 100 MHz (Figures 1,2). These velocity are approximately 1/3rd of the speed of light which is accurate for a GPR survey. Based on the GPR data collected, the water table is estimated to be about 50-60 ns into the Earth (Figures 3-6). The water table occurs slightly shallower during the 200 MHz survey as opposed to the 100 MHz survey. The 100 MHz and 200 MHz were collected at about 7:30-8:oo am and 11:30-noon, respectively. The change in water level is confirmed by the USGS National Water Information System gage height graph (Figure 7). The gage height is the height of the water surface above the datum. Some errors in the seismic data occurred from decoupling of the antennae to the ground. During processing, we make an assumption that the velocity does not change with depth. This is in fact false. However, since we are only looking at a shallow depth, the error from this assumption is not substantial. During the 100 MHz data collection, a longer time window should have been used.
First of all,
My apologies for the late response, I have been dealing with a toothache/extraction and got behind on my blog posts.
Lees Ferry was quite the trip. We enjoyed camping and hiking for the entire week of Spring Break. We mapped the assigned area and tried to let the great outdoors sink in.
As far as the GPR survey, the data is still being processed. The first day of the survey had an abundance of technical and operational errors. First, we spent about 2-3 hours trying to get the computer to see the antennae. Final, after fixing this problem, the antennae were extremely difficult to couple to the ground with the large cobble/boulder sized rocks. Overall, the first day of acquisition was unfortunately a dud. We returned to camp with heads held low, spirits broken and sunburns a-blazed. That night at camp, Lynh and I continued to try to work out the bugs that occurred during the first day so they would not occur on the second day. We practiced using the equipment, timed set-up and break-down, discussed how we were going to change our survey since we had limited time and learned how to perform a common source point (CSP) survey to get the velocity of the outcrop. We enjoyed steaks the last night which lifted our heavy hearts and mentally prepared for the daunting task ahead.
We awoke early the next day, hoping for pancakes but got a grease fire. I was hoping this was not a sign of things to come. We packed up camp, put the gear into the van and took off to the GPR survey location. This time, Sam and Thao joined us to make the survey go faster. We were out in the field at about 7 in morning. After the equipment was set up, we began with the 100 MHz survey for line 1. This line went for the unconsolidated, eolian sand dunes onto the consolidated sand/gravel river deposits. After the completion of the 100 MHz line, we did the 100 MHz CSP to get the velocity. At this point, we decided to take a quick break and eat some tuna sandwiches. The funny part is how much sand we actually ate; it was probably more sand than tuna or wich :-). We decided that it was better to do the 200 MHz CSP survey since all we had to do was change the antennae. After this, we completed the 200 MHz survey line. At this point, it was already about noon and we knew we needed to head back to camp to help pack up. Lynh and I decided that one survey line with to frequencies and two CSPs was quite the accomplishment. This GPR survey was an incredibly frustrating yet rewarding experience. Hopefully, after the data is processed, we will have a story to share. I’m so proud of Lynh and I for our hard work and would like to thank Sam and Thao for helping us out. I would also like to thank Jefferson for helping us figure out the bugs that night at camp. We couldn’t have done it without you guys!
Problem: We would like to look at the water table resolution and location in coarse-grained and fine-grained sediments using GPR.
Location: An area where the Paria River and Colorado River join (Figure 2). At this location, there is a gravel unit and fine-grained sand dunes unit. GPS location: N36°51´40´´, W111°35´51´´
Unit Descriptions (Figure 1):
- Gravel deposits, undivided (gv), peach color on geological map
- Local Mesozoic formations: Sub-rounded to rounded pebbles to small cobbles
- Colorado River origin: minor well-rounded cobbles of far-traveled porphyritic rocks
- Paria River origin: Pinkish pebbles of Claron Formation
- Thickness: 3-4 m
- Present only at mouth of Paria River where it forms a broad fan that deflects Colorado River to south.
- Extensive reworking by Colorado River
- Active sand dunes (ea), green color on geological map
- Very fine grained to fine-grained
- Well sorted
- Lightly vegetated
- Thickness: 1-3 m
- Source: USGS 2000
- Three survey lines (A-A´, B-B´, C-C´) run parallel to each other from northwest to southeast and are spaced about 25-35 meters apart. One survey line (D-D´) runs southwest to northeast perpendicular to the other survey lines (Figure 1). The survey lines will be about 100 meters in length. This survey layout allows us to image the gravel unit and fine-grained sand dunes unit fully and allows us to monitor the transition between the two units.
- We will do the survey lines with the 100 MHz antennae. We will be looking for a definitive water table reflection in both the gravel and fine-grained units and possibly (hopefully) a well-recorded change in water table level and resolution between the coarse and fine-grained sediments. We will repeat the survey lines with the 200 MHz antennae to monitor the difference in resolution.
- The monostatic setup of the GPR will allow us to have data with co-located sources and receivers, so we will not have to account for offset when processing the data which will increase our signal/noise ratio.
- The data collect occurs very quickly (partly because of the co-located source and receiver) so we can do surveys of 2 fold, 4 fold or 8 fold (think CMP stacks). We will need to decide hope often we want to shoot.
- We will take GPS readings at the beginning of the survey line and about every 10 m (about 10 recording per survey line).
- During acquisition, we will be about to see the raw data as it is recorded. This will help decide whether we will need to shoot a line again and will help guide us through the processing.
- The software that Dr. Rich uses to process GPR data is Matlab. The system comes with a program called Precan.
- We will need to do some basic field statics (topography correction, AGC).
- We´ll need to filter out any noise in our data. Because the antennae are unshielded, we may get reflections off of the above surface features, so we´ll have to keep that in mind when analyzing our data.
- Since we will be looking at changes in water saturation, we may end up looking for changes in attenuation.
- Possibly doing some Spectral Decomposition.
- Dr. Rich has agree to assist us with hands-on training with the GPR on the Week of March 4th at the Canadian riverbed in Norman, OK to get practice collecting and processing the data.
- I plan on bringing a car converter just in case we need to charge the equipment and laptop.
- Literature research is currently in progress to better fine-tune the survey and get a better grasp on GPR in different grain sizes and in sedimentary river structures.
What is Ground Penetrating Radar? And how does it work?
Yesterday (2/14/2013), I had an interesting conversation with Dr. Keller to get a better idea of what my partner, Linh, and I are getting into.
After a few short anecdotes (which if you know Dr. Keller, you know that this is usual), we finally got onto the subject of the Field project. I described to him what we were thinking of doing and asked him a few questions about the project. He suggested talking to Dr. Rich about using the GPR equipment and getting trained on it. I then asked him about the accuracy of the GPS. He explained that in North America the GPS system has a correction automatically applied to the values. This correction is called WAAS (Wide Area Augmentation System) which uses 25 ground reference stations to correct for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere (garmin).
We have two proposed locations for a GPR survey:
1. Around 36°52’19.48″N, 111°35’57.15″W, there is a bend in the Paria River which grains into the Colorado River just to the south. We would like to look at the water levels in this area to possibly understand what makes the river bend.
2. Around 36°51’51″N, 111°35’54″W, there is an area with what looks like debris flows. Since debris flows coursen upward, we might be able to get good reflections to indicate the tops of the debris flows.
This year the Intro to Field class will be doing an additional project on top of the mapping project. My partner Linh and I are very interested in the geophysics of the area. The research I have done with regards to the seismicity of the area has been a dead end. The closest significant normal fault that is exposed near Lees Ferry is approximately 50 miles away (Table 1; Ch. 3: Tectonic Evolution). The faults are generally N to NW striking and are located near the western margin of the Colorado Plateau.
Table 1. Significant normal faults that are exposed in the Grand Canyon
(Source: Ch. 3: Tectonic Evolution, Table 3.1)
|Fault||Approx. Vertical Offset (m)||Location (miles from Lees Ferry)|
|Bright Angel fault||145||88.5|
|Hurricane fault||400||191 and 221-224|
|Grand Wash fault||>5,000||278|
*downriver of Grand Wash cliffs, at upper end of Lake Mead reservoir
We are considering a GPR (ground-penetrating radar) study to look at the water content of the sediment and the structures within the flood deposits (point bars, etc). In an open-file report by USGS, GPR was used to image the internal structure of sand bars on the Colorado River, Grand Canyon. These surveys took place south of Lees Ferry on the Colorado River (Figure 1).
In order to set up a survey, we will need to determine the optimal location(s) at Lees Ferry. To do this, we need to look at the already established geologies obtained in previous studies. We should decide if we will be taking cores in order to better correlate our findings.
I would also like to look further into a possible magnetic study. However, even though igneous intrusions are common in parts of the Colorado Plateau, there is little signs of igneous activity or structures in the Lees Ferry area (USGS Bulletin: Geology of Lees Ferry area).
We will have to borrow the necessary equipment from the department in order to perform the geophysical survey.
A magnitude 8.0 earthquake occurred at 1:12 UTC on February 6 just west of the Santa Cruz Islands (Solomon Islands province) in the Pacific Islands. The focal depth was 28.7 km +/- 5.2 km according to the USGS. Along with this large earthquake, a tsunami advisory was put into effect for most of the islands in the south Pacific (Tsunami Bulletin Number 002). However, they were cancelled since most of the damage was on a local scale. According to the focal mechanism, the earthquake had a thrust/reverse slip (USGS: Moment Tensor). The earthquake occurred near or on the subduction zone between the Australian and Pacific plates. Up to and since the main shock, many smaller M5.0-M6.0 earthquakes have been occurring in the area. Although the damage from the earthquake and tsunami was minimal, 70-80 homes and properties located on Santa Cruz Island near the epicenter were destroyed (Huffington Post), and 5 people have been confirmed dead (Fox). “Four villages on Santa Cruz were hit by the waves, with two facing severe damage, [Solomon Islands Police Commissioner John] Lansley said. Other areas of the Solomons did not appear to have been seriously affected” (Associated Press).
Good luck to those who are still missing and to the Solomon Islands for a quick recovery.
The 2013 Spring semester Intro to Field Mapping course is going to be different than any other. During Spring break, the class will be taking a trip to Lees Ferry, Arizona. While there, the class will be mapping to the area and camping The camping activity is rather unusual for an undergraduate geoscience course at the University of Oklahoma.
Lees Ferry is located within a national park in northern Arizona. The Grand Canyon begins at this location just north of the Colorado River. The class will be mapping formations from the Permian and the Triassic.
Formations to be mapped:
-Kaibab limestone (Permian)
-Moenkope formation (Early and Middle(?) Triassic)
-Chinle formation (Late Triassic)
-Moenave formation (Triassic(?))
I must say I am excited about this trip.