The use of Ground Penetrating Radar (GPR) on Alluvial Fans South of Carson City, Nevada.
In Nevada, alluvial fans are very important areas to be studied when flooding is concerned because there are not many large rivers in the region. Basin and Range topography dominates the region, so when government agencies such as the Federal Emergency Management Agency (FEMA) assess flood risk for developers, homeowners, and insurance companies other hazardous areas such as alluvial fans must be evaluated. Residential areas such as that in the figure to the right, have a much greater flooding hazard than the homeowners might have believed after seeing the desert landscape that surrounds their property.
|Alluvial Fan Formation
Alluvial fans are formed as water deposits sediment that was eroded from the drainage basin, or catchment area, transported by the flow of water. Often, the catchment area of an alluvial fan is a valley, canyon, or an intermountain basin. The water load will increase, along with sediment, and flow down the steep gradient of the valley or canyon floor. As this happens, more and more sediment will gather with the stream-flow until it reaches its carrying capacity. When the flow of water and suspended sediment reaches the mouth of a canyon, which is known as the hydrographic apex, the main channel of water and sediment will move laterally and may begin dividing into several smaller, distributary channels.
|Sediment Deposition Pattern on an Alluvial
The deposition of sediment on alluvial fans is an interesting process because the pattern of sediment deposition results in a natural sorting of materials by size. Coarser rocks and gravel are deposited near the mouth of the canyon, near the apex of the fan. Coarser material is also found in and along the main channel beds further down the fan from the apex as is seen in the figure below.. These materials can be carried further down the channel because the carrying capacity of the stream flow does not decrease as rapidly in the channel as it would elsewhere on the fan. As the distance from the head of the fan increases, the size of the materials continues to decrease, from rocks and gravel, to small gravel and sand, and finally to fine sands and silts.
|Objective of Using GPR
on Alluvial Fans
Analyzing and interpreting data collected in the field using GPR and other methods should uncover evidence that may help determine whether or not there are serious hazards associated with increased human land use of alluvial fans. GPR profiles will be analyzed to search for evidence like former channel beds throughout different parts of alluvial fans. The presence of older channel beds on alluvial fans, like in the figure to the right, would suggest that the surface of alluvial fans is still developing and unstable, with a variety of potential paths for floodwaters.
|The GPR System
Most digital GPR systems, such as the pulseEKKO 100 which is the system that was used to collect data in Western Nevada, consist of a console with a portable laptop computer and 12 volt battery that is all contained in a backpack and two antennae. One antennae transmits the signal while the other antennae is the receiver, and they are connected to the console by fiber optic cables. Each of the antennae are powered by a 12-volt battery. The figure below shows the GPR system in use during the research trip to Western Nevada.
|How GPR Gathers Data
GPR works by using electromagnetic energy (EM) to transmit a signal into the ground and then recording it as it is reflected off of subsurface features. Some of the energy is reflected back to the surface as the transmitted energy contacts sediment of different grain size, mineralogy, density, bedrock contact, and water content. One important characteristic of GPR systems is that they are more effective in quartz-rich sand and gravel, while sediments with clay minerals or saline water limit signal penetration because of high conductivity.
Antennae frequencies vary depending upon the application and data being collected, however 50, 100 and 200 MHz seem to be the most common frequencies used for research. The usefulness of each antennae frequency depends on the type of data that is sought. If a data with a high resolution is required then a higher frequency antennae is required, however the signal will not penetrate as great a depth as a lower frequency. Transmitter voltage can also be increased to provide greater continuity of reflection and depth of penetration.
When GPR data is gathered, it is collected along a specific transect or line that can be plotted on a map or with a global positioning system. As the data is collected and organized by the computer, a profile showing the horizontal survey distance (m) versus vertical two-way travel time in nanoseconds is created. This can then be converted to determine the depth of reflectors by measuring the EM propagation velocity in sediment.
|Analyzing GPR Profiles on Alluvial Fans
By using this data, the subsurface structure of alluvial fans can be analyzed. Forms like older channels that have not been recently invaded by water or debris flows may be possible to interpret from the GPR profiles. If the GPR signal is able to locate former channel beds on alluvial fans, then additional evidence would exist that could help demonstrate the instability of alluvial fans and the risks associated with flooding. The two GPR profiles below were collected on the Buck Brush Alluvial Fan in Johnson Way, which is just south of Carson City, Nevada.
|Profile of the Buck Brush fan GPR transect, which extended a total of 440 meters due north from Gordon Avenue, just west of the Johnson Way Fire Department. The alluvial fan channel is visible as a depression in the topography at the 280 meter point of the transect. Note how the elevation of the fan near the channel is higher than on either side.|
|The profile of the Wade Street transect. The transect extended north 406 meters from the intersection of Gordon Avenue and Wade Street in Johnson Way, Nevada. The channel of the alluvial fan can be seen at the 280-meter mark.|
The Buck Brush transect was the first GPR transect completed. It was located just west of the Johnson Way Fire Department, and extended north from Gordon Avenue for a total of 406 meters. The transect was perpendicular to the main channel of the fan, which can be seen on the GPR profile (Figure 5) at the 280 meter point of the transect. The second transect ran alongside Wade Street heading north from the intersection with Gordon Avenue (Figure 6). It was located two blocks (about ¼ to ½ a mile) east of the Buck Brush transect. Both of these profiles were completed using the same GPR set up. The antennae had a 1 meter separation, and were moved along the transects in 1 meter steps. The profiles were collected using a 400 V transmitter. Although the depth of signal penetration varied from 3-3.5 m in the Buck Brush profile and 4-4.5 m in the Wade Street profile, both profiles successfully displayed several layers of flooding deposits on the fans. Other features of alluvial fans such as former channel beds were also present on the GPR profiles.
|Prospects for Future GPR Applications on
Judging from the performance on the alluvial fans at Johnson Way, GPR has the capability to be a successful method of examination that can be used for more intensive research on alluvial fan development. Although the signal penetration of some GPR lines was significantly lower in areas with more silts and clay, the signal penetration still located enough sediment layers to acquire significant data about fan histories. The discovery and locating of former channel beds using GPR is beneficial for researchers as they attempt to determine an alluvial fans history of flooding. Studying the history of channel movement on an alluvial fan can result in a better understanding of how stable alluvial fan surfaces are, and whether residential development is advisable. By gathering more data on alluvial fans, researchers and government agencies can more accurately assess the flood risk of surrounding areas. The success of GPR in studying alluvial fan flooding is important, because it would be a less time consuming and more cost-effective alternative to trenching(The figure below shows a trench already dug) and other more traditional modes of examination.The GPR data collected is proof that more research should be done to confirm that these alluvial fans are still shifting and developing. If and when that confirmation is made from research, it is then up to researchers and government agencies such as the Federal Emergency Management Agency (FEMA) to assess how severe the flood risk is for areas on and near each of the alluvial fans.