Electromagnetic Technology and Levee Assessment | |
|
Author DeAndra DiBene American River College, Geography 350: Data Acquisition in GIS; Fall 2006 Contact Information: ddibene@tremaine.us | |
|
Abstract The information in this project is my own personal experience and observation performing a geophysical electromagnetic survey to determine the actual stability of a delta levee. | |
|
Rindge Tract |
 |
|
Introduction The delta levee system needs rehabilitation to improve its flood protection and reliability. Argus Technologies was contracted by an engineering firm to perform a geophysical scoping survey to detect anomalies in a delta island using electromagnetic technology. The findings of our survey would be investigated by the engineering firm before any attempts to repair the levee. The procedure of this survey will be discussed in a comprehensive manner to the reader. | |
|
Background On June 9th, 2006 Argus Technologies conducted a geophysical investigation on Rindge Tract (RD 2037), of which I took part. The 15.8 mile delta island is located approximately 30 miles south of Sacramento, California and 5 miles to the north of Stockton, California. This was a levee assessment that represents Phase I work funded by the FEMA Hazard Mitigation Grant Program to better determine the integrity of the levee. In order to understand what levee integrity means, I’ve listed a few key factors that warrant levee failure: Stability is an important factor, what threatens the stability of a levee are rodent burrows. From my own personal observation, beaver dens nestled into a levee weakens the structure of a levee. I have personally seen evidence of sinkholes on a levee crown that were a direct result of beaver dens. Water seepage is when the soils of the levee can dissipate with water seeping through or beneath the levee. For instance, surrounding soils of a rusted pipe can threaten the levee structure. If any of these factors are neglected, the levee becomes at risk as it simply washes away. These were key factors that became the focus of our survey. | |
|
Methods The purpose of the survey was to supply baseline electromagnetic data for locating, identifying and ranking potential deficiencies to determine levee integrity. Below is a summarization of this technical procedure. | |
| Argus Technologies has developed the EM3 to perform mobile, non-evasive underground surveys. As with all electromagnetic induction surveys, the Em3 transmits an electromagnetic field to induce a response from materials under the earth's surface, without direct contact with the earth. The instrument passes a current through the transmitter coil creating a magnetic field. This electrical current then comes into contact with conductive subsurface features such as water or metals. These electromagnetic anomalies give a distinctive reading during analysis of the data collected. Incoming data from three receivers on the EM3 are tagged with real-time Kinematics (RTK) GPS coordinates with centimeter-level accuracy. The receiver, magnetic coils, and and GPS are transported behind an ATV traveling approximately 5-10 miles an hour. Three to four readings of data are collected per meter using the Allegro Field PC (Juniper Systems, Inc., Logan UT). ArcGIS ArcVIEW 9.1 (ESRI, Redlands, CA), and the EMIGMA 7.8 (Petros Eikon, Inc.,Concord, Ontario, Canada), are used to process and display the raw data. | |
|
Here I am riding the EM3 |
 |
|
Results The combined results of data collection
and analysis reveal anomalies below the surface. These anomalies with exact coordinates
now become what we call stations. These stations are given to an engineering firm to do
the excavation of our data. Excavation at these stations revealed large amounts of
metallic debris, voids caused by rodents, and water seepage. The large amounts of rusted
pipe found tend to weaken the the strength of the levee. No longer in use, these pipes
erode and leave a void in the levee. These findings are crucial when the next phase of
levee repair begin. |
Processed data in ArcMap. The engineering firm will drill at these stations to validate our findings. |
 |
|
Drilling and excavation at a station produced eroding pipe. |
 |
|
Analysis Not all data collected and analyzed turned out to
be what we had thought. Some of the features excavated turned out to be nothing more than a
small strand of copper wire. Which in a way was good news for the levee, bad news for the
excavators. | |
|
Not Exactly Payday |
 |
|
Conclusions The EM3 instrument is valuable in
assessing levee conditions and can lead to prioritization of levee repair. Although
electromagnetic assessment is a powerful method to determine the structural stability
of a levee, there are other necessary surveys to succeed Phase I, these are ground penetrating
radar and/or targeted drilling and excavation, in order to validate the analysis of data
collected, as in this case. This was my first geophysical survey and I actually witnessed the important
role electromagetic induction scoping can provide. Our findings proved to save the engineering firm
time and money | |
|
References Liao, Yi et.al., 2006. Geophysical Investigation of Rindge Tract (RD 2037), Sacramento-San Joaquin Delta, California. Rec District 2037 Report, Argus Technologies, LLC. (4). | |