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Assessing Canal Seepage Variability using Geophysical Methods

This presentation describes a USGS study on assessing canal seepage variability using various geophysical methods such as resistivity, capacitively-coupled resistivity, and time-domain electromagnetic induction. Preliminary results and future steps are discussed.

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Assessing Canal Seepage Variability using Geophysical Methods

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  1. NDNR Modeling Group MeetingJanuary 25, 2007 Welcome to the Nebraska Department of Natural Resources Modeling Group meeting. Doug Hallum will describe a USGS study to assess canal seepage variability with various geophysical methods.

  2. Resistivity: Concept • Inject a known current into the ground C1 & C2 (transmitter) • Measure resulting potential or voltage difference P1 & P2 (receiver) • Apparent resistivity is calculated using the geometric factor

  3. Elkhorn Loup Model (ELM) – Canal Geophysical Techniques • Direct-Current Resistivity • Staked Survey • Land-Streamer • Capacitively-Coupled Resistivity • OhmMapper • Time-Domain ElectroMagnetic Induction • TerraTEM (TDEM)

  4. Resistivity Arrays • Arrays used in the ELM study: • Dipole dipole • Wenner -Schlumberger Figure from: http://www.cflhd.gov/agm/engApplications/RoadwaySubsidence/513ResistivityMethods.htm

  5. DC Resistivity • Staked Survey • Provides highly repeatable “base” dataset • High signal to noise ratio • Time and labor intensive Electrodes Electrode + Multiconductor cable Resistivity meter

  6. DC Resistivity • Land-Streamer • Based on the staked survey methodology • Collects data in motion • Ground contact a potential problem Electrodes

  7. D-C Resistivity – Staked Survey Preliminary Results Dipole-Dipole Survey

  8. Capacitively Coupled Resistivity • Uses antenna dragged along the ground • Induces a current through capacitance • Uses a dipole - dipole array • Collects data in motion • Often used in concert with GPR Slides modified from: http://www.geometrics.com/OhmMapper/ohmmap.html

  9. Capacitively-Coupled Resistivity • Ohmmapper • Rapid continuous data collection • Direct formation contact unnecessary • Less time and labor than staked survey • Array spacing determines depth of investigation • Can have trouble with high resistivities From: Lunar and Planetary Science XXXVII (2006) 1410.pdf

  10. Capacitively-Coupled Resistivity Preliminary Results

  11. Time Domain ElectroMagnetic Induction • Uses a Tx loop to generate an electromagnetic field • Generates measurable responses (eddy currents) • Currents decay and are measured by the Rx loop • Great depths can be studied by increasing transmittance time • Continuous data collection • High conductance decreases depth of study transmitter loop (Tx) receiver loop (Rx) induced eddy currents at progressively later turnoff times Figure from: http://www.ncwater.org/Education_and_ Technical_Assistance/Ground_Water/TDEM/

  12. TDEM – Tx and Rx Waveforms • Waveform of the transmitted signal is a train of pulses • Step-functions, ramps, or other waveforms and measurements are made in the off-times between pulses, usually after the primary has stopped charging (Sheriff, 2002)

  13. Time-Domain ElectroMagnetics • TDEM • Rapid data collection • Direct formation contact unnecessary • Less time and labor than staked survey • Short arrays achieve deeper depths • Post processing currently limited www.spectrum-geophysics.com www.gpx.com.au www.fhwa.dot.gov

  14. Time-Domain ElectroMagnetics Preliminary Results 01/11/2007

  15. Summary of Preliminary Results • All techniques produced similar results. (2 layers) • DC staked surveys yielded low noise, high qualitydatasets, Land-Streamer did not produce usable data. • OhmMapper produced somewhat noisy data. • The OhmMapper can only log 5 receivers at a time. • Multiple passes necessary to obtain near surface and deeper data. • Requires long arrays to achieve maximum depth of interest. • Longer offsets (increased depth of interest) produced noisy data. • TDEM produced comparable results • Data processing is somewhat difficult at this point. • Evaluating new software which will reduce data processing time. • Engineer from Australia modified the TerraTEM for continuous data acquisition and filtering.

  16. Conclusions • Post-processing is not complete. • Preliminary results are available for some transects. • Location of data and correlation between methods is not currently available. • Geoprobe data is not currently available. • Documentation will be in peer review by fall, 2007.

  17. Whats next? • Further analysis of TDEM data processing software. • Make comparisons to lithologic data. • Build more robust sled to eliminate mechanical problems. 01/11/2007

  18. Questions: James C. Cannia P.G. Hydrologistjcannia@usgs.govU.S. Geological SurveyUSGS Nebraska Water Science Center5231 South 19th St.Lincoln, NE 68512-1271 Office 402-328-4128 Cell 402-416-5321 Fax 402-328-4101

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