Wide area soil moisture estimation using the propagation of low frequency electromagnetic signals
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Wide-Area Soil Moisture Estimation Using the Propagation of Low-Frequency Electromagnetic Signals. William Scheftic (Graduate Student, Atmospheric Sciences, University of Arizona) Kenneth L. Cummins and E. Philip Krider (Atmospheric Sciences, University of Arizona)

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Wide-Area Soil Moisture Estimation Using the Propagation of Low-Frequency Electromagnetic Signals

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Wide area soil moisture estimation using the propagation of low frequency electromagnetic signals l.jpg
Wide-Area Soil Moisture Estimation Using the Propagation of Low-Frequency Electromagnetic Signals

William Scheftic (Graduate Student, Atmospheric Sciences, University of Arizona)

Kenneth L. Cummins and E. Philip Krider (Atmospheric Sciences, University of Arizona)

David Goodrich, Susan Moran, and Russell Scott (USDA Southwest Watershed Research Center)


Discussion outline l.jpg
Discussion Outline Low-Frequency Electromagnetic Signals

  • Finite soil surface conductivity has a quantifiable effect on surface-wave radio propagation in the 100 kHz to 1 MHz range

    • Brief theoretical explanation

  • Lightning Based method

    • Methodology

    • Preliminary results

    • Some Limitations

  • Radio transmission method

    • Overview of summer 2007 field campaign


Slide3 l.jpg

Effect of finite conductivity on surface-wave signal propagation

(propagation model of Norton, 1937):

Conductivity = 10 mS/m; Propagation distance = 100 km

Signal Attenuation

Phase [rad]

Freq [Hz]


Variation with conductivity l.jpg
Variation with Conductivity propagation

  • Soil – Low pass filter on propagating fields

  • Smaller conductivity or larger distance: lower cutoff-frequency


Electrical conductivity varies with soil moisture content l.jpg

%W=0.34 propagation

Electrical Conductivity (mS/m)

σ=2.

Percent soil saturation

Electrical conductivity varies with soil moisture content


Lightning background l.jpg
Lightning Background propagation

Cloud-to-ground lightning electromagnetic fields: (b) First, and (c) Subsequent strokes



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Propagation Animation - 75->450 km distance propagationConductivity = 5 mS/m

75 km








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Rudimentary LTG Method propagation

  • Select desired path for analysis

    • Starting at a polygon that defines the lightning-observation region

    • Ending at a sensor location (e.g., Lordsburg; Williams)

  • Evaluate risetime of lightning waveform measured by the selected sensor.

  • Convert risetime to apparent electrical conductivity

  • Convert apparent conductivity to soil moisture

  • Used North American Regional Reanalysis (NARR) as validation

Window Rock

Williams

Polygon

Yuma

Tucson

200 km

Lordsburg

Sensor

Path




Slide18 l.jpg

Soil Moisture vs. Conductivity propagation(loose criteria)


Slide19 l.jpg

Soil Moisture vs. Conductivity propagation(weighted count > 35)


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Limitations propagation

  • Other factors affect conductivity changes

    • Soil temperature, soil salinity, conductivity gradient over a several-meter depth

  • No perfect set of wide-area data for validation

    • NARR can miss precipitation events as occurred for at least one of the 2005 lightning events.

  • The size of the lightning region being analyzed determines how similar lightning to sensor paths really are.

  • Must have lightning!!


An alternate signal source is man made narrow band radio signals loran ndb am radio stations l.jpg

d propagationab

Sb

da

Sa

Tx

Sc

Latitude

dc

Longitude

An alternate signal source is man-made narrow-band radio signals(LORAN, NDB, AM radio stations)


The effect can be seen as changes in magnitude and phase for narrow band radio signals l.jpg

E propagation

μSec

φ

The effect can be seen as changes in magnitude and phase for narrow-band radio signals


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RadioTX Field Campaign ‘07 propagation

  • June->October Field Campaign in San Pedro Basin

    • Three broadband sensors

    • Remote control from PAS

    • Measure mag/phase vs. fq.

    • Derive conductivity

  • Correlate with WG and San Pedro in-situ measurements, and NARR

Note: 18 AM transmitter within 100km of Tombstone


Slide25 l.jpg

420 kHz NDB propagation


Slide26 l.jpg

790 kHz AM station propagation


Questions l.jpg
Questions propagation?


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Supporting propagation Material


Wide area validation north american regional reanalysis l.jpg
Wide-area Validation: propagationNorth American Regional Reanalysis

  • Specifications

    • 32 km resolution, every 3 hours

    • Available from Jan 1979 through Feb 2007

    • Adequate representation of hydrologic balance

      • Uses NOAH LSM ver. 2.6

      • 4 soil depth layers

      • Uses hourly rain gauge data and PRISM technique to assimilate precipitation



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