Estimation of Subsurface Moisture Variation in Layered Sediments Using Ground Penetrating Radar By Matthew Charlton King’s College London Ground Penetrating.

Estimation of Subsurface Moisture Variation in Layered Sediments Using Ground Penetrating Radar
By Matthew Charlton King’s College London Ground Penetrating Radar Research Group Department of Geography King’s College London (U.K.)

1) Development and testing of direct GPR methods for VMC estimation.
STRUCTURE 1) Development and testing of direct GPR methods for VMC estimation. 2) GPR response in dry layered materials. 3) GPR response and VMC estimation after addition of water to layered materials

Subsurface-moisture Estimation
Variation of subsurface moisture Space and time Existing Techniques (examples) gravimetric time domain reflectometry neutron moderation method Limitations of existing techniques Invasive Point Measurements Time Consuming Calibration

GPR AND SOIL MOISTURE ESTIMATION
TRADITIONAL APPROACH 1) Derive Velocity CMP / WARR Depth Correlation Borehole (Transillumination) 2) Derive Dielectric Constant 3) Calculate VMC

SOME LIMITATIONS OF CMPs
1) Inappropriate for large areas 2) Time consuming (data acquisition and analysis) 3) Often limited for depth 4) Back-breaking

RESEARCH OBJECTIVES Use GPR to quantitatively determine Volumetric Moisture Content (VMC) Accurately, efficiently. Reflection Profiling Mode (direct data acquisition). Different depths beneath the surface. Spatially distributed. Hillslope hydrology and water leak detection applications

DIRECT GPR DATA VISUALIZATION
A-Scan

A-Scan B-Scan

A-Scan B-Scan Fourier Transform Amplitude Spectra

A-Scan B-Scan Fourier Transform Amplitude Spectra Hilbert Transform Instantaneous Amplitude

A-Scan B-Scan Fourier Transform Amplitude Spectra Hilbert Transform Instantaneous Amplitude Instantaneous Phase

A-Scan B-Scan Fourier Transform Amplitude Spectra Hilbert Transform Instantaneous Frequency Instantaneous Amplitude Instantaneous Phase

MATERIAL PROPERTIES gravel, coarse, mixed, fine, clay
hydraulic conductivity decreases with particle size water added every 25mins from dry to saturated Porosity generally decreases with particle size Increase in dielectric constant with decreased particle size Material / Hydraulic Experiment D50 (mm) Porosity Conductivity (mmhr-1) Dielectric Constant (m3/m3) Mean Standard Mean Standard Deviation Deviation M M M M M M

METHODS: VMC ESTIMATION
Small Test Facility

GPR Data 20 Minutes after water addition Bistatic PulseEKKO 1000A 30ns Time Window 50ps Sampling Interval 32 Stacks

METHODS: VMC ESTIMATION Experimental Hydrology
Drainage Experiments Wetting Experiments

Analysis of GPR Data Process original time-domain data. Export to Microsoft Excel

Analysis of GPR Data Process original time-domain data. Export traces to Microsoft Excel. Determine analysis start time.

Analysis of GPR Data Process original time-domain data. Export traces to Microsoft Excel. Determine analysis start time. Determine analysis end time.

Analysis of GPR Data Process original time-domain data. Export traces to Microsoft Excel. Determine analysis start time. Determine analysis end time. Calculate selected statistic between the start and end times. Calculate Statistic

Analysis of GPR Data Process original time-domain data. Export traces to Microsoft Excel. Determine analysis start time. Determine analysis end time. Calculate selected statistic between the start and end times. Develop relationship between observed VMC and selected statistic

MEAN INSTANTANEOUS AMPLITUDE
Selected Signal Statistic Ease of determination. Strongest relationships. Associated with reflection strength. Large value associated with major subsurface changes. Describes waveform shape. Encompasses all sources of change. Decline in MIA with increase in VMC.

THE MIA-VMC RELATIONSHIP

ALL MATERIALS

COMPARING DRY RAW AMPLITUDE TRACES: M1

COMPARING SATURATED RAW AMPLITUDE TRACES: M1

COMPARING WET RAW AMPLITUDE TRACES: M1

ALL MATERIALS

MOISTURE DISTRIBUTION: HIGH HYDRAULIC CONDUCTIVITY (M2 at 0 litres)
VMC (m3/m3) A-Scan Amplitude (uV) Depth (m) Two Way Travel Time (ns) Horizontal Position (m) Envelope Amplitude (uV) Two Way Travel Time (ns)

MOISTURE DISTRIBUTION: LOW HYDRAULIC CONDUCTIVITY (M4 at 0 litres)

THE DIRECT ARRIVALS Non-stationary
Ground-wave changes in timing and magnitude Transmitted pulse should be constant has high magnitude reduces effect of late-time variability Transmitted pulse does vary dry: combined with ground-wave saturated: combined with a positive wavelet

TESTING THE MIA-VMC RELATIONSHIP
The Original Relationships Overall logarithmic relationship Estimations accurate for most materials Linear form if M4 results are excluded Estimation error up to 0.443m3/m3 for M4 Wrong functional form for M4 Next best-fit linear relationship for M4: VMC = E-05MIA

TESTING THE MIA-VMC RELATIONSHIP
Adjusted M4 Relationship Maximum error only 0.062m3/m3 Are the relationships linear? Error introduced by variation in recorded MIA values

VMC ESTIMATION CONCLUSIONS
Mean Instantaneous Amplitude estimates VMC. Textural dependence is exhibited. Measures attenuation patterns. Maximum error is 0.07m3/m3. Limited to intermediate VMCs. Need for site-specific calibration. Further work Extend analysis depth. Clarify ambiguity of relationships. Assess potential for a combination relationship. Investigate additional effects of other attenuation sources.

LARGER SCALE LABORATORY INVESTIGATIONS
Dry Grids: 450 and 900MHz and 0.1 and 0.2m station spacing E3: water into clay material with no rock fragments E4: water into clay material with rock fragments E5: deeper water in coarse sand with brick pavement identify variability in dry GPR response validate relationship as a means of estimating subsurface VMC

METHODS: LARGER SCALE Practicalities of mains water leak detection
Ability of GPR to detect water bodies Develop analytical techniques for VMC determination Suggest operational, technical or analytical improvements necessary for routine detection

Large Test Facility - 2m3

METHODS: LARGER SCALE GPR Data Bistatic PulseEKKO 1000A
450 and 900MHz antennae 50ns Time Window 10ps Sampling Interval 32 Stacks

Large Test Facility - 2m3

LARGE TEST FACILITY: GPR RESPONSE IN DRY LAYERED MATERIALS

0.2m station spacing at 900 and 450MHz

900MHz Depth Estimation Depth (m) Horizontal Position (m)

450MHz Depth Estimation 450MHz worse than 900MHz (less accurate mean; greater range) Especially near surface (due to antenna offset and footprint)

0.1m station spacing at 900 MHz: no gain and AGC gain images

Profile at y = 0.5

for each material layer Semivariogram of Mean Trace Amplitude
Layer Dielectric Constant m m m m

LARGE TEST FACILITY: GPR RESPONSE IN DRY LAYERED MATERIALS AFTER WETTING

E3 DRY (AGC GAIN)

E3 WET (AGC GAIN)

E4 DRY (AGC GAIN)

E4 WET (AGC GAIN)

E5 DRY (AGC GAIN)

E5 WET (AGC GAIN)

Laboratory Transects E3 and E4 at 900MHz Full Profile Dry

Laboratory Transects E3 and E4 at 900MHz Full Profile Dry and Wet

Dry, Wet and VMC Difference
Laboratory Transects E3 and E4 at 900MHz Full Profile Dry, Wet and VMC Difference

Laboratory Transects E3 Water Location Approximately m

Laboratory Transects E4 Water Location Approximately m

Laboratory Transects E5 at 900MHz Full Profile Dry

Laboratory Transects E5 at 900MHz Full Profile Dry and Wet

Dry, Wet and VMC Difference
Laboratory Transects E5 at 900MHz Full Profile Dry, Wet and VMC Difference

Moisture Variation: E3-E5 at 900MHz using full profile
Coefficients of Variation E5 Dry: 0.075 E5 Wet: 0.068 E5 E3 and E4 Coefficients of Variation E3/E4 Dry: 0.191 E3/E4 Wet: 0.082

Moisture Variation: E3-E5 at 900MHz using only M6 layer

Moisture Variation: E3-E5 at 450MHz using only M6 layer

Non-detection of E4 leak at 450MHz
At 900MHz we can see both E3 and E4 leaks. Why not at 450MHz? Rock Fragments: pathways for more rapid drainage Time between profiles: 40mins after event

VMC ESTIMATIONS FULL PROFILE E3 and E4 Overestimated by up to 0.322m3/m3 E5 Overestimated by up to 0.173m3/m3 M6 LAYER E3 and E4 Overestimated by up to 0.178m3/m3 E5 Overestimated by up to 0.011m3/m3 Generally, 450MHz VMC estimates are more accurate than 900MHz due to a tendency to underestimate MIA

FACTORS CONTRIBUTING TO VMC OVERESTIMATION
Anything which produces low MIA values will produce higher than expected VMC estimates. Principally Absence of coherent Direct Arrivals Antenna Coupling Long time window decreases MIA Non-linear function of predictive relationship Conversely 450MHz underestimate broader wavelets produce higher MIA

Comparison of Direct Arrivals
Comparison of mean dry (thin full line) and wet (broken line) direct arrival response at 900MHz for E3 with dry trace from original M6 experiments (thick full line). Comparison of mean dry (full line) and wet (broken line) direct arrival response at 900MHz for E4 Experimentally derived Transmitted Pulse (full line) at 900MHz acquired by suspending the antennae at 0.95m above a dry surface. Compared with dry (faint line) and wet (faint dashed line) direct arrival response for E5. Traces aligned to match in time.

Antenna Coupling Sensitive to elevation of decoupled antenna
which antenna (transmitter or receiver) has become decoupled Variation of response with antenna ground-coupling. Transmitter lifted 0m (full line), 0.15m (thick dashed), and 0.25m (thin dashed) off the ground whilst Receiver remains in contact

Antenna Coupling: E3 and E4

CONCLUSIONS MIA detects moisture in layered materials
Relationship is very sensitive to other system characteristics Surface Roughness Dielectric Characteristics of Material Antenna Frequency Depth of Investigation Require physically-based solutions Field calibration is necessary Further research to facilitate regular application

Questions? Thank you...

Estimation of Subsurface Moisture Variation in Layered Sediments Using Ground Penetrating Radar By Matthew Charlton King’s College London Ground Penetrating.

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Estimation of Subsurface Moisture Variation in Layered Sediments Using Ground Penetrating Radar By Matthew Charlton King’s College London Ground Penetrating.

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