Measurement of a Temporal Sequence Of DInSAR Phase Changes Due to Soil Moisture Variations Keith Morrison 1, John Bennett 2, Matt Nolan 3, and Raghav Menon 1 1 Department of Informatics & Systems Engineering, Cranfield University, Shrivenham, UK 2 Department of Electronic & Electrical Engineering, Sheffield University, Sheffield S1 3JD, UK 3 Institute of Northern Engineering, 455 Duckering Bldg, University of Alaska Fairbanks, AK, USA

What is the physical cause of DInSAR phase changes over a soil ? In particular, does it relate to soil moisture ? Questions

Presentation Background of the DInSAR–Soil Moisture relationship Laboratory measurements Data Analysis Interpretation

Presentation Background of the DInSAR–Soil Moisture relationship Laboratory measurements Data Analysis Interpretation

The 2008 Sichuan Earthquake in China as Mapped by L-Band Satellite DInSAR

ΔΦ α Δh ΔhΔh

Surface static, but penetration depth changes Penetration depth related to moisture content DInSAR could be proxy for soil moisture

Experimental Imaging Scheme Target Parameters Sandy soil 176cm x 195cm x 20cm VWC Radar Parameters C-Band SAR Imaging VV Polarisation Bandwidth 4-6GHz Aperture 114cm

Soil Sample

Image Resolution Positioniº (degrees)Range (cm)Cross-Range (cm) Near Box Edge8707 Box Centre30208 Far Box Edge451410

Optical Imagery

VWC Moisture Variation Measurements at roughly weekly intervals, over a 50-day period

SAR Image Day 1, VWC ~ 0.4

Data Quality - Calibration

DInSAR Development Φ = Φdiff + Φtopo + Φatmos + Φnoise

Feature Tracking Centre of mass Large processing task – 599 images, each with 93 balls. Automated scheme required.

Optical Result A ±0.1mm change would have produced only a maximum phase change of ±1ºcosi Physical soil surface was static during the experiment - but classic interpretation of phase indicates surface movement > 1.7cm

Mean Phase Change 1.5m x 1.5m Indicates sensitivity of 2.6º per 1% change in VWC

Review Observed the differential phase change over a sandy soil surface. The only changing variable was soil moisture, which changed 40% to 10% as the soil dried. Large phase variations >180º were observed, which showed a smooth and continuous temporal variation. Simultaneous optical monitoring of the soil surface showed it to be static. The phase change was indicative of increasing range.

Changing Penetration Depth, ΔP

ε is the dielectric constant (from Ulaby et al., 1982) λ is the free-space wavelength where m v represents VWC (from Hallikainen et al., 1985 )

sub in for P n = P m + ΔP

Modelled change in the depth of the reflecting layer required to produce the observed bulk phase changes over the central 150cm x 150cm

Summary Seven-week experiment looked at C-band differential phase behaviour of a sandy, non-expansive soil in response to varying moisture content. Large phase changes were seen over the soil in the absence of any physical movement of the surface. Complex, phase patterning was smooth and continuous temporal development, with some features displaying changes greater than 180º. Linear relationship between phase and soil moisture over the VMC range was found by spatially averaging over a 1.5m x 1.5m region.

Summary With the notion of a single reflecting layer related to penetration depth, entire phase history could be understand by ~1cm change. However, the phase signal is a summation of surface and volume returns, so likely an interference effect. The sample area is representative of the resolution footprint of some current airborne SARs, and perhaps future spaceborne SARs. The derived phase response of 2.6º per 1% change in soil moisture indicates a feasible measurement accuracy for soil moisture change at the level of several percent.