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  1. 1. Image Distortion Effects in Subsurface SAR Imaging of Deserts and Their Correction Technique<br />Adel Elsherbini and Kamal Sarabandi<br />Radiation Laboratory, <br />University of Michigan – Ann Arbor<br />1<br />
  2. 2. Outline<br />Motivation<br />Subsurface InSAR Operation<br />Scattering Phenomena<br />Processing Technique<br />Image Distortion Effects<br />Proposed Iterative Corrective Approach<br />Simulation Results<br />Verification using Scaled Model<br />Conclusion & Future Work<br />2<br />
  3. 3. Motivation<br />Deserts cover ~ 20% of the earth’s land surface.<br />Estimation of the subsurface topography and the top sand layer thickness is very useful in:<br />Oil fields and ground water explorations<br />Study of sand dunes formation & migration<br />Planetary explorations<br />Deep mine fields detection<br />Archaeological surveys<br />…. etc.<br />Current technology is GPR<br />Sand dunes advancing towards the city of Nouakchott, the capital of Mauritania*.<br />Sand dunes on Proctor Crater on Mars **<br />3<br />
  4. 4. High Cost<br />Would not penetrate through the sand<br />Low Vertical Accuracy<br />The Proposed Technique<br />Common Fast Top Surface Height Retrieval Techniques*<br />4<br />(*) Richards, M.A.; , "A Beginner's Guide to Interferometric SAR Concepts and Signal Processing [AESS Tutorial IV]," Aerospace and Electronic Systems Magazine, IEEE , vol.22, no.9, pp.5-29, Sept. 2007<br />
  5. 5. Background:Conventional InSAR<br />SAR Image 1 (Master)<br />Phase Interferogram ~(R1 – R2)<br />Prop. Path<br />Noise<br />Fringes<br />Coregister the two images and generate the phase difference<br />R2<br />R1<br />SAR Image 2 (Slave)<br />Unwrapped Interferogram<br />Ref. Points => Heights<br />5<br />
  6. 6. Scattering Phenomena for Sand Covered Bedrock<br />Sand Layer<br />Roughness ~few mm<br />Surface and volume scattering negligible up to ~1GHz<br />Low propagation loss at VHF (~150 MHz)<br />Bedrock Layer<br />Roughness ~few cm<br />Higher backscattering<br />Low Frequency InSAR (150 MHz)<br />High Frequency InSAR (35 GHz)<br />Sand Layer<br />Bedrock<br />6<br />
  7. 7. Proposed ApproachDual Frequency InSAR Subsystems<br />Ka-InSAR for mapping the sand surface<br />The backscattering is mainly from the sand surface.<br />Conventional InSAR processing<br />VHF-InSAR for obtaining the bedrock height information<br />The backscattering will be mainly from the bedrock.<br />Subsurface InSAR processing<br /><ul><li>Refraction
  8. 8. Different propagation velocity</li></ul>7<br />
  9. 9. Modifications in Subsurface VHF InSAR Processing<br /><ul><li>Conventional SAR focusing
  10. 10. Worse azimuth resolution
  11. 11. Conventional 2D Phase Unwrapping
  12. 12. Use the same techniques as conventional InSAR
  13. 13. Top Surface Height information from KaInSAR is used together with the VHF InSAR data to obtain the correct subsurface height using a new inversion algorithm (*).</li></ul>(*) A. Elsherbini, and K. Sarabandi, “Mapping of Sand Layer Thickness in Deserts Using SAR Interferometry”, TGRS 2010.<br />8<br />
  14. 14. 3D Top Surfaces<br />Top sand surface is not flat.<br />Need to verify the performance for 3D surfaces<br />A new subsurface SAR simulator is developed.<br />Inputs:<br />Top Surface<br />Target(s) Locations<br />Radar Path<br />Output:<br />Raw SAR Image<br />Range to target<br />Amplitude change due to attenuation<br />Requires solution of nonlinear equations for each target and radar combination (Very time consuming)<br />New acceleration technique is developed using 3D interpolation*.<br />Radar<br />Target<br />9<br />(*) Presented by the authors at IGARSS 2010<br />
  15. 15. Actual Height (-5m)<br />Targets Height<br />Example:Barchan Dune Simulation<br />Path 2<br />Proposed Algorithm<br />Path 1<br />Targets Height<br />Geometric Distortion<br />Generating the phase difference<br />and Applying the proposed Algorithm<br />Conventional InSAR<br />Targets Height<br />10<br />
  16. 16. Subsurface Image Distortions<br />Geometric Distortion<br />SAR Image Defocusing<br />Full aperture <br />Best resolution <br />Quick defocusing as the target gets deeper (small depth of focus)<br />Partial aperture<br />Worse resolution<br />Long depth of focus<br />Subsurface focusing can be used if we know the target height.<br />To know the height, we need to focus !<br />Iterative Focusing Algorithm<br />11<br />
  17. 17. Subsurface Correction Algorithm<br />Iter. #1<br />Iter. #3<br />Conventional<br />Subsurface Height<br />Focusing Surface<br />12<br />
  18. 18. Experimental Verification<br />10 GHz scaled model measurements in the Lab<br />10 GHz Horn Antennas<br />2.5 m<br />25 cm<br />NWA<br />Flat sand with rough metallic horizontal bedrock<br />Corner reflectors for calibration<br />13<br />
  19. 19. Experimental Verification<br />Azimuth Sections<br />Different azimuth sections are shown aside.<br />Height errors conventional InSAR processing:<br />40%<br />Artificial slope<br />Using the proposed algorithm, the measured R.M.S error is 4mm (0.3m in the actual model) and the mean error ~1-2mm.<br />About 20% improvement in the coherence is also obtained.<br />14<br />Iter. Approach.<br />Conventional<br />
  20. 20. Conclusion<br />Subsurface InSAR Importance and Operation.<br />Image distortion effects include:<br />Geometric Distortion<br />Azimuth Defocusing<br />Image distortion limits the use of subsurface InSAR in many applications<br />New Iterative Image Correction Algorithm.<br />Order of magnitude improvement in the resolution.<br />20-40% Improvement in coherence.<br />Verification using Simulation and Scaled Model Measurements<br />15<br />
  21. 21. Future Work<br />Multiple Look Angles<br />Allows inversion for the sand dielectric constant in addition to the subsurface height<br />Can enable height retrieval under inhomogeneous sand layer.<br />Can be used to detect and correct some layover effects.<br />Multiple operating frequencies<br />Scattering from both top and bottom surface at some frequencies.<br />Allows estimation of the subsurface roughness.<br />Scattering from Dense Random Media (Sand):<br />Used semi-empirical formulas to estimate the backscattering at Ka band.<br />More rigorous approach is desired.<br />16<br />
  22. 22. Thank you<br />Question ?<br />17<br />
  23. 23. Thank you<br />Question ?<br />18<br />
  24. 24. Thank you<br />Question ?<br />19<br />
  25. 25. Scattering Phenomena: Previous Measurements*<br /><ul><li>110 x 100 km area in southern Egypt
  26. 26. L-band radar reveals previously unknown faults covered by sand.
  27. 27. 200 x 190 Km area in northern Sudan.
  28. 28. L-Band radar could penetrate the superficial sand layers up to several meters</li></ul>Optical Image<br />Optical Image<br />Radar Image<br />Radar Image<br />(*) Paillou, P.; Rosenqvist, A.; , "A JERS-1 radar mosaic for subsurface geology mapping in East Sahara," Geoscience and Remote Sensing Symposium, 2003. IGARSS '03. Proceedings. 2003 IEEE International , vol.3, no., pp. 1870- 1872, 21-25 July 2003<br />20<br />
  29. 29. Current Technique for Subsurface Topography Estimation<br />GPR: Ground Penetrating Radar<br />Operation<br />Disadvantages<br />Ground Based<br />Very Labor Intensive<br />Very Time Consuming for mapping large areas<br />A new remote sensing technique is required !<br />21<br />
  30. 30. Modifications in Subsurface VHF InSAR Processing<br /><ul><li>Conventional SAR focusing
  31. 31. Only a portion of the synthetic aperture can be used to avoid defocusing
  32. 32. Worse ground resolution
  33. 33. Conventional 2D Phase Unwrapping
  34. 34. Subsurface height at the ground control points is obtained using GPR.
  35. 35. Use the same techniques as conventional InSAR
  36. 36. Top Surface Height information is used together with the VHF InSAR data to obtain the correct subsurface height using a new inversion algorithm</li></ul>22<br />
  37. 37. VHF InSAR <br />Phase Interferogram [Rad]<br />Inversion Algorithm<br />Based on:<br />Geometrical Optics<br />Uses:<br />Ka-InSAR top surface map<br />VHF-InSAR Interferogram<br />Generates:<br />The bedrock height map<br />Knowns:<br />Phase Interferogram (VHF)<br />Electrical Range to Bedrock (VHF)<br />Flight Paths (VHF)<br />Top surface map (Ka-InSAR)<br />Unknowns:<br />Radar look angle<br />Bedrock Height<br />Iterative approach is used<br />Azimuth [Km]<br />Electrical Range [Km]<br />23<br />
  38. 38. Future Work:Investigation of Ka band backscattering models for the sand volume scattering.<br />Current techniques:<br />Wave Approach<br />Very complex and requires certain simplifying approximations.<br />Some of these approximations are not valid for sand.<br />Radiative Transfer Theory (RT)<br />Calculating the phase and extinction matrices for sand is not straight forward<br />Possible Approach:<br />Hybrid Numerical-RT Model<br />Use of the previously developed packing algorithm to calculate the pair distribution function.<br />A numerical technique is used to compute the effective permitivity of the dense medium using Monte-Carlo simulations as well as phase and extinction matrices.<br />Examine the interaction between volume scattering and surface scattering for natural sand surfaces.<br />24<br />
  39. 39. Future WorkAntenna design for near ground operation<br />Antenna will be placed closed to a dielectric half space<br />Using dielectric filling of the waveguide.<br />Artificial dielectrics to reduce the weight.<br />25<br />
  40. 40. Sensitivity Analysis<br />The effect of each of the following on the estimated subsurface height error was analyzed:<br />Range Resolution<br />Need good range resolution<br />BW >~ 60 MHZ (40% @ 150MHz, UWB)<br />Interferogram phase error<br />Very sensitive for small baselines<br />Baseline has to be > ~100 m<br />Two path interferometry<br />Sand Height & Slope (from Ka-InSAR)<br />Relatively sensitive but within the achievable accuracies using Ka-InSAR<br />Sand Dielectric Constant<br />Not sensitive to moderate variations<br />Baseline Distance<br />26<br />
  41. 41. Doppler History<br />27<br />
  42. 42. Coherence Improvement<br />28<br />
  43. 43. Caustic Effects<br />29<br />
  44. 44. Subsurface Height Estimation Using Conventional InSAR<br />Sample Scenario<br />Assumed low frequency to penetrate the top layer.<br />Conventional InSAR height estimation does not account for propagation through the top layer:<br />Surface undulations is transferred to the estimated bedrock height<br />Calculated height is incorrect.<br />A new subsurface height estimation system is required !<br />Sand<br />Bedrock<br />Estimated Height using Conventional InSAR<br />30<br />
  45. 45. Algorithm Results<br />Simulated several sand and bedrock scenarios<br />The algorithm was found to converge within a few iterations (~5 iterations).<br />31<br />