DInSAR Measurement of Soil Moisture

Soil moisture is a key player in global climate, agricultural yields, fire danger, engineering analyses, and military off-road vehicle tracking, yet is particularly difficult to measure over broad areas. Over the past few years I have led a team investigating the use of differential SAR interferometry (DInSAR) for the measurement of soil moisture. We now believe that DInSAR has more potential than any other technique currently being developed for this purpose.

This image shows what we believe is drying (blue) and moistening (yellow/red) between two SAR acquisitions in an arid region of Colorado. Black lines are stream channels. Note that ridges (where stream branches start) often show differences in color, which we believe likely indicates a difference in soil moisture due to different soil properties in this mesa topography. Pixel size is 50 m and image width is several kilometers.

The basic idea behind our technique is fairly simple. Because soil moisture affects the dielectric properties of the soil, it must also affect the penetration depth of microwaves into the soil. We believe that these changes in penetration depth are measurable with DInSAR as a path length change. Because these changes are small (on the order of millimeters), a high resolution DEM or appropriately small baseline offset must be used to detect them.

After the first phase our research, my colleagues and I published several papers documenting our results:

Nolan, Matt, 2003. DInSAR measurement of soil moisture. IEEE Transactions in Geoscience and Remote Sensing, IEEE Transactions in Geoscience and Remote Sensing, 41(12), 2802-2813.

Nolan, Matt and Dennis R. Fatland, 2003. New topographic maps may stimulate advancements in the DInSAR measurement of soil moisture and surface deformation. EOS, 84(25), 24 June 03, p 235.

Nolan, Matt and Dennis R. Fatland, 2003. Penetration depth as a DInSAR observable and proxy for soil moisture. IEEE Transactions in Geoscience and Remote Sensing, 41(3), p532-537.

Following this work, I received funding from the Army Research Office to pursue the idea further. My current collaborators are Bernhard Rabus at MDA, Alessandro Ferretti at TRE, and Keith Morrison at U Cranfield. Together we have started from scratch to explore the fundamentals of the phenomena described in the previous work, conducted finite element modeling of electro-magnetic wave propogation, used insar in an anechoic chamber to study the phenomenon in highly controled situation, used field-based insar to scale up to the plot scale, and conducted further satellite remote-sensing using both ERS and Radarsat (in standard and fine beam modes) to understand the current limits to the technique. Thus far we have found that there is indeed a relationship between soil moisture and insar phase, but that the dominant phase-change signal seems to be most dependent on sharp gradients in soil moisture within the soil column.