Comparison of L- and C-Band SAR data in the Saar Mining District, Germany

The interferometric utilization of Synthetic Aperture Radar data from L-band and C-band has an important role for the monitoring of land surface deformations like former evaluations have proven [1]. Meanwhile several multi-sensor ground-stations are available, equipped with bi-directional artificial corner-reflectors (CR) and permanent GNSS stations, attached to fine leveling baselines. The long wavelength of L-band SAR missions like ALOS-2 (λ = 22.9 cm) provides highly coherent interferograms, but here large-sized CR are required e.g. for absolute motion calibration. SAR missions with shorter wavelengths, like the C-band onboard the Sentinel-1 mission (λ = 5.6 cm) provide, in general, less coherent interferograms, but a smaller CR size is sufficient. In order to assess the capabilities of L- and C-band SAR data the impulse response function will be calculated at corner-reflector sites and the coherence will be estimated in rural areas of the Saar test site.

The test site is located in the Saar-Lorraine coal basin at the French-German border, a nowadays post-mining district with highly urbanized settlements as well as large stretches of forested and rural areas. The area is characterized by century long active deep mining – mainly for hard coal – including extensive groundwater management measures. Here, the active coal mining started in the 18^th century and ended in 2006 (Lorraine) and 2012 (Saar) [2]. Meanwhile some of the underground mines got progressively flooded. As a consequence surface uplift occurred and is expected to be ongoing in the near future [3]. For a 12 by 14 km area in the Saar district dense and highly accurate leveling campaigns have been performed bi-annually since 2013. Thus, besides good knowledge of subsurface geology and mining activities also precise in-situ measurements of the ground motion are available. The recent and ongoing surface deformations will be monitored using multiple methods including a network of CR at multi-sensor ground stations [4] and publicly accessible Persistent Scatterer Interferometry datasets from the Sentinel-1 based Ground Motion Service Germany [5].

In late 2020 first ALOS-2 acquisitions of the Saar area from the ESA-JAXA cooperation were made available to the authors. The ALOS-2 data are evaluated and placed in relation to Sentinel-1 acquisitions. Finally, an outlook on the possible complementary use of geodetic and C- and L-band data in the Saar district as well as for other mining areas in Germany is given.

[1] Wegmueller et al. 2005: Monitoring of mining induced surface deformation using L-band SAR interferometry. IGARSS 2005; DOI: 10.1109/IGARSS.2005.1526447

[2] Corbel et al. 2017: Coal mine flooding in the Lorraine-Saar basin: experience from the French mines. IMWA 2017. https://www.imwa.info/docs/imwa_2017/IMWA2017_Corbel_161.pdf

[3] Heitfeld-Schetelig 2016: Gutachten zu den Bodenbewegungen im Rahmen des stufenweisen Grubenwasseranstiegs in den Wasserprovinzen Reden und Duhamel. http://www.bid.rag.de/bid/PDFs/SA//GWA_Reden_Duhamel/3_IHS_Bodenbewegungen/IHS_Saar_Gelaendehebungen_WH_Reden_Duhamel_2016_04_20.pdf

[4] Spreckels et al. 2020: GNSS, Nivellement und Radar – einheitliche Multisensor-Standorte als Referenzpunkte zur Überwachung von Bodenbewegungen. Geomonitoring 2020. DOI: 10.15488/9351

[5] BGR, 2021: https://bodenbewegungsdienst.bgr.de