Pablo J. González

Radar interferometry techniques for the study of ground subsidence phenomena: a review of practical issues through cases in Spain

Subsidence related to multiple natural and human-induced processes affects an increasing number of areas worldwide. Although this phenomenon may involve surface deformation with 3D displacement components, negative vertical movement, either progressive or episodic, tends to dominate. Over the last decades, differential SAR interferometry (DInSAR) has become a very useful remote sensing tool for accurately measuring the spatial and temporal evolution of surface displacements over broad areas. This work discusses the main advantages and limitations of addressing active subsidence phenomena by means of DInSAR techniques from an end-user point of view. Special attention is paid to the spatial and temporal resolution, the precision of the measurements, and the usefulness of the data. The presented analysis is focused on DInSAR results exploitation of various ground subsidence phenomena (groundwater withdrawal, soil compaction, mining subsidence, evaporite dissolution subsidence, and volcanic deformation) with different displacement patterns in a selection of subsidence areas in Spain. Finally, a cost comparative study is performed for the different techniques applied.

The 2014–2015 eruption of Fogo volcano: Geodetic modeling of Sentinel‐1 TOPS interferometry

After 20 years of quiescence, Fogo volcano erupted in November 2014. The eruption produced fast-moving lava flows that traveled for several kilometers and destroyed two villages. This event represents the first episode of significant surface deformation imaged by the new European Space Agencys Sentinel-1 satellite in its standard acquisition mode, Terrain Observation by Progressive Scans (TOPS), which differs from that of previous synthetic aperture radar (SAR) missions. We perform a Bayesian inversion of Sentinel-1 TOPS SAR interferograms spanning the eruption and accurately account for variations in the TOPS line-of-sight vector when modeling displacements. Our results show that magma ascended beneath the Pico do Fogo cone and then moved laterally toward its southwestern flank, where the eruptive fissure opened. This study provides important insights into the inner workings of Fogo volcano and shows the potential of Sentinel-1 TOPS interferometry for geophysical (e.g., volcano monitoring) applications.

Surface uplift and time-dependent seismic hazard due to fluid injection in eastern Texas.

Observations that unequivocally link seismicity and wastewater injection are scarce. Here we show that wastewater injection in eastern Texas causes uplift, detectable in radar interferometric data up to >8 kilometers from the wells. Using measurements of uplift, reported injection data, and a poroelastic model, we computed the crustal strain and pore pressure. We infer that an increase of >1 megapascal in pore pressure in rocks with low compressibility triggers earthquakes, including the 4.8–moment magnitude event that occurred on 17 May 2012, the largest earthquake recorded in eastern Texas. Seismic activity increased even while injection rates declined, owing to diffusion of pore pressure from earlier periods with higher injection rates. Induced seismicity potential is suppressed where tight confining formations prevent pore pressure from propagating into crystalline basement rocks.

Drought-driven transient aquifer compaction imaged using multitemporal satellite radar interferometry

In unconsolidated, heterogeneous aquifer systems, low rates of pore-pressure diffusion of applied effective stresses due to the drainage of thick, low-permeability, clay-rich layers with time constants of decades to centuries cause delayed, residual permanent compaction and land subsidence. Current application of satellite differential radar interferometry (DInSAR—differential interferometric synthetic aperture radar) to estimate aquifer hydraulic properties (compressibility and/or storage) in these systems is limited by the temporal availability of synthetic aperture radar data (1992–present). In this paper we study the degree of aquifer compaction due to water extraction using DInSAR through an example in southeast Spain. Ground deformation data indicate large-scale deformation and in particular the discovery of the highest rates of groundwater-related land subsidence recorded in Europe (>10 cm/yr), affecting the Guadalentin River basin (>200 km 2 ), the largest tributary of the Segura River. Modeling of the ground surface time series of the Guadalentin Basin indicates that deformation is mainly driven by nonlinear time-delayed flow processes in the underlying aquifer. After a drought period (1990–1995), the aquifer responded with an exponential decay of the land subsidence (lasting ∼8 yr), suggesting transient groundwater pore-pressure flow. We show that multitemporal satellite radar interferometry analysis and its modeling can be a stimulating way to study nonlinear soil mechanics and groundwater flows at aquifers. A deeper understanding of such processes could help the management of water resources and land subsidence of unconsolidated coastal and Quaternary alluvial aquifers in a highly evolving climate region (the Mediterranean Sea and elsewhere).

Spatiotemporal analysis and interpretation of 1993–2013 ground deformation at Campi Flegrei, Italy, observed by advanced DInSAR

Campi Flegrei is one of the most hazardous volcanic areas in the world because of its close proximity to the city of Naples. Here we apply the multidimensional small baseline subset (MSBAS) differential interferometric synthetic aperture radar (DInSAR) technique to obtain vertical and horizontal components of ground deformation for Campi Flegrei at high spatial and temporal resolutions that span, for the first time, 20 years. The area underwent continuous subsidence from 1993 through 1999. Moderate uplift began in 2010 and substantially increased through 2012, reaching approximately 13 cm by 2013. We model the observed deformation to determine source parameters for subsidence and uplift epochs. Both the inflation and deflation mechanisms involve large, extended sources in a layered hydrothermal system whose location is controlled by the caldera structure and stratigraphy. The temporal resolution of MSBAS approaches that of GPS daily time series, with superior precision and spatial resolution, making it an excellent alternative for volcano monitoring.

High‐resolution digital elevation model from tri‐stereo Pleiades‐1 satellite imagery for lava flow volume estimates at Fogo Volcano

Resolving changes in topography through time using accurate high-resolution digital elevation models (DEMs) is key to understanding active volcanic processes. For the first time in a volcanic environment, we utilize very high-resolution tri-stereo optical imagery acquired by the Pleiades-1 satellite constellation and generate a 1 m resolution DEM of Fogo Volcano, Cape Verde—the most active volcano in the Eastern Atlantic region. Point cloud density is increased by a factor of 6.5 compared to conventional stereo imagery, and the number of 1 m2 pixels with no height measurements is reduced by 43%. We use the DEM to quantify topographic changes associated with the 2014–2015 eruption at Fogo. Height differences between the posteruptive Pleiades-1 DEM and the preeruptive topography from TanDEM-X give a lava flow volume of 45.83 ± 0.02 × 106 m3, emplaced over an area of 4.8 km2 at a mean rate of 6.8 m3 s−1.

Coseismic Horizontal Offsets and Fault-Trace Mapping Using Phase Correlation of IRS Satellite Images: The 1999 Izmit (Turkey) Earthquake

On August 17, 1999, a strong earthquake (Mw ¿ 7.4) occurred along the western sector of the North Anatolian Fault system in Turkey. The epicenter was located near the city of Izmit, 50 km east of Istanbul. Previous works determined the coseismic surface displacements by satellite synthetic aperture radar (SAR) interferometry (InSAR) and satellite optical-image correlation. In 1999, the highest spatial resolution orbiting camera was the panchromatic sensor (PAN), a 5.8-m pixel sensor (SPOT 2 was a 10-m pixel sensor) onboard the Indian Remote Sensing (IRS) satellite. We propose to apply a new phase-correlation method to PAN images to study the coseismic rupture due to the Izmit earthquake. The phase-correlation method does not need phase unwrapping and was proved to be robust under a wide variety of circumstances. Image correlometry deals with the quantification of the subpixel offsets over the whole image, allowing displacement measurement with an accuracy that is proportional to the pixel size. We measured the near-field deformations exploiting two geometrically corrected IRS images with similar look angles. A quality check of the derived offset map was performed by comparison with GPS benchmarks and SPOT offsets. The results show that IRS PAN images can be correlated to derive coseismic slip offsets due to a large earthquake (and to map its fault trace).

Systematic InSAR monitoring of African active volcanic zones: What we have learned in three years, or an harvest beyond our expectations

We present here a brief overview of some findings and preliminary results obtained after almost three years of systematic monitoring of active volcanic areas in Africa by means of differential synthetic aperture radar interferometry (InSAR). With a database rich of more than 400 SAR scenes of Fogo (Cape Verde), Ol Doinyo Lengai (Tanzania), Nyiragongo-Nyamulagira (DR of Congo) and Mount Cameroon volcanoes, we processed more than 2000 interferograms among which we could detect significant and major geophysical processes: the first dyking event ever captured geodetically in a continental rift (Lake Natron; Northern Tanzania), the co-eruptive deformations of the Lengai, Nyiragongo and Nyamulagira volcanoes, the co-seismic displacements associated to the mb 6.1 February 3rd 2008 Bukavu earthquake as well as the identification of atmospheric induced phase delays over Fogo and Mount Cameroon volcanoes to be attributed to the seasonal oscillations of the inter-tropical convergence zone (ITCZ). These results have been reached given the abundance of data that increases the chances to capture unpredictable events, and capture them with the most favorable interferometric conditions as possible (e.g. in terms of geometrical and temporal baselines that minimized the vegetation-induced decorrelation). They provided strong scientific material as well as tools for hazard assessment.

Real Time Tracking of Magmatic Intrusions by means of Ground Deformation Modeling during Volcanic Crises

Volcano observatories provide near real-time information and, ultimately, forecasts about volcano activity. For this reason, multiple physical and chemical parameters are continuously monitored. Here, we present a new method to efficiently estimate the location and evolution of magmatic sources based on a stream of real-time surface deformation data, such as High-Rate GPS, and a free-geometry magmatic source model. The tool allows tracking inflation and deflation sources in time, providing estimates of where a volcano might erupt, which is important in understanding an on-going crisis. We show a successful simulated application to the pre-eruptive period of May 2008, at Mount Etna (Italy). The proposed methodology is able to track the fast dynamics of the magma migration by inverting the real-time data within seconds. This general method is suitable for integration in any volcano observatory. The method provides first order unsupervised and realistic estimates of the locations of magmatic sources and of potential eruption sites, information that is especially important for civil protection purposes.

Removal of systematic seasonal atmospheric signal from interferometric synthetic aperture radar ground deformation time series

Applying the Multidimensional Small Baseline Subset interferometric synthetic aperture radar algorithm to about 1500 Envisat and RADARSAT-2 interferograms spanning 2003-2013, we computed time series of ground deformation over Naples Bay Area in Italy. Two active volcanoes, Vesuvius and Campi Flegrei, are located in this area in close proximity to the densely populated city of Naples. For the first time, and with remarkable clarity, we observed decade-long elevation-dependent seasonal oscillations of the vertical displacement component with a peak-to-peak amplitude of up to 3.0 cm, substantially larger than the long-term deformation rate (<0.6 cm/yr). Analysis, utilizing surface weather and radiosonde data, linked observed oscillations with seasonal fluctuations of water vapor, air pressure, and temperature in the lower troposphere. The modeled correction is in a good agreement with observed results. The mean, absolute, and RMS differences are 0.014 cm, 0.073 cm, and 0.087 cm, respectively. Atmospherically corrected time series confirmed continuing subsidence at Vesuvius previously observed by geodetic techniques.