Yngvar Larsen

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.

InSAR Deformation Time Series Using an

Satellite synthetic aperture radar interferometry (InSAR) is an invaluable tool for land displacement monitoring. Improved access to time series of satellite data has led to the development of several innovative multitemporal algorithms. Small baseline (SB) is one such time-series InSAR method, based on combining and inverting a set of unwrapped interferograms for surface displacement. Two-dimensional unwrapping of sparse data sets is a challenging task, and unwrapping errors can lead to incorrectly estimated deformation time series. It is well known that L1-norm is more robust than L2-norm cost function minimization if the data set has a large number of outlying points. In this paper, we present an L1-norm-based SB method using an iteratively reweighted least squares algorithm. We show that the displacement phase of both synthetic data, as well as a real data set that covers the San Francisco Bay area, is recovered more accurately than with L2-norm solutions.

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The synthetic aperture focusing technique (SAFT) is used to create focused images from ultrasound scans. SAFT has traditionally been applied only for imaging in a single medium, but the recently introduced phase shift migration (PSM) algorithm has expanded the use of SAFT to multilayer structures. In this article we present a similar focusing algorithm called multi-layer omega-k (MULOK), which combines PSM and the ω-k algorithm to perform multilayer imaging more efficiently. The asymptotic complexity is shown to be lower for MULOK than for PSM, and this is confirmed by comparing execution times for implementations of both algorithms. To facilitate the complexity analysis, a detailed description of algorithm implementation is included, which also serves as a guide for readers interested in practical implementation. Using data from an experiment with a multilayered structure, we show that there is essentially no difference in image quality between the two algorithms.

-Norm Small-Baseline Approach

Following earthquakes, faults are often observed to continue slipping aseismically. It has been proposed that this afterslip occurs on parts of the fault with rate-strengthening friction that are stressed by the main shock, but our understanding has been limited by a lack of immediate, high-resolution observations. Here we show that the behavior of afterslip following the 2014 South Napa earthquake in California varied over distances of only a few kilometers. This variability cannot be explained by coseismic stress changes alone. We present daily positions from continuous and survey GPS sites that we remeasured within 12 h of the main shock and surface displacements from the new Sentinel-1 radar mission. This unique geodetic data set constrains the distribution and evolution of coseismic and postseismic fault slip with exceptional resolution in space and time. We suggest that the observed heterogeneity in behavior is caused by lithological controls on the frictional properties of the fault plane.

Synthetic aperture focusing of ultrasonic data from multilayered media using an omega-K algorithm

Abstract: Alpine topography in Norway is largely fault-controlled. Linear and asymmetric ranges developed in the footwalls of normal faults that were reactivated after the main phase of Mesozoic rifting, but prior to the Late Cenozoic glaciations. Stark geomorphological contrasts developed across the faults, reflecting differential glacial exploitation of the pre-glacial drainage pattern. Alpine topography developed preferentially in the footwalls. Triangular facets mark the traces of the most recently active faults. At the base of deeply incised, alpine range-front escarpments, the best-exposed faults display metres-thick fault-rock successions and record multiple phases of fault movement. Juxtaposition of Precambrian and Caledonian basement rocks with Jurassic or Cretaceous sedimentary rocks provides evidence for fault activity in or after the Mesozoic for some of the faults. Late Cretaceous or younger reactivation is indicated by jumps in apatite fission-track apparent ages across the faults, and interferometric synthetic aperture radar and earthquake data attest to normal faulting at the present day. Two of the areas described host anomalous clusters of rockslides that may relate to tectonic activity. The most distinct landscape-forming faults in western Scandinavia were probably active in the Cenozoic, and imposed asymmetric landscape patterns from the scale of single mountain ranges to the whole of Scandinavia.

Spatial variations in fault friction related to lithology from rupture and afterslip of the 2014 South Napa, California, earthquake

Europes Sentinel-1A spacecraft and its extraordinary images of slip from the South Napa earthquake herald a new era of space-based surveillance of faults.

Fault-controlled alpine topography in Norway

The main operational mode of the European Space Agencys upcoming Sentinel-1 operational satellite will be the Terrain Observation by Progressive Scans (TOPS) imaging mode. This paper presents a very efficient wavenumber domain processor for the processing of TOPS mode data. In particular, a novel signal transform, called a moving band chirp Z-transform, is introduced in order to allow the entire azimuth aperture to be focused simultaneously without any need for temporary unaliasing, which requires upsampling, or subaperture processing.

Earthquake monitoring gets boost from a new satellite

The subaperture cross-correlation magnitude (SCM) has previously been proposed as a statistic that improves the contrast between small ship targets and the surrounding sea in synthetic-aperture-radar images. This preprocessing technique utilizes the fast decorrelation of open-water surface ripples on the scale of the SAR wavelength relative to coherent targets such as a ship. However, optimization of the bandwidth splitting in the subband extraction has not received any attention. The aim of this letter is twofold: 1) to describe the technical details of the algorithm, including modifications that are necessary to allow overlapping subapertures; and 2) to study the effect of splitting the bandwidth into two azimuth subapertures with respect to varying bandwidths and subaperture overlap. The impact on the SCM is investigated in terms of measures of speckle reduction and target-to-clutter contrast. Experiments are performed on real single-look complex SAR data containing repeated acquisitions of a vessel in open sea. The results indicate that the subband extraction strategy has a clear impact on performance.

Efficient Full Aperture Processing of TOPS Mode Data Using the Moving Band Chirp

The synthetic aperture focusing technique (SAFT) has been shown to significantly improve the lateral resolution of monostatic imaging systems. To apply SAFT to interior imaging of cylindrical structures such as pipes or blood vessels, algorithms adapted to cylindrical measurement surfaces are needed. In this paper, we present a new algorithm for SAFT applied to such cylindrical scans. The derivation of the algorithm is based directly on the Fourier domain solutions to the wave equation in cylindrical coordinates, and in this sense, the algorithm is exact. Compared with existing methods, the proposed algorithm yields lower side lobes and better resolution for wide-beam transducers. The attainable angular resolution is shown to depend on the size of the transducer and the scanning radius R. For a flat, circular transducer of diameter D, the angular resolution is approximately D/(2R). It is also shown that using an angular step size of D/(4R) will keep grating lobes at least 40 dB below the main lobe, and we therefore recommend D/(4R) as a suitable step size for practical applications.

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We estimate two-dimensional (2D) glacier surface motion using synthetic aperture radar (SAR) X-band intensity tracking. It has been observed that the viability of SAR interferometry (InSAR) is often limited by coherence loss over glaciers in landlocked regions using SAR data pairs of more than 1 day temporal baseline. An alternative to InSAR is the intensity-tracking approach, which relies on intensity cross-correlation for the estimation of subpixel surface motion in range and azimuth direction. In this work, we apply this approach for 2D glacier surface motion estimation in the north-western (NW) Himalayas, using TerraSAR-X (TS-X) spotlight mode high-resolution data pairs of 11, 22, and 33 day temporal separation. The results are in good agreement with total station surveying measurements synchronous with the satellite data acquisition period. The technique is found to be highly appropriate for monitoring the flow rate of glaciers in the Himalayas on a multitemporal basis.