Joint Terrestrial and Aerial Measurements to Study Ground Deformation: Application to the Sciara Del Fuoco at the Stromboli Volcano (Sicily)

The 2002–2003 Stromboli eruption triggered the failure of part of the Sciara del Fuoco slope, which generated a tsunami that struck the island and the northern coastline of Sicily. The Sciara del Fuoco is a very steep slope where all lava flows from the craters’ emplacement; most lateral eruptions usually take place from fissures propagating in this sector of the volcano. The eruption went on to produce a lava field that filled the area affected by the landslide. This in turn led to further instability, renewing the threat of another slope failure and a potentially related tsunami. This work describes a new joint approach, combining surveying data and aerial image correlometry methods, to study the motion of this unstable slope. The combination has the advantage of very precise surveying measurements, which can be considered the ground truth to constrain the very-high-resolution aerial photogrammetric data, thereby obtaining highly detailed and accurate ground deformation maps. The joint use of the two methods can be very useful to obtain a more complete image of the deformation field for monitoring dangerous and/or rather inaccessible places. The proposed combined methodology improves our ability to study and assess hazardous processes associated with significant ground deformation.


Introduction
Active volcanoes represent a rapid temporally changing morphological terrain. Local topography is modified due to new material deposition, rapid erosion, compaction, and/or mass-wasting processes. Some of those morphological processes are associated with hazardous events, such as tsunami-generating landslides. Therefore, keeping up-to-date topographic maps and tracking its temporal changes is important to evaluate natural hazards. Rapid updates of topographic maps, digital elevation models (DEM), and terrain displacement maps have been made in the past to characterize the morphological changes of very active volcanoes [1,2]. The most popular technique is photogrammetry, as it has been widely used to determine topography and its temporal changes with a precision on a scale of meters [3]. Radar techniques have been also used to determine topography with a comparable precision. In addition, the applications of differential radar techniques are able to track changes in a narrow range, from millimeters to centimeters [4]. However, sudden topographic changes on a scale of tens of centimeters are difficult to capture, either with photogrammetric or with radar technology, when they produce a decorrelation between the two images. Optical imagery cross-correlation or optical correlometry can be used to measure topographic changes, particularly horizontal ground displacements with sub-pixel precision [5][6][7][8][9]. Optical correlometry can therefore provide the capability of partially filling the observational gap. Here, we propose using optical correlometry of high-resolution aerial orthoimagery to determine surface ground displacements on a scale from tens of centimeters to several meters. Then, using a multisensor approach, namely a combination of techniques, would allow for the extension of a range of track changes in topography.
Stromboli is the northernmost island of the Aeolian Archipelago volcanic arc (Southern Tyrrhenian Sea, Italy). It is a composite volcano, rising about 2500 m above the sea floor, with its summit at about 1 km above sea level. The volcanoes of this arc developed between 1.3 Ma and the present [10]. The central part of the archipelago is still active and represents a transition zone between two geodynamically different domains with a continental collision and inactive volcanoes in the west and a subduction process and active volcanism in the east [11,12]. Stromboli has a complex construction process characterized by repeated sector collapses that have occasionally affected large portions of the edifice [13]. The overall conical shape of the volcano is broken by a large depression in the northwestern flank of the edifice: the Sciara del Fuoco (SdF). The SdF continues below the sea, down to 700-750 m b.s.l.; this scar was produced by a huge sector collapse, on a scale of 1 cubic km, that very likely originated during the Holocene [14]. The sliding of the SdF is constant, also producing continuous rockfalls [15,16].
On 30 December 2002, two landslides occurred on the northern part of the SdF and generated a tsunami. The total volume of the landslides was estimated at about 17 million cubic meters from photogrammetric and bathymetric data [17]. The landslides took place where the new lava flow had emplaced a few days before. After the landslide, several flows overflowed on the same sector, forming a new lava terrace; this morphological element was a major concern after the end of the eruption, as it made the NE sector of the Sciara del Fuoco unstable once again.
The aim of this work is to test, compare, and integrate two independent datasets of ground deformation measurements, coming from a robotized surveying system and from orthoimages. One objective is to assess the suitability of this joint approach for monitoring the motion of this unstable slope of the volcano. Terrestrial techniques are more precise, but data are limited to sparse locations. Conversely, remote sensing techniques (though not so precise) allow covering wider areas with a higher spatial resolution. They help define the shape and dimension of the moving body and can provide important parameters, together with the kinematics of the motion, for hazard evaluations. For a correct estimation of the applicability, a comparison and joint interpretation of results is needed with both techniques, allowing for the validation of remote sensing. Once the technique is validated, it can also be used as a unique observation technique to monitor hazardous and remote active areas.

Terrestrial Ground Deformation Monitoring System
During the 2002-2003 eruption, a terrestrial monitoring network was installed to routinely measure the motion of benchmarks inside the SdF [15,16,18,19]. This network progressively evolved to an automatic system, named THEODOROS (THEOdolite and Distancemeter Robot Observatory of Stromboli), based on a remotely controlled robotized Total Station. It was aimed at measuring the pattern of the deformation from upper to lower altitudes (see Supplementary Material).

Aerial Image Correlometry
Stromboli volcano has been routinely imaged using aerial photogrammetric surveys during the last several eruptions for the estimation of volume changes, the detection of motion, and the rapid response to volcanic crises [3,20]. From the available dataset (Table 1) [21]. The resulting orthoimages were generated using standard photogrammetric methods, but with high performance, due to the in-situ collection of data at 20 GPS ground control points and the inertial system and GPS technology to control the camera position and orientation with high accuracy [20]. Quality and geometric control of the orthoimagery is presented in detail in [3,20], which follows the methodology of [22] (see Supplementary Material for a more detailed description of the procedures).

Results
Independent ground deformation measurements using the THEODOROS system and 3D point coordinates measured manually on the photogrammetric stereo models are available for the studied period ( Figure 1). Horizontal displacements obtained using those systems are shown in Figure 1b methods, but with high performance, due to the in-situ collection of data at 20 GPS ground control points and the inertial system and GPS technology to control the camera position and orientation with high accuracy [20]. Quality and geometric control of the orthoimagery is presented in detail in [3,20], which follows the methodology of [22] (see Supplementary Material for a more detailed description of the procedures).

Results
Independent ground deformation measurements using the THEODOROS system and 3D point coordinates measured manually on the photogrammetric stereo models are available for the studied period ( Figure 1). Horizontal displacements obtained using those systems are shown in Figure 1b         Both datasets (see Supplementary Material for data correlation) agree in showing a significant seaward motion of the ground on the upper northeastern part of the SdF. The maximum displacements seem to be located by both techniques at an elevation of about 550 m, just below the slope rupture bordering the terrace affected by vents opening during the 2002-2003 eruption, where the GBInSAR system also detected even larger displacements [23,24]. THEODOROS benchmark SDF16 clearly limits the instability towards SW, confining the sliding part on the north-easternmost side of the SdF; this sector could represent the downward prolongation of the most mobile sectors, detected just below the summit craters by the ground-based InSAR [25]. Benchmark SDF7 also confirms the stability of the central part of the SdF. Downslope, SDF5, and SDF6 reflectors reveal that the flank instability does not affect the entire side of the SdF down to the coastline but is confined to its upper part. Furthermore, both datasets concur in showing a decreasing motion in time, suggesting an evolution of the phenomena toward a progressive stabilization of the flank.

Discussion
We produced very dense ground deformation maps with the resolution of the aerial images; these images were also validated and constrained by ground truth, as, for example, is usually done on Etna [26,27]. Figure 3 displays the measured deformation patterns using the proposed correlation method. The smooth deformation pattern for the 2003-2004 period (upper row) may indicate the prevalence of the thermal-contraction of the lava terrace volume with respect to the slope movement (as also observed after the 2007 eruption by the authors of [18]). As cooling of the lava terrace continues, it seems that the slope movement became the dominant deformation process in the SdF (2004-2005 period), confirming what was observed by the authors of [28], and both the shape of the unstable portion and the sharp upper border of the detachment along a NE-SW line (perpendicular to the maximum slope) are well defined. During this period, a two-fracture system was developed: a main NE-SW fracture system in line with the main slope rupture of the pre-lava terrace emplacement topography (see the AA' profile in Figure 3 parallel to the maximum slope direction) and a secondary parallel NE-SW fracture in another minor buried topography slope rupture; similar ground cracks have also been detected in successive periods and imputed to shallow-seated gravitational motion of the volcanoclastic cover of the northernmost part of the SdF [25]. The profile BB' in Figure 3 also indicates the importance of the thickness in the deformation rates-the thicker the cover of lava flows, the higher the deformation rates.

Conclusions
The cumulative displacement (2003)(2004)(2005) is dominated by the larger failure process deformation of the 2004-2005 period. The first period seems to be controlled by the lava thickness contraction. The results of this work image the ground motion affecting the SdF from 2003 to 2005 well, highlighting the shape and borders of the incipient slope failure and are in good agreement with InSAR data collected just upwards towards the summit crates area [25]. The direction of the motion is also in agreement with InSAR and field data, producing NE-SW ground cracks and fractures that, when deep-seated, could act as a preferential means of lateral intrusions from the summit craters, feeding lower eruptive vents in the SdF [25].
The optical correlometry results provide centimeter-to-meter displacement maps with very high spatial resolution (few meters), although mainly in the horizontal components; therefore when combined with additional techniques, i.e., THEODOROS, a wide range of topographic changes can be tracked. This combined approach successfully defines the shape and dimension of the slope-moving materials, providing important parameters, together with the kinematics of the motion, for hazard evaluations.
Multisensor combinations are a powerful approach to obtaining very dense ground deformation maps (see also [26,27,[29][30][31]), with the spatial resolutions of a few meters. The proposed integrated approach may prove very useful for monitoring hazardous processes producing significant surface deformation in highly inaccessible places such as the SdF at Stromboli, where implementing ground-based systems is difficult and even dangerous (e.g., [31]). This combination of ground-based and remote sensing techniques can be useful in other deformation environments as well, such as earthquakes (see e.g., [5,7]) and landslides (see e.g., [31][32][33]). The validation of the correlation methodology means it can be used as the sole technique if necessary in very remote or dangerous active areas.
Supplementary Materials: The following is available online at www.mdpi.com/2072-4292/8/6/463/s1. It describes in detail the surveying network, orthoimagery processing, and correlation of measured displacement results.