Anomalous thermal infrared (TIR) emissions have widely been detected by satellite sensors before the major earthquakes. A recent processing technique for geostationary thermal data, developed for the case of the 2009 April 6, magnitude 6.3 L'Aquila earthquake, makes it possible to identify areas of enhanced TIR emissions around the epicentral region at a mean distance of less than 50 km but inside a radius of about 100 km. The index, called Night Thermal Gradient (NTG), derived from 4-D time-series data (two spatial and two temporal coordinates), identifies TIR anomalies by following the temperature trend during night, when the surface of the Earth is expected to cool. Leading up to the L'Aquila earthquake, an anomalous warming trend was observed. In this study, the anomalous NTG pattern is compared to the expected normal trend, taking into account the seismogenic faults, the overall tectonic setting, lithological spatial features, the orography and world stress map near the epicentral region. Main results are that a certain lithological selectivity can be recognized and that the known main stress field and seismogenic faults seem to be less important than certain tectonic lineaments, which are classified as non-seismogenic. The strong correlation between the topography and the TIR anomalies is in agreement with proposed physical mechanism for the generation of TIR anomalies. This relation is, in turn, present mainly in correspondence to two tectonic lineaments which in particular are thrusts: therefore, strong compressive states seem to be a positive condition for the generation of TIR anomalies. The temporary modification of these stress fields have triggered the Paganica Fault to its normal rupture mechanism. It is important to note that the distances, over which the TIR anomalies occurred, are an order of magnitude larger than the estimated length of the main fault rupture. Pixel-by-pixel time-series comparisons between the maximum TIR anomaly area and the epicentre of the main shock show that the increase in radiative emission occurred in the areas of maximum TIR anomalies and did not start by spreading outward from the epicentral region.
Settlements of building foundations are generally due to water content changes in the shallow subsurface, both by natural and man‐made causes. Although resin injection is revealed to be a satisfactory solution for ground consolidation, a continuous monitoring of the process is needed to achieve optimal results. In order to control the injection of expanding resins, a field procedure is developed, based on the use of time‐lapse three‐dimensional (4D) Electrical Resistivity Tomography (ERT). The choice of electrical resistivity, as a parameter for designing and monitoring the consolidation work, is based on the key assumption that this physical property is the most sensitive to water content changes in soils. During the injection stage, repeated ERT acquisitions allow the injection process to be controlled and the injection schedule and its parameters to be modified, whenever necessary.
In this paper the procedure and its results are illustrated, through a case history in Venice (Italy), where salt‐water bearing soils also had to be taken into account. Careful analysis of electrode array configurations and parameters had therefore to be performed in advance. Horizontal and vertical sections from the resulting 3D resistivity models show, through a noticeable local increase of the resistivity at and nearby the injection points, that re‐homogenization of soil is successfully achieved. Repeated 3D ERT measurements, carried out three and a half years after the consolidation work, show that stabilization of the subsoil below and around settled foundations is achieved, as also confirmed by comparing suitable extensimeter measurements on overlying structures, carried out before and after the treatment.
A geophysical investigation that included seismic-reflection surveying and time-domain electromagnetics (EM) was carried out in the Flumendosa River Delta plain in southeastern Sardinia, Italy. The objective was to improve knowledge of geologic and hydrogeologic controls on a highly productive aquifer hosted in thick Quaternary deposits and known to be affected by an extensive saltwater intrusion. The seismic reflection survey, whose results are reported here, aimed to image the Paleozoic bedrock topography and to obtain detailed structural and stratigraphic information on the sequence of largely fluvial sediments extending from the surface down to bedrock. The survey consisted of two inline profiles, nearly parallel to the coastline and 1 km inland. The sources (0.25 kg of explosives buried at 2 m depth) and receivers (50-Hz vertical geophones) produced a twelvefold data set with common midpoints every 2.5 m. Detailed integrated velocity analysis (constant velocity gathers, constant velocity stacks, and semblance plots) and frequency-wavenumber-domain dip moveout (DMO) proved to be the most important processing steps in producing the two stacked sections. Both sections were characterized by distinct seismic units bounded by quasi-continuous reflectors. The lowermost reflection, imaged on section 1 at two-way traveltimes between 310 ms (~300 m) and 580 ms (~530 m) and on section 2 between 200 ms (~190 m) and 65 ms (~52 m), was interpreted to be Paleozoic bedrock. A maximum depth twice as deep as expected was the primary and somewhat surprising result. Imaging of oblique progradational facies — another major finding — proved the existence of undocumented marine Pleistocene sediments that could help in revising the area's geology
The design and execution of consolidation treatment of settled foundations by means of injection of polyurethane expanding resins require a proper investigation of the state of the foundation soil, in order to better identify anomalies responsible for the instability. To monitor the injection process, a procedure has been developed, which involves, in combination with traditional geotechnical tests, the application of a noninvasive, geophysical technique based on the electrical resistivity, which is strongly sensitive to presence of water or voids. Three-dimensional electrical resistivity tomography is a useful tool to produce effective 3D images of the foundation soils before, during, and after the injections. The achieved information allows designing the consolidation scheme and monitoring its effects on the treated volumes in real time. To better understand the complex processes induced by the treatment and to learn how variations of resistivity accompany increase of stiffness, an experiment was carried out in a full-scale test site. Injections of polyurethane expanding resin were performed as in real worksite conditions. Results confirm that the experimented approach by means of 3D resistivity imaging allows a reliable procedure of consolidation, and geotechnical tests demonstrate the increase of mechanical stiffness.
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