The aim of this project is to use ambient noise recordings to study the structure of the crust, focusing on the subsurface structures of the upper crust, at the so-called “geotechnical” scale (a few tens of metres to about one kilometre). The main method studied is Ambient Noise Tomography and its combination with independent data derived either from the noise data itself (HVSR ratios, Rayleigh wave ellipticity) or from independent data (active geophysical methods, reliable reference models). The main objective of this study is to extract detailed information on the subsurface structure of surface formations (e.g. seismic wave velocities, anisotropy, etc.).
The main objective of the project is to develop a reliable tool, which can be used to determine the geophysical structure of the most heterogeneous surface geological formations, especially in urban environments, where the greatest challenges are presented by the limitations in the application of large-scale geophysical methods and the spatial location of classical geotechnical tests. The fundamental objective of the PROTECTANT project is to investigate in detail the potential and limitations of the application of environmental (ground) noise tomography and its combined interpretation with other data under different conditions and environments.
To achieve the above objectives, the project implements two (2) main Work Modules:
Α) The first one (WP2) studies the use of environmental noise tomography in urban and peri-urban environments to generate three-dimensional (3D) simulations of the velocity distribution of transverse elastic waves, VS. The tomographic inversion considers the independent, and in the final stage of the project the combined inversion of the Rayleigh and Love wave dispersion curves, combined with the information from the HVSR curves and the extracted Rayleigh wave ellipticity curves, with the final goal of creating a three-dimensional (3D) imaging tool in the study areas. In the framework of this EU project, measurements have already been carried out in one (urban) test field (Thessaloniki), initial data processing has already been performed and a publication with preliminary results has already been submitted, while measurements in the second test field are planned.
B) The second (WP3) aims to compare different techniques for one-dimensional inversion of ground noise data, and their calibration using a new base of one-dimensional structure models recently presented for the Greek site (Stewart et al., 2014), with the aim of assessing the reliability of the methods and proposing techniques to reduce the uncertainty of each method. In the framework of this EU, measurements have already been made at several points in the baseline, and the results for the Santorini area, which has particular geological-geophysical characteristics, are in the process of being published. At the same time, test schemes of combined inversion on synthetic and real data have already been implemented, which are expected to lead to a new approach to the interpretation of the above mentioned environmental noise data in challenging geological environments (e.g. urban areas).

Both main objectives of the project address the same theoretical and application questions, as both proposed approaches serve the same goal: Reducing the inherent uncertainties in the environmental noise inversion model. This is an important step in addressing many future challenges, such as:

α) Making environmental noise methods a reliable tool for upper crustal subsurface geophysical studies

b) Achieving the long-term goal of making such techniques the tool that can be reliably used in everyday engineering practice, in addition to its use in relevant geoscience studies. Applications that exploit environmental noise still require additional developments in methodology and implementation, as additional automation and tuning features need to be developed to allow their application by non-experts.

c) The results will also contribute to a better understanding of the nature, waveform content and patterns of environmental noise, which is a major theoretical challenge for geoscientists, as our knowledge is still rather poor.

In order to achieve the above, it is of primary importance to collect environmental noise data in urban and semi-urban environments. The data collection requires measurements to be carried out which is implemented in the WP1 which a significant exemplary environmental noise data set has been collected.