Geophone networks and environmental studies: Application to landslides Konstantinos Marmarokopos Electrical and Computer Engineer National Technical University of Athens, Department of Informatics, Ionian University, Corfu George Efremidis Department of Civil Engineering, University of Thessaly, Pedion Areos, GR – 38334 Volos, Greece, email:gefraim@uth.gr Markos Avlonitis Department of Informatics, Ionian University, 7 Tsirigoti Square, GR – 49100 Corfu, Greece, Email: avlon@ionio.gr I. INTRODUCTION Seismic surveys are a non-invasive method for exploring the geological structure of the Earth. Their purpose is to identify the composition of the subsoil. This information can be used to indicate the presence of potential mineral deposits, geotechnical studies, underground tomography, earthquake prediction and various other applications. A survey is performed by sending seismic waves deep into the subsurface and recording the image of different rock formations based on the reflection and refraction of the waves returning to the surface. The acquisition of seismic data requires specialist equipment including vibration trucks and geophones, i.e. special equipment for the registration of waves, which work in a similar way as a microphone does. Following processing and interpretation, the acquired data provide an insight into the geological structure of the Earth, which is a basis for drafting geological maps and sections. Thanks to seismic surveys via geophone networks, which are preceded by the analysis of historical geological data, it is possible to map the underground without using the costly drilling methods. Active and passive surface wave techniques are relatively new seismic methods for determining shear wave velocity (VS) profiles. Testing is performed on the ground surface, allowing for less costly measurements than with traditional borehole methods. The basis of surface wave techniques is the dispersive characteristic of Rayleigh waves when traveling through a layered medium. Rayleigh wave velocity is determined by the material properties of the subsurface to a depth of approximately 1 to 2 wavelengths. Longer wavelengths penetrate deeper and their velocity is affected by the material properties at greater depth. Surface wave testing consists of measuring the surface wave dispersion curve at a site and modeling it to obtain the corresponding shear wave velocity profile. The technique used in this study is the Multichannel Analysis of Surface Waves (MASW), which involves a source such as a sledgehammer impact on the ground. Vibrations generated due to sledgehammer impact are gathered by interconnected geophones (receivers) set up in the vertical direction and in a linear array with a constant spacing at the ground surface to obtain the experimental Rayleigh wave phase velocity dispersion curve. Then, an inversion technique is applied to calculate underground layer parameters, in our case thickness and shear wave velocity. It is our aim to explore the possibility to use geophone networks for environmental studies and more specifically for monitoring and predicting landslide stability. Landslides affect many elements of the environment, such as the topography of the earth’s surface, the character and quality of rivers and streams and groundwater flow, the forests that cover much of the earth’s surface and the habitats of natural wildlife that exist on the earth’s surface, including its rivers, lakes, and oceans and last but not least human lives and infrastructure. Although landslide avalanches are well known, the formative mechanism of slip surfaces is usually not well understood. The main failure mechanism during landslides is that shear takes place along either a discrete sliding surface, or within a zone, underneath the face. If the shear force (driving force) is greater than the shear strength of the interface (resisting force) then the slope will be unstable. Instability could take the form of displacement that may or may not be tolerable, resulting in the slope’s collapse either suddenly or progressively. Recently a series of papers emerged where the stability of landslides was tested by means of the distribution of micro- slip event in the slope’s interface. [9][10]. To this end, self- organized critical models were used in order to detect precursor activity in landslide failure and more specifically it was shown that the on-set of landslide failure can be mapped to the decreasing values of the exponent b in the power law relation of the micro-slip distribution. Based on these studies, the stability of a given slope can be robustly predicted if data acquisition of micro-slip events within the slope’s interface can be accurately monitored. In this paper we propose a specific framework in order to study landslide stability by means of a network of geophones where the analysis of the corresponding spatiotemporal data, in the light of the previously mentioned recent self-organized critical models, may serve as a valuable and robust tool towards the early prediction of landslides.