Scholarly record
RELATIONSHIP OF SHALLOW SEISMIC REFRACTION RESULTS VERSUS LITHOLOGY IN A RAILWAY PROJECT PLAN (NW PORTUGAL)
Abstract
The method of seismic refraction is widely used in many applied Geology fields and problems today. Although it has some limitations, in the case of detecting a lower velocity bounded layer, this method is well tailored to a crystalline environments where, more often, weathering degree is highest on the surface and gradually decreases in depth and thus can aid in associating the weathering degree to velocity. Some relationships have been made to establish a connection between velocities and the elastic properties of rocks. In recent years seismic refraction methods have evolved in terms of improved equipment, especially by means of better seismographs, but particularly due to better inversion techniques that consider the subsurface as a more heterogeneous environment. The later are commonly known as travel time tomography techniques. In crystalline environments this is useful due to the occasional heterogeneity of the near surface but also because of the gradual character of velocity change as opposed to sudden velocity breaks at boundaries that were associated with intercept time methods and even GRM. With this in mind we sought, over the years, to apply this method to projects throughout Portugal. In the northern part it is even more adequate due to the dominant granitic and schistose environments that we encounter. In the past few years High Speed railway networks have been planned to integrate with the European network, already existing in some countries namely Spain and France among others. The project requires detailed planning for excavation in hilly and mountainous terrain due to both engineering and environmental considerations. We had access to a seismic refraction dataset, acquired by a local geophysical company, comprising of around a 190 individual 60m profiles and we interpreted them with a travel time tomography technique. Each section easily permits the filtering of velocity domains and we considered the 800m/s as an empirical limit to separate geotechnical soil from soft rock. Afterwards, by georeferencing in GIS every test over the corresponding lithology, we were able to establish, through simple descriptive statistical parameters, defining characteristic relationships between each lithological group and the geophysical results. These relationships could surely be useful for the sustainable development of the project in this highly variable geologic environment.
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References7
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