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RELATIONSHIP OF SHALLOW SEISMIC REFRACTION RESULTS VERSUS LITHOLOGY IN A RAILWAY PROJECT PLAN (NW PORTUGAL)

AUTHOR/S: R. MOURA, H. C. RIBEIRO
Sunday 1 August 2010 by Libadmin2009

9th International Multidisciplinary Scientific GeoConference - SGEM2009, www.sgem.org, SGEM2009 Conference Proceedings/ ISBN 10: 954-91818-1-2, June 14-19, 2009, Vol. 1, 693-700 pp

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.

Keywords: Seismic refraction, GIS, rock mass, travel time tomography