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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS015 Oral Presentation 2329
Single-Station Passive Seismic Stratigraphy to 2km depth in sedimentary
basins
Prof. Francesco Mulargia
Fisica, settore Geofisica Universit di Bologna
Silvia Castellaro
Passive seismic 3D stratigraphy is commonly effected by reconstructing the noise wavefield that crosses
an array of seismometers. The range of wavelengths that can be analysed is tied to the size and layout
of the array, with bandwidth and resolving power generally proportional to the degree of complicacy of
the measuring network. At the other extreme, maximum simplicity is achieved by the Single-Station
Passive Seismic Stratigraphy (S-SPSS) approach, which has been so far essentially based on the
Horizontal to Vertical Spectral Ratio (HVSR). The latter, in spite of its hazy theoretical bases, has been
successful in 1-D subsoil mapping up to depths of some tens of meters. In light of tremor composition,
both a theoretical analysis as well as numerical simulations suggests that the Airy phases are a
fingerprint more reliable and effective than HVSR for stratigraphic applications. In a series of tests on a
variety of sites on large sedimentary basins of the Italian territory, where direct deep well data are
available from oil exploration, we positively identified the bedrock interface down to depths of 1700 m.
Tremor illumination was found to be sufficient under all weather conditions, although the reflections
from interfaces deeper than 500 m were more evident under stormy weather and became very clear in
concomitance with sea storms. Under good weather conditions in the whole Mediterranean basin we
failed to identify the bedrock interface when sited at 2000 m or deeper. A tremor acquisition for passive
stratigraphy requires typically 30 minutes, with only marginal improvement brought in by longer
recording times. Several strata at different depths can be usually resolved at each site. We find that
inverting tremor data for depth by using a fixed general parametrization leads to an accuracy on depth
estimates around 20%. This suggests the use of S-SPSS as a fast and inexpensive tool to build starting
models for accurate stratigraphic analysis. Alternatively, by tuning the parameters at a site where
stratigraphy is known by direct exploration, S-SPSS can accurately extend subsurface mapping to all the
regions where a similar structural pattern applies and where the depth of the interfaces may vary. In
this case, the velocity profiles can also be extracted rapidly and at low cost.
Keywords: passive seismic stratigraphy, sedimentary basin