Abstract
The CROP 04 line extends from the Tyrrhenian coast (Agropoli) to the
Adriatic one (Barletta) cutting across the entire thrust belt-foredeep-foreland
system. The total length approximates 160 kilometres. In the thrust
belt, the line crosses most of the tectonic units that form the roof
of the Apennine duplex system in Campania and Basilicata. Answers
to the following questions were considered major aims of the experiment:
- What is the depth of the sole thrust beneath the Apenninic chain
and where is the backstop of the orogenic system? - Is it possible
to recognise the lower crust of Apulia? If a lower crust is recognisable
in Apulia, how far does it extend westwards beneath the thrust belt?
- Was the crystalline basement of Apulia involved in the post- Tortonian
compressional deformation? The acquisition of the CROP 04 seismic
line lasted from December, 1989 to April, 1990. Vibroseis and dynamite
were both used as source types. The processing of the whole line
was completed in March, 1991. Due to the poor quality of the field
data, the processing was not successful in providing an interpretable
seismic profile. After a pause in which frustration and discouragement
prevailed, an additional effort was planned, aimed at determining
the causes of the very low signal/noise ratio of the field data.
In addition, a new processing was required, aimed at exploiting as
much as possible the signal content also making use of some a priori
geological knowledge of the area. The analysis of the field data
has revealed the occurrence of three negative factors: - Excessive
spread length (about 20 km) in situations where lateral variations
in the P- wave velocity exceed 1000 m/sec near the surface and lateral
variations in the ground elevation reach several hundred metres within
a single spread length; - Crooked acquisition of the seismic data
due to the rough topography that made difficult the accessibility
for the vibroseis trucks. The high tortuosity in many parts of the
line caused problems in CMP sorting and degraded the seismic image;
- Large occurrence of incoherent noise both in the dynamite and vibroseis
data. At large offset (> 4000 m) the signal was often completely
hidden by the noise. The reasons for this incoherent noise consist
of recording inadequacies and wrong choices of the seismic-profile
course in areas affected by severe tectonic deformation. As concerns
the new processing, a quality threshold defined by amplitude and
frequency indicators was preliminary established and the traces whose
quality fell below that threshold (30-40\% on average) were removed
from the dataset. After having removed the 'bad' traces, the following
processing steps were applied: - Computation of the static corrections
by means of inversion of refracted arrivals from a single refractor.
First break picking was carried out manually on both vibroiseis and
dynamite data; - Optimisation of the CMP sorting in order to limit
as much as possible the deleterious effects of the line tortuosity;
- Careful stack velocity analysis. Presently, the line has been entirely
reprocessed in the interval 0-10 seconds TWT. The reprocessed section
is comparable to the best commercial lines in the region. Nevertheless,
records suitable for geological interpretation usually do not exceed
5-6 seconds in commercial lines whilst in the new stack continuous
and well-structured events are recognisable up to 8-9 seconds. A
key point for any geological interpretation is represented by the
existence of a west-dipping package of faint, but well-organized
reflectors evident between 6 and 8 seconds in correspondence to the
Alburni Mountains. These reflectors fix an important constrain for
the sole thrust of the Apenninic tectonic wedge that in this region
reaches a depth of 20-25 kilometers. This depth coincides with the
depth of the so-called 'Tyrrhenian Moho' revealed by seismic refraction
experiments already in the seventies.
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