Incollection,

Line CROP 04: Southern Apennines

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Memorie Descrittive della Carta Geologica d'Italia, 62, APAT, Rome, Italy, (2003)

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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