Characterization of shallow volcanoclastic deposits by turning ray seismic tomography: an application to the Naples urban area Giovanni Bais a , Pier Paolo G. Bruno b, * , Vincenzo Di Fiore c , Antonio Rapolla c a Dipartimento di Geofisica e Vulcanologia-Largo San Marcellino 10, 80138 Naples, Italy b Osservatorio Vesuviano, INGV, Via Diocleziano 328, 80124, Naples, Italy c Dipartimento di Scienze della Terra-Largo San Marcellino 10, 80138 Naples, Italy Received 15 February 2002; accepted 7 October 2002 Abstract Several bidimensional seismic tomography surveys were carried out in urban areas of the Campi Flegrei and Somma- Vesuvius volcanic districts, Naples, Italy, with the aim of investigating the shallow subsurface and of detecting small-scale variation and heterogeneities within pyroclastic deposits. The method employed in this study is turning ray tomography (TRT). With accurate field acquisition and travel time picking and correct choice of inversion parameters, it was possible to obtain detailed P-wave tomographic models in areas characterized by thick beds of pyroclastic deposits. We were able to identify zones with heterogeneous P-wave velocities caused by the presence of buried paleochannels, or by differential welding and zeolitization in the subsurface of the investigated areas. TRT appears to be a suitable method to investigate deposits for engineering applications, at sites where the bedrock is too deep to be reached by foundations and the overburden is characterized by much heterogeneity. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Seismic tomography; Pyroclastic rocks; Somma-Vesuvius; Campi Flegrei; Shallow geophysical survey 1. Introduction In volcanic areas, stratigraphy is frequently com- plex because morphology, facies and elastic properties vary due to deposition and transport processes and paleotopography (e.g. Cas and Wright, 1987). Unlike for the well-known regularly widespread ‘‘pyroclastic fall’’ deposits, the above statement is particularly true for ‘‘pyroclastic density current’’ deposits. We use this term following Allen and Cas (1998), who described a ‘‘gas-supported, density-graded, particulate transport system, irrespective of particle concentration or flow state’’. Pyroclastic surge and pyroclastic flow deposits are the products of pyroclastic density currents. Pyro- clastic surges are diluted and turbulent high-velocity currents that frequently form thick and massive depos- its. Pyroclastic flow is a high concentrated gas/solid dispersion with high temperature and high density; its flow regime may be laminar or turbulent and may vary in different areas of a pyroclastic density current (Allen and Cas, 1998). 0926-9851/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0926-9851(02)00230-6 * Corresponding author. Tel. +39-81-6108437; fax: +39-81- 6100811. E-mail address: bruno@ov.ingv.it (P.P.G. Bruno). www.elsevier.com/locate/jappgeo Journal of Applied Geophysics 52 (2003) 11 – 21