SURVEYING METHODOLOGIES IN THE PHLEGREAN VOLCANIC DISTRICT (NAPLES - ITALY) 
Achilli V. 1 ' 2 , Borgstrom S. 2 , Capone M. 2 , Del Gaudio C. 2 , De Martino P. 2 , Klees R. 3 , Ricciardi G.P. 2 , Ricco C. 2 , Sepe V. 2 , 
Usai S. 3 , Vettore A 1 
1 Dipartimento Costruzioni e Trasporti - University of Padua (Italy) 
2 Osservatorio Vesuviano - Naples (Italy) 
3 Delft University of Technology - The Netherlands 
Commission VI, Working Group 3 
KEY WORDS: Spirit leveling, Tiltmetry, GPS, INSAR 
ABSTRACT 
The surveillance of the neapolitan volcanic area is carried out by the Osservatorio Vesuviano also by means of the study 
of ground deformations. In the work, the results obtained from the application and comparison of different surveying 
techniques in the Phlegrean Volcanic District (Phlegrean Fields and Ischia island) will be presented and discussed. In 
particular, the Phlegrean Fields area is marked also by strong vertical ground displacements. Two different spirit leveling 
networks are regularly measured in the Phlegrean Volcanic District by several years; from the beginning of the nineties, 4 
tiltmetric stations at the Phlegrean Fields were installed, together with a 30 vertices GPS network in 1997. Starting from 
December 1996, on the island of Ischia a 20 vertices GPS network was materialized and measured two different times. 
In the frame of an international collaboration, some interferograms for the Phlegrean Fields area were obtained: they 
were compared with the results of classical surveying techniques, giving a good agreement. 
1. GEOLOGICAL SETTING OF THE PHLEGREAN 
VOLCANIC DISTRICT 
The Phlegrean Volcanic District (Naples - Italy) includes 
the volcanic areas of the Phlegrean Fields and the 
islands of Ischia and Procida. 
The Phlegrean Fields caldera is located in the NE part 
of the district and the magmatic system is still active, as 
testified by Mt. Nuovo eruption in 1538, by the recent 
bradyseismic crises of 1969/72 and 1982/84 and by the 
diffuse fumarolic and hydrothermal activity. From the 
geological point of view, the caldera is mainly formed by 
volcanic rocks and subordinate^ by clastic sea 
sediments; from the structural point of view, the 
configuration of the phlegrean area is the result of 
deformation events related to the regional tectonics and 
to the volcano-tectonics. The regional tectonics is the 
cause of direct faults with NE-SW and NW-SE direction 
and subordinate^ with NS direction. 
The magmatic chamber is located at low depth (about 
4-5 Km). 
Considering that inside the phlegrean caldera and in the 
surrounding areas live about two million people, it is 
possible to deduce that the volcanic risk of the area is 
extremely high. 
The island of Ischia is located in the NW sector of the 
gulf of Naples and is mainly formed by volcanic rocks, 
by landslide deposits of different type and subordinate^ 
by sea sediments. 
Also the structural setting of the island of Ischia has 
been determined by deformations induced both by the 
regional tectonics and by the volcano-tectonics. 
The regional tectonics is the cause of two main fault 
systems with NW-SE and NE-SW direction. 
At the moment, the presence of a magmatic system still 
active and potentially able to give future eruptions is 
testified, besides by the last eruptive event occurred in 
1302 (the Arso eruption), also by an intense fumarolic 
activity, by seismic activity and by vertical ground 
displacements. 
2. SURVEYING METHODOLOGIES APPLIED IN THE 
PHLEGREAN FIELDS AREA 
2.1 High precision spirit leveling 
The altimétrie network of the Phlegrean Fields (fig. 1 ), 
considering also the lines in common with the Vesuvian 
and the regional networks, is formed by 300 
benchmarks distributed on about 120 km of leveling 
line, with a mean distance of about 400 m on eleven 
loops. 
Fig. 1 Geodetic surveillance networks of the Phlegrean 
Fields