Tunnel tiltmeters, Pozzuoli, Italy
Volcanic deformation near the Bay of Naples
The Earth's surface near Pozzuoli in the Bay of Naples lies above an active volcanic system that has erupted violently in the past and will do so in the future. See geological summary pdf article by Helen Brand. One million people live near a vast volcanic caldera and several smaller volcanic vents and cones.Pozzuoli is famous for its archaeological evidence for uplift and subsidence. A reduction of magma pressure beneath the Roman port c.500 AD, flooded the coastal market to a depth of tens of meters. Several Roman villas remain submerged despite inflation of the region c.1500 which raised the coastline 2-5 m. The depth of submergence is recorded by barnacles on the pillars of the Temple of Serapis (now known to be the market place) in the present port of Pozzuoli (see picture at bottom of page). The most recent erupton occurred at Monte Nuovo, Sept.-Oct. 1538 which covered much of Pozzuoli with a thick layer of ash. The energy flux from Solfatara, a region of active gas emissons NE of the Pozzuoli harbour, is estimated currently to be 1 million megajoules/day (equivalent to a kiloton of TNT each day!).
Numerous sensors now monitor the region of subsidence and inflation near Pozzouli. They sense its vertical motion (GPS), surface tilts and flexure(tilt-meters and leveling) and subsurface strain (borehole strainmeters) as the region responds to changes in subsurface magma pressure. (click on map for enlarged views)
In the four decades culminating in 1985 uplift amounted to 3.5 m, inflation since then ceased until Nov 2004 when uplift renewed. Inflation since then has amounted 0.5 m. In early 2009 the harbour of Pozzuoli was rising at a rate of almost 0.4 mm each week according to the GPS unit at RITE (operated by Istituto Nazionale di Geofisica e Vulcanologia Naples (see map above and data below).
A magma chamber is believed responsible for the current inflation. It is located 4 km below the Bay and can be modeled as a point source when allowance is made for distortion to the surface strain-field caused by caldera fractures.
This web page describes the long, water-pipe, tilt-meters that were installed 2008/9 by the Universita di Salerno and the University of Colorado.
The image below shows one of the columns of the market square with sea water corrosion assisted by barnacles indicating the 5 m depth of post-Roman submersion prior to uplift in the 15th century and recently.
Eastern column of the Temple of Serapis. THe stained areas indicate former levels of the sea.
Tilt-meter Operation
A water-pipe tilt-meter consists of two sensors, one at each end of a sealed rigid pipe half-full of water. Under isothermal conditions the water surface is an equipotential reference, and tilt of the ground causes one end to fall and the other to rise an equal amount. Tilt is derived from the difference in height change detected by the two sensors, divided by the length between them. Temperature variations underground, and pressure variations in the pipe, are negligible, hence the water surface, in principle, provides a drift-free horizontal datum: tilt stability each year is typically 0.1 microradians and tilt precision at weekly periods is about 1 nanoradian. Tilt resolution at one minute intervals is 0.1 nanoradian
The sensors can detect water level changes of 0.1 micron (roughly 1/500 of the thickness of a human hair). Because the Pozzuoli tunnels are vented, temperature variations exceeding 1 deg C occur, which adds noise to the measured tilt signal. By adding a third sensor to the center of the water pipe it is possible to obtain three measures of tilt, permitting signal integrity to be verified. To further discriminate real tilt from local noise temperature is measured at each sensor to millidegree precision.
Clusters of tilt-meters have been installed in two tunnels. In the northern tunnel tiltmeters operate in three azimuths. In the southern tunnel three pipes measure two azimuths. Each sensor detects a combination of body tide, load tide and volcano inflation. Data are sampled 16 times per second and averaged for 90 s. The data are recorded each minute with 16 bit A/D converters on a local data logger. An independent 12-bit data logger averages the data for 30 minutes and these data are transmitted using the Iridium satellite every 120 minutes (programmable remotely). An email alert is sent to local scientists when the tilt signal exceeds a prescribed level, or in the event of a local power failure.
Data
Location of tiltmeters
click on map for enlarged views
Water level sensors
Each of the 12 sensors in the tiltmeter array consists of four 5-cm-diameter, chromium-plated, copper balls supporting a central ferromagnetic core whose vertical position is monitored to a precision of 0.1 micron by an LVDT fastened to a stable pillar 20 m underground.
Microwatt heaters prevent moisture condensing on the floats and lid, and a 6 micron thick flexure strip guides the core within the body of the LVDT. The tiltmeter sensors are calibrated to 1% accuracy. Intercalibration accuracy can be verified by adding or removing 1 litre of water to the pipe.
Pozzuoli Tiltmeters North, PTNO
Two 12 cm diameter pipes are strapped to the stone wall of the tunnel at 1.5 m intervals. The photo above shows the north water-level sensor at the end of the 241-m-long north/south pipe.The figure below illustrates in map view the geometry of the three pipes and the sensor naming scheme in the northern Pozzuoli tunnel. A single pipe links the NW, SE & NE sensors, a geometry limited by the lengths of the side tunnels. The length of the north/south pipe is limited to 240.6 m by the slope of the tunnel and by the floor-to-ceiling height.
Pozzuoli Tiltmeters South PTSO
The figure below illustrates the geometry of the three pipes and the sensor naming scheme in the southern Pozzuoli tunnel. Separate pipes link the NW & CH sensors, the CL & SE sensors, and the NE and SW sensors.The steep slope of the tunnel down to the NW limits the longest horizontal pipe to 70 m. The CH (centre low) sensor is 1.5m below the CH (centre high) sensor.
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