Seafloor deformation monitoring is now performed in the marine sector of the Campi Flegrei volcanic area. MEDUSA infrastructure consists of 4 buoys at depths of 40-96m equipped with cGPS receivers, accelerometers and magnetic compasses to monitor buoy status and a seafloor module with a bottom pressure recorder. We study the seafloor deformation in the caldera. Previously we show that cGPS onland network and MEDUSA timeseries for the years 2017-2020 are in agreement with the deformation predicted by a Mogi model describing the observed deformation of an active volcano. Only for buoy A data differ significantly from model, at 6.9sigma and 23.7sigma for the horizontal speed (v) and direction. We devised a new method to reconstruct the sea bottom displacement including cGPS and compass data. The method, applied to buoy A and validated also on C, uses compass data to correct cGPS positions accounting for pole inclination. Including systematic errors, the internal consistency, within 3sigma (2sigma) for the speed (angle), between the results derived for different maximum inclinations of the buoy pole up to 3.5deg shows that the method allows to significantly reduce the impact of the pole inclination which can alter the estimation. We find good convergence of the velocity and deformation angle for increasing values of the buoy pole inclination. We found v=3.521+-0.039(stat)+-0.352(syst)cm/yr and an angle -115.159+-0.670(stat)+-7.630(syst)deg. The relative impact of potential systematics (statistical) effects increases (decreases) with cutoff. Our analysis gives v consistent with Mogi at 5.2sigma(stat) or 0.5sigma(stat and syst), and a deformation angle consistent at 4.3sigma(stat) or at 0.3sigma. The module of the vectorial difference between v from the data and Mogi diminishes by a factor 7.65+-1.23(stat) or +-5.78(stat+syst) compared with previous work. Potential improvements are discussed.
SUMMARY Ultraslow seafloor deformations are frequent in several marine environments like volcanic calderas, offshore oil and gas extraction fields subject to subsidence and river delta regions; they can exhibit unpredictable behaviours, particularly in caldera systems situated along coastlines. However, offshore monitoring of seafloor uplift and subsidence is still very challenging. Here, we present a new method to recover vertical seafloor deformation at Campi Flegrei caldera, Southern Italy, using bottom pressure recorder (BPR) sensors, tide gauges and a barometer. Using data from two BPRs installed on the seabed within the multiparametric elastic-beacon devices and underwater sensors acquisition (MEDUSA) marine infrastructure of the Istituto Nazionale Geofisica e Vulcanologia (INGV) -‘Osservatorio Vesuviano’, we transform pressure measurements into equivalent water level changes to derive the vertical seafloor displacements. We obtain high-accuracy vertical deformation records from the BPR measurements by taking into account the high-resolution mean seawater density variation over time, estimated by applying an innovative procedure to the BPRs data and using additional barometric and sea level data. We obtained for the two BPRs an uplift of 22.8 cm over about 2 yr and 7 cm over about 18 months, respectively. We compare the results with data acquired by GPSs installed on the top of MEDUSA buoys, deployed at the same sites as the BPRs, which recorded the vertical seafloor deformation values of 22 and 6.9 cm, respectively, over the same periods. These independent data sets show a strong correlation, with correlation coefficient values of 0.98 and 0.87, and very good agreement in both the trend and amplitude of vertical motion, proving the reliability of BPRs in accurately measuring vertical seafloor deformation. The methodology we developed allows a cost-effective implementation of high-accuracy seafloor vertical deformation monitoring networks.
Abstract Measuring seafloor motion in shallow coastal water is challenging due to strong and highly variable oceanographic effects. Such measurements are potentially useful for monitoring near‐shore coastal subsidence, subsidence due to petroleum withdrawal, strain accumulation/release processes in subduction zones and submerged volcanoes, and certain freshwater applications, such as volcano deformation in caldera‐hosted lakes. We have developed a seafloor geodesy system for this environment based on an anchored spar buoy topped by high‐precision GPS. Orientation of the buoy is measured using a digital compass that provides heading, pitch, and roll information. The combined orientation and GPS tracking data are used to recover the three‐dimensional position of the seafloor marker (anchor). A test system has been deployed in Tampa Bay, Florida, for over 1 year and has weathered several major storms without incident. Even in the presence of strong tidal currents which can deflect the top of the buoy several meters from vertical, daily repeatability in the corrected three‐component position estimates for the anchor is 1–2 cm or better.
Abstract We show the equivalence of earthquake-induced ground acceleration and water-pressure waveforms for the case of collocated hydrophones and seafloor seismometers installed in shallow water. In particular, the comparison of the waveforms and amplitude spectra of the acceleration and water-pressure signals confirms the existence of a frequency range of “forced oscillations” in which the water-pressure variations are proportional to the vertical component of the ground acceleration. We demonstrate the equivalence of the acceleration and water-pressure signals for a set of local earthquakes (epicenter distance of a few tens of kilometers) and regional earthquakes with a wide range of magnitude (2.7<Mw<6.8), recorded by seismometers and hydrophones operating in shallow water (depth less than 80 m) in the Campi Flegrei caldera (southern Italy). We describe the “forced oscillations” theory, and we demonstrate the signals equivalence in the frequency range 0.1–10 Hz, thus extending the frequency range of application of the hydrophones as accelerometers. The high correlation between the ground acceleration, derived from the ground velocity, and hydrophone pressure signals in the mentioned frequency range enables the use of the hydrophone waveforms for standard seismological studies (i.e., earthquake source). The calibration of hydrophones by comparison with collocated accelerometers, or seismometers, is also enabled in a range of frequencies that is very difficult to reproduce in a laboratory. The results of our work also open the possibility of hydrophones being more extensively used in place of accelerometers in marine environments where accurate installation of seismic sensors is difficult or unaffordable.
Abstract We present a new methodology using bottom pressure recorder (BPR) measurements in conjunction with sea level, water column, and barometric data to assess the long‐term vertical seafloor deformation to a few centimeters accuracy in shallow water environments. The method helps to remove the apparent vertical displacement on the order of tens of centimeters caused by the BPR instrumental drift and by seawater density variations. We have applied the method to the data acquired in 2011 by a BPR deployed at 96 m depth in the marine sector of the Campi Flegrei Caldera, during a seafloor uplift episode of a few centimeters amplitude, lasted for several months. The method detected a vertical uplift of the caldera of 2.5 ± 1.3 cm achieving an unprecedented level of precision in the measurement of the submarine vertical deformation in shallow water. The estimated vertical deformation at the BPR also compares favorably with data acquired by a land‐based GPS station located at the same distance from the maximum of the modeled deformation field. While BPR measurements are commonly performed in deep waters, where the oceanic noise is relatively low, and in areas with rapid, large‐amplitude vertical ground displacement, the proposed method extends the capability of estimating vertical uplifts from BPR time series to shallow waters and to slow deformation processes.
