Sergio Zerbini

Influence of soil consolidation and thermal expansion effects on height and gravity variations

Abstract The daily GPS height series of the Medicina station were analyzed for the period July 1996–September 2001. The station is located in the middle Po Plain on fine-grained alluvial deposits. A seasonal oscillation in the order of 18 mm (peak-to-peak amplitude) is present in the data. This crustal deformation has been modeled by including variations in the atmospheric, oceanic and hydrologic mass. The vertical positions can also be affected significantly by soil consolidation. Geotechnical parameters derived by in situ tests and laboratory analyses of the clayey soil collected at Medicina allowed the estimate of the soil settlement relevant to the seasonal oscillation of the surficial water table. Thermal expansion of the geodetic monument has to be taken into account in the case of high-precision vertical positioning. In this work models both for the soil consolidation and the thermal expansion effects are provided. The continuous gravity observations collected at Medicina by means of a superconducting gravimeter also exhibit a marked seasonal oscillation, which has been interpreted as the sum of loading and Newtonian attraction effects, as well as of the contribution due to soil consolidation. Especially the study concerning the soil consolidation effect has allowed a better insight on the seasonal vertical movements occurring at the Medicina station by providing quantitative information on soil behavior due to change of effective pressures. The results can be applied to those stations characterized by similar fine-grained soils and surficial hydrogeology.

Assessment of height variations by GPS at Mediterranean and Black Sea coast tide gauges from the SELF projects

Abstract In the framework of the European Union SEa Level Fluctuations (SELF) I and the SELF II Projects designed to study sea level variations around the Mediterranean and Black Seas, the Global Positioning System (GPS) technique was adopted to measure the ongoing crustal movements at tide gauge stations. Tide gauges measure sea-level variations with respect to a ground benchmark. In order to determine true sea-level variations of a few millimeters per year, it is necessary to estimate the ground vertical movement to a high degree of accuracy. Countries involved in the projects were Spain, France, Italy, Greece, Bulgaria and Russia. In the period from 1993 to 1998 repeated observations at more than 28 tide gauges and at a number of reference and intermediate stations were conducted. GPS measurements were complemented by Water Vapor Radiometers (WVR) at selected sites. Individual campaigns were evaluated and analyzed and showed height variations in the millimeter–centimeter range. All data were processed following common standards using the Bernese Software in a unified analysis to generate a combined solution. Based on the normal equations of all of the campaigns, a combined solution for the SELF Projects sites was generated. In the analysis of multi-technique anchor sites to the International Terrestrial Reference Frame ITRF97, it is shown that vertical rates of different techniques may be of different sign with respect to each other and also with respect to the ITRF97 combination. Vertical rates are not reliable in general and even the restriction to the use of long the long-time series sites KOSG, ONSA, WTZR, MADR, GRAZ and MATE did not ensure significant and unique vertical rates. Nevertheless, they had to be used for the reference frame definition. The results of repeated gravity observations and the continuous GPS observations at Porto Corsini and Medicina were compared to the epoch-wise GPS campaigns and revealed that the vertical rates are in conformance with each other if taken over the same period in time. The impact of the WVR observations on tide gauge position variation estimation is shown to improve the vertical component in the case of baseline-wise observations with two WVRs by up to 50%. It is shown, that the time span is too short for epoch-type observations to resolve significant height variations. The estimated rates of about 0–20 mm/year are most likely explained by systematic and random errors in the GPS observations. The absolute gravity observations at selected tide gauges are accurate to ±3 μGal and are in general agreement with the zero result from GPS. The main outcome of the SELF campaign observations therefore was the determination of a homogeneous zero-epoch data set and the creation of a database that includes the complete link from the GPS sites to the sea level in the five Mediterranean countries involved.

Multi-parameter continuous observations to detect ground deformation and to study environmental variability impacts

Abstract In the framework of the European Union SELF II project, a study was developed in order to assess the accuracy with which vertical crustal movements could be determined by means of continuous GPS and gravity observations in a relatively short timespan of a few years. The reliable knowledge of vertical rates at tide gauge stations is necessary to properly interpret sea level variations. For height determinations, continuous GPS and gravity measurements started in mid-1996 at Medicina, in the southern Po Plain. Additionally, continuous GPS observations have also been performed at Porto Corsini, on the Adriatic coast, where a tide gauge station is located which belongs to the Italian tide gauge network, and at a station in Bologna. Negative linear trends, of different magnitude, have been identified at the three stations. The time variability of gravity and GPS heights in relation to variations of several environmental parameters was investigated. A marked seasonal signal has been identified in both data series. It has been interpreted as the sum of different loading and Newtonian attraction effects modeled on the basis of the relevant environmental data series. At Medicina, the comparison between height and gravity series has shown that the seasonal variations are quite comparable both in amplitude and phase. The only remarkable difference between the two data sets is a sudden increase in gravity, in the order of 3 μgal, observed in mid-1997. This has been attributed to mass/density increase associated with uprising of deep-seated salty (connate) waters, likely triggered by local stress field changes. A simple model is proposed to check the feasibility of the suggested mechanism and the magnitude of the relevant gravity anomaly. This study demonstrated, among other aspects, the importance of collecting continuous, high accuracy, multi-parameter data series for an appropriate interpretation of signals related to environmental variability. For sea level fluctuation studies, the need for determining reliable long-term linear trends in station heights has been demonstrated for the Porto Corsini station. The sea level trend estimated by means of the tide gauge data and the GPS vertical crustal rate has been compared with the absolute sea level trend for the Northern Adriatic provided by the Topex–Poseidon satellite altimetry mission. Over the time frame of the satellite altimetry data set, the results provided by the two different measuring approaches agree within the errors of the estimated sea level trends.