S. R. C. de Freitas

Tidal gravity anomalies as a tool to measure rheological properties of the continental lithosphere: application to the South American Plate

Characterization of the bulk physical properties of individual tectonic units is important for understanding the dynamics of continental evolution. One important parameter is effective elastic thickness (Te), a measurement of the flexural strength of the lithosphere. Te is traditionally estimated by examination of the transfer function between gravity and topography. Sparse gravity data coverage limits the application of this method in South America. Instead, we use an empirical correlation (as defined by data from Australia and by the world databank) between tidal gravity anomalies and Te to estimate Te for tectonic units of South America. Our results are consistent with independent determinations of Te in several sub-regions of South America. Although the empirical correlation appears to be quite strong, further research needs to be done to develop a physical theory for the connection between gravity tide anomalies (which sample an essentially instantaneous rheological response of the Earth) and Te (which measures the rheological response of the Earth at geological time scales).

Review: Using an artificial neural network to improve the transformation of coordinates between classical geodetic reference frames

Advances in technology have allowed for the improvement of geodetic reference systems (GRSs). Relating different GRSs can be done by employing transformation parameters which may not, however, be satisfactory in certain applications due to the heterogeneous and local character of deformations caused by the procedures adopted in classical networks. Classical networks were established basically by procedures of triangulation and traverse survey, and the existence of these deformations justifies the search for new transformation methodologies. This study evaluated artificial neural networks (ANNs) as a tool for the transformation between GRSs. Frames points with known geodetic coordinates (latitude and longitude) in the South American Datum of 1969 (SAD69) system and in the older Corrego Alegre system, both still in use in Brazil, were chosen for this study. The SAD69 coordinates of the frame points were transformed into Corrego Alegre coordinates and then the computed coordinates were compared with known ones. Four tests were carried out in order to transform the coordinates. The first test involved the use of official transformation parameters and the formulas of Molodensky. In the second test, new transformation parameters were employed. In the third test new regional transformation parameters were determined, while the fourth test employed an ANN to predict the Corrego Alegre coordinates of the check points. Results indicated that the employment of an ANN transformed the coordinates most accurately, and indicated that they can be useful in modeling deformations in classical networks.

Local Effects in the Brazilian Vertical Datum

Nowadays, determining the topography of Mean Sea Level (Sea Surface Topography — SST) at the tide gauge station and its temporal variation is one of the most important aspects of defining a vertical Datum. Different approaches are used for this purpose: Oceanographic approach; Satellite altimetry associated with geostrophic levelling; Gravity associated with satellite positioning; Geodetic boundary-value problem approach (GBVP). In this sense, time series for observing tide gauge geocentric position permits to fix its geometric position and to detect local dynamic effects on the crust and in the SST. The main purpose of Work Group III in SIRGAS project (Geocentric System for the Americas) is to establish the link of all vertical reference systems in South America. In general, each national vertical network in that region was established under three particular conditions: local vertical datum; height system without physical meaning; and different spirit levelling standards. The unification of South American vertical datums, is a fundamental step to understand the observed off set among the networks in the region. The achieved results of two multiparametric on the brazilian vertical datum and their analysis are presented in this paper.

Associated Problems to Link South American Vertical Networks and Possible Approaches to Face Them

The national South American vertical networks were established under particular national definitions. The main problem to link them is the sparse gravity information. This lack leads generally to the existence of only levelled height data at the borders. These conditions cause an uncertainty to explain the observed offsets among networks. Due the uncertainties from the spirit leveling and global geopotential models, link solutions must be improved using other techniques. Some approaches based on determination of regional gravity disturbances patterns and gravity profiles with INS/GPS to determine geopotential numbers or local geoid tendencies are proposed.

Artificial Neural Network: A Powerful Tool for Predicting Gravity Anomaly from Sparse Data

Generally, the gravity surveys are developed along roads or other communication ways. This leads to an irregular space distribution and lacking data in large areas, like that containing high mountains, wetlands, lakes and forests. The usual methods for geoid computation from gravity data need a regular grid of gravity anomalies. Numerous methods have been developed for gravity anomalies interpolation at regular distribution. This paper reports on implementation of an interpolation method by using techniques for learning and training of Artificial Neural Networks (ANN) in predicting both free-air and Bouguer gravity anomalies from irregular and sparse data. The method was applied for a region in the Ecuador (5°S - 1°N and 75°W – 81°W) that has strong variations in crustal density and morphology. The free-air gravity anomalies prediction results were compared with the method of Kriging interpolation. The ANN method presented better results in predicting gravity anomalies in the considered region.

Predicting Free-Air Gravity Anomaly Using Artificial Neural Network

Generally, the values of the gravity are obtained on roads, and near rivers and valleys. This kind of distribution may produce irregular space data and lacking of data in large areas. Geoid heights are usually estimated in regular grids using spectral techniques. Numerous methods have been used for free air gravity anomalies interpolation. This paper reports an Artificial Neural Network (ANN) implementation for predicting free air gravity anomaly. The results were compared with both Kriging and Minimum Curvature. The ANN shows better results in the prediction.

Brazilian Vertical Datum Monitoring - Vertical Land Movements and Sea Level Variations

UFPR’s Geodetic Instrumentation Laboratory carried out several geodetic campaigns at Brazilian Vertical Datum during the last nine years. The last campaign performed in the first semester of 2005 had as main goals: calibration of two sea level sensors of the tide gauge station; periodic GPS observations at one SIRGAS station, used to materialize the tide gauge geocentric position; and leveling of the benchmarks in the harbor area. In this work there are presented results obtained with this campaign and some comparisons to past campaigns. The main objective is to check the stability of the GPS station in order to estimate vertical land movements and sea level variations. GPS results indicate a tendency of subsidence of 0.2mm/a with a standard deviation of 0.1mm/a at the station.

Tests on Integrating Gravity and Leveling to Realize SIRGAS Vertical Reference System in Brazil

The integration of gravity data within the Brazilian leveling network is very difficult due to the historical dissociation between leveling and gravity surveys. This research was started, in the context of SIRGAS Project, to resolve this problem by evaluating different strategies and procedures with the aim of establishing some kind of gravity coverage over vertical reference stations. Data employed in this article comes from one of the few areas where the points of the Brazilian fundamental leveling network are entirely covered with gravity surveys and, in addition, connect three permanent GPS stations, two of which belong to the SIRGAS 2000 Reference Network. This will allow for several analyses regarding the Brazilian realization of SIRGAS Vertical System, including: an evaluation of the effects of adopting different types of heights; and the investigation of strategies and procedures to solve the absence of gravity values over benchmarks. Values of geopotential differences were computed and adjusted, for a 2300 km network consisting of six loops with perimeters ranging from 136 km to 690 km. Dynamic, Helmert, Normal and Normal-Orthometric reductions were generated and compared. The status of the development of a GIS designed for the tasks involved in geopotential numbers computation to be applied in the studies related to the SIRGAS Vertical Reference System is also presented.

Deformations Control for the Chilean Part of the SIRGAS 2000 Frame

With the adoption of the Geocentric Reference System for the Americas (SIRGAS), the determination of temporal variations and deformations in its frame are a necessary task. These tasks are fundamental in order to maintain the consistency of the frame with respect to the definition of SIRGAS, especially in the Chilean deformation area, where the variations are larger compared with the rest of the plate due to the convergence between the South American and Nazca plates. 21 permanent GPS stations was used. 14 of them are regional stations of the International GNSS Service (IGS), and 7 permanent GPS stations are from different regional networks. 10 of these 21 stations are located in the Chilean Andes were used to study the kinematic effects in this area. Some of them are also SIRGAS stations. Five years of GPS observations from 2000 to 2004 were selected in order to have a sufficient time period with enough coverage for the considered stations. Daily solutions were generated with the Bernese GPS Software 5.0 using IGS precise ephemeris, clocks and ERP. The normal equations were then accumulated and solved in a final combination where the coordinates and velocities of selected stations are weighted for the datum definition compatible with ITRF2000. The estimated velocities of the north Chilean Andes regions are ∼26–30 ± 2mm/yr, decreasing up to ∼16–25 ± 2mm/yr and for the south Chilean Andes the estimated velocities are ∼9–12 ± 2mm/yr. For the stable part of the continent the estimated velocities are ∼8–12 ± 2mm/yr. The final results are discussed and compared with the NUVEL-1A and SIRGAS velocity models (SIRGAS-VM).

New analysis of a 50 years tide gauge record at Cananéia (SP-Brazil) with the VAV tidal analysis program

A homogeneous high quality tide gauge hourly record covering the period 1954–2004 was obtained at Cananeia (SP-Brazil). A previous analysis of a 36 years data set has shown many interesting features, especially very long period signals. This re-analysis benefits from a 40% longer time series and the powerful program VAV for tidal data processing is used to determine the parameters of the tidal constituents derived from the tidal potential, including the long period tidal waves, and of the shallow water and radiation tides. Long period terms are determined from the tidal residues by a semi-automatic research algorithm. The variation of the mean sea level is estimated after subtraction of the ocean tides and estimation of the long period terms. The mean sea level rate of change is estimated to 0.5666 ± 0.0070 cm/year. Special attention is given to the determination of the ocean pole tide as well as the 11 years term directly related to the solar activity.