This work reflects the evolution of the various geodetic measurement techniques that have been applied in monitoring the subsidence phenomenon that occurs on the Eastern Coast of Lake Maracaibo (COLM). From the discovery of subsidence in this area (1927) to the present, we can summarize this geodetic evolution in 4 stages, starting with the Leveling technique, followed by the incorporation of GPS and then GNSS, and finally, in the most recent years the incorporation of DInSAR. All these techniques in themselves have been evolving and improving each time. Highlighting its optimized combination (last stage) has allowed better monitoring and understanding of this phenomenon.
Phase Leveling surveys
The first stage began in 1927, with the first topographic survey in the Lagunillas region, due to continuous flooding in the area (Trutmann, 1949). In 1929, measurements were repeated, and subsidence was detected for the first time when comparing the values between the two campaigns. It was decided to install a geodesic vertical control network (Leal, 1989).
By 1932 the first vertical control points (BM) were installed in the Cabimas region, and in 1933 subsidence values were detected in this area, which made it necessary to extend the network therefore, in 1934, there were 269 BMs in the entire land area and 475 in the entire lake.
Until 1936 the campaigns were carried out annually; from there on, biennial monitoring of the phenomenon was implemented, extending the network to the places where new production wells were drilled. For this reason, in 1937 and 1938, the leveling network was installed in Tía Juana and Bachaquero, and the first measurements were made.
To continue monitoring subsidence and the geophysical and geological processes that this implies, in 1977, the first regional network of gravimetric measurements was installed with an agreement between MARAVEN and the School of Geodetic Engineering of the University of Zulia (EIG-LUZ) (Henneberg et al, 1980).
Precision leveling measurements have continued over the years, with some campaign interruptions, with 2,282 vertical control points distributed throughout the area.
Phase GPS/GNSS surveys
In 1988, with the rise of the Global Positioning System (GPS), the first GPS network was installed, and test measurements were carried out in the Tía Juana section, simultaneously with precision leveling, obtaining differences in millimeter heights between the two techniques (Murria, 1991; Chrzanowski et al. 1988).
Given these high-quality results, in 1990, GPS was included as part of the techniques for monitoring this phenomenon (Walford, 1995).
The GPS campaigns were carried out parallel to the conventional measurements in 1992, 1994, 1996, 1998, 2002, 2005, and 2007. Until 2005, the GPS monitoring network consisted of 29 vertices (Hoyer et al., 2005 ) and for the 2007 measurement campaign, it had 94 stations regularly distributed in the area (Suárez and Higuera, 2007).
The GPS campaigns from 1992 to 1998 were measured by PDVSA and processed by the Laboratory of Physical and Satellite Geodesics of LUZ (LGFS-LUZ), while those of 2002 and 2005 were measured and processed in their entirety by the LGFS, including the gravimetric measurements.
Finally, starting in 2007, the GNSS and gravimetry campaign was executed and processed jointly by PDVSA personnel and the Venezuelan Institute of Technology for Petroleum (INTEVEP) (Suárez and Higuera, 2007).
Phase DInSAR surveys
In 2012, the Differential Interferometry by Synthetic Aperture Radar (DInSAR) technique was incorporated to complement and strengthen subsidence studies. In total, four projects have been carried out using this technique; the details of each campaign are presented in Table 1.
From all these DInSAR studies carried out, it was found that this technique offers high accuracy results under ideal conditions, and may not be as effective according to the conditions of the vegetation and the activities on said surface, for example where there is agricultural activity. Another aspect to highlight from these studies is that in some areas vertical deformation foci (uplift) were detected in the opposite direction to the phenomenon of subsidence.
Finally, one of the greatest contributions of this technique is that it allowed generating highly accurate results both on land and in the oil fields located in Lake Maracaibo. Here the high number and density of wells and fixed platforms installed on the lake served as ideal reflectors for the radar signals. By evaluating the potentialities and the high qualities obtained, the possibility of expanding its use to other areas of research such as the study of deposits was opened, the work presented by González, 2016, is an example of this.
By 2015, significant maturity and technical expertise had been reached in both GNSS and DInSAR applied to the study of this phenomenon, which allowed the development of a proposal that included both in its monitoring; this phase includes:
- Install a network of GNSS stations for continuous monitoring and a network of permanent SAR reflectors (CRInSAR, for its acronym in English).
- Optimize SAR image capture frequency and classic leveling campaigns.
- Add LiDAR data collection for the generation of digital elevation models.
- Combine all available geodetic information to create more robust prediction models.
- Create a processing group dedicated to maintaining said geodetic infrastructure and generating the corresponding results and analyzes.
Unfortunately, for budgetary reasons, this project could not be implemented.
As a summary of what was exposed and analyzed during this historical trip, Table 2 is presented, where the monitoring of subsidence in the COLM is unified through the different geodetic techniques applied, each adapted to the corresponding technological generation.
It is of special interest to highlight that despite the fact that they are tools that are different from each other, as regards their physical foundation, instrumentation and processing, they have all generated high quality results to give continuity to the monitoring of this phenomenon. In addition, these projects are a concrete example where the different branches of classical geodesy, satellite and remote sensors coexist and interact perfectly.
As a final comment, it is important to highlight that each of the geodetic techniques have had a great contribution to achieve a better understanding of this phenomenon, complementing each other and showing that making optimal use of their combination can achieve much more robust results.
The authors thank all the people and institutions that collaborated in the compilation of this historical information as well as the graphic support presented here, especially the School of Geodesic Engineering of the University of Zulia through its Laboratory of Physical and Satellite Geodesy, Petróleos de Venezuela SA, Airbus and the European Space Agency
– Arenas I. Hernández B. Royero G. Cioce V. Wildermann E. (2019). Subsidence detection due to oil extraction effect applying the DInSAR technique in Venezuela. Mapping Vol. 28, 195, 18-26. May-June 2019. ISSN: 1131-9100
– Chrzanowski A., Chen Y., Leeman R., Leal J. (1988). Integration of the Global Positioning System with geodetic leveling surveys in ground subsidence studies. Proceedings of the 5th International (FIG) Symposium on Deformation Measurements. ONE B. Fredericton, Canada. Pp. 142-151.
– González D. (2016). Comparison of the geomechanical model of the Lagunillas inferior 07 reservoir with the petrophysical model to explain the subsidence phenomenon. Degree work. Graduate Studies Division. Faculty of Engineering. University of Zulia
– Henneberg H., Badell C., Drewes H. (1980). Recent research on the subsidence of the eastern shore of Lake Maracaibo. Technical Magazine of the Faculty of Engineering. Vol 3, n ° 1.
– Hoyer M., Wildermann E., Leal J., Gallardo J., Rothe C., Suarez H. (2005). Geodetic measurements for controlling subsidence effects at Lake Maracaibo zone. 19th Colloquium on Latin American of Geociences, April 18-20, 2005. Potsdam, Germany
– Leal J. (1989). Integration of Satellite Global Positioning System and Leveling for the Subsidence Monitoring Studies at the Costa Bolivar Oil Fields in Venezuela. M.Sc.E. thesis. Department of Surveying Engineering Technical Report No. 144. University of New Brunswick. Fredericton, Canada.
– Murria J. (1991). Subsidence Due to Oil Production in Western Venezuela: Engineering Problems and Solutions. Proceedings of the Fourth International Symposium on Land Subsidence. IAHS Publ. not. 200.
– Suárez H., Higuera M. (2007). GNSS measurements for subsidence control in the COLM (2007). Geophysics and Geodesy Management. Petroleos de Venezuela S.A. Zulia, Venezuela
– Suárez H., González D. and Rothe C. (2013). DInSAR study (ALOS-PALSAR) for the determination of subsidence in the COLM (2007-2011). Remote Sensing Department. Petroleos de Venezuela S.A. Zulia, Venezuela
– Suárez H., González D., Rothe C. and Ramos, F. (2014). DInSAR study (TerraSAR-X) for the determination of subsidence in the COLM (2011-2013). Remote Sensing Department. Petroleos de Venezuela S.A. Zulia, Venezuela
– Suárez H., González D., Rothe C., Ramos, F. and López A. (2015). DInSAR study (TerraSAR-X) for the determination of subsidence in the COLM (2011-2014). Remote Sensing Department. Petroleos de Venezuela S.A. Zulia, Venezuela
– Trutmann O. (1949). Report on the activities of topographical Dept. Department of Toporafía. Shell Company. Caracas Venezuela
– Walford J. (1995). GPS Subsidence Study of the Costa Bolivar Oil Fields, Venezuela. M.Sc.E. thesis. Department of Geodesy and Geomatics Engineering Technical. Report No. 174, University of New Brunswick. Fredericton, Canada.