Oil production from shallow (300 to 1000m), unconsolidated reservoirs in the Costa Oriental (Eastern Coast) of Lake Maracaibo (COLM) in western Venezuela has caused significant ground subsidence over an area of more than 2.000 Km2. Subsidence has been monitored since the late 1920's and has reached, as of April 2006, as much as 6.5 m with rates as high as 0.26 m in areas of intensive oil production.
The topography of the area (low laying, swampy ground barely above Lake level) has resulted in the need of progressively building earthen coastal protection dikes which, together with inner (diversion) dykes and an elaborated drainage system to protect more than 60.000 inhabitants and the industrial facilities needed to produce and/or transport more that 1500.000 barrels of oil.
The coastal dikes are located in a seismic area of low to moderate intensity. Seismic geology and seismicity studies were carried out from 1985 to 1988. These studies showed a moderate seismic risk, mainly due to the possibility of liquefaction of the dike foundation soils. Mitigative measures have been implemented in about 25 of the total 47 Km of thee three coastal dikes in the oilfields of Lagunillas, Tia Juana and Bachaquero
A leveling network was established in Lagunillas in 11929 and later extended to other oil fields. To date network covers an area of about 1.600 Km2 and comprises 1.889 bench marks in land as well as 329, in the Lake in near shore oil well platforms. The subsidence monitoring surveys are conducted at two-year intervals. There are more than 60 years of subsidence history, a significant treasure from a scientific point of view.
In 1988 GPS (Global Positioning System) techniques were incorporated in the leveling campaign with mixed results given the, then, poor satellite geometry as well as possible tropospheric effects. It was, therefore, decided to continue with both conventional leveling and GPS measuring until the ptoblems were resolved
Preliminary research undertaken in 1998 PDVSA, the Venezuelan Oil Company, the Canadian Centre for Geodetic Engineering (CCGE) at the University of New Brunswick, Canada and the Department of Land Surveying and Geo-Informatics at the Hong Kong Polytechnic University has shown promising results and, at the time of the writing of this paper consideration is being given the use of Interferometric Synthetic Aperture Radar (InSAR) and other novel remote sensing technologies for ground subsidence measurements.
The final paper will describe in detail the history and development of subsidence measuring in the COLM as well as future plans thereto.