Integrated Geophysical Methods for LNG Site Characterization in a Jungle Environment

Abstract

The contribution of integrated geophysical survey methods to site characterization in difficult and challenging environments is extremely valuable to design engineers and planners of new Liquid Natural Gas (LNG) facilities, especially if the site is located in a remote and geologically complex area. In particular, subtropical jungle environments present unique equipment and instrument preparation requirements, field operation problems, and have certain human risk factors that must be taken into account. Qualifying and characterizing jungle sites for installation of an LNG facility is costly, time consuming, and requires the application of a variety of geoscientific investigation methods, including geophysics. In conjunction with geotechnical and other site investigation methods, data from continuous resistivity profiling, seismic refraction tomography, P‐S wave downhole seismic, and Wenner sounding surveys are used as an aid the design and planning of an LNG expansion facility located on Bioko Island, Equatorial Guinea. The LNG expansion site, referred to as Phase III LNG, is located at the northwest end of Bioko, an Equatorial Guinea island territory. The investigation areas include steep coastal slope and inland plateau sites, which are the locations for a jetty pipeline structure and LNG tanks with supporting facilities. Although standard geotechnical methods are traditionally the dominant method used for characterization of LNG site foundation soils and rock formations, the drilling of boreholes to obtain core samples is very costly, time consuming, and often dangerous in subtropical jungle environments. On the island of Bioko, steep coastal slopes are inaccessible to large drill rigs, and only limited dozing is permissible to gain access to LNG tank and other planned plant facility locations. In addition to these logistical constraints, the island of Bioko is home to several of the world's most venomous snakes and insects. Malaria is a prevalent problem in this region of the world, which becomes a more serious concern in areas where the jungle is disturbed by cutting and dozing operations. Analysis of the resistivity and seismic refraction tomography give foundation and facility construction engineers valuable insight regarding site soil profiles, and confirmation of the depth to competent base rock at, and between, LNG tank locations. Along the steep coastal slope lines where little or no geotechnical information is available, the geophysical data provide valuable information for slope stability model analysis. When combined with data from geotechnical and other geo‐engineering studies, the application of geophysics is shown to be indispensable, particularly in challenging and difficult jungle environments where access to investigation sites is limited and conditions are hazardous.