Case study in NW Greece of passive seismic tomography: a new tool for hydrocarbon exploration
S. Kapotas, G.A. Tselentis and N. Martakis
Journal name: First Break
Issue: Vol 21, No 12, December 2003 pp. 37 - 42
Info: Article, PDF ( 162.24Kb )
We have learned more about the structure of the Earth and its crust from earthquake seismology in all its facets than from any other single geophysical or geological method. The use of the vast amount of information provided by the natural seismicity of the earth has been largely restricted to the investigation of classical seismological problems or to large scale investigations of the earth’s interior, with relatively little attention being paid towards small scale hydrocarbon exploration. Listening to the earth passively, and using the collected seismological information wisely, can be successfully applied to hydrocarbon exploration, as is shown in the present investigation. Controlled source seismology uses conventional surface sources such as vibroseis, explosives or airguns to generate seismic waves whose travel times and amplitude distribution through the earth are used to determine structural images and bulk physical properties of the subsurface. In contrast, passive seismic tomography uses micro-earthquakes as an energy source to probe earth structure. It is a fairly simple concept, based on the fundamental principle that all small movements and ‘roars’ in the earth are actually seismic sources. Both compressional and shear waves are emitted from an earthquake source and can be used for independent estimates of compressional (Vp) and shear (Vs) velocities of the various geological formations. The definition of velocity structure in a complex tectonic environment is a challenging task for conventional reflection velocity analysis based on NMO methods. In recent years, there has been an increase in exploration activity in geologically complex areas, such as fold and thrust belts, and even seeking good seismic images beneath high impedance layers such as basalt. Exploration in these areas is challenging, as well as expensive, and is driving the oil exploration industry towards the application of state of the art techniques. A key aspect of tackling the ‘complex geology’ problem has been the design and implementation of new types of seismic acquisition and processing strategies. The passive seismic tomography method falls into this category. Conventional land seismic is a labour-intensive business with expensive recording crews and set-ups of hundreds of miles of cable out on the ground, while geophones have to be deployed and retrieved manually. Surface access is needed for vibrators and shot-hole rigs, while permitting and other environmental issues mean high costs. The rationale for the application of passive tomography as a complementary imaging tool is multifold. It is a cost effective manner to image a large area where the terrain is difficult (mountainous or even shallow water) and, as a consequence, conventional seismic is expensive and may be of poor quality. Since the seismic energy from microearthquakes comes from below the target of interest it can be easily used to map complex tectonic regions (i.e. overthrust belts, subbasalts, shallow carbonates etc.) characterised by seismic energy penetration problems. Another advantage of passive seismic tomography is the capability to measure accurately an intrinsic Vp/Vs ratio. This is a direct consequence of the production of high amplitude shear waves by small earthquakes that are reliably recorded by (3-component) surface receivers. In contrast, 3D reflection seismic methods employ man-made sources (e.g. explosions) that do not produce large shear waves, so that detecting and identifying the weak shear waves reflected deep in the medium is not generally reliable, and often not even possible. Consequently, the active seismic reflection methods currently employed by the petroleum industry do not adequately provide material parameter information related to the shear velocities in the medium. Typically, a passive seismic 3D survey will cost less than a conventional 2D survey by several orders of magnitude. Drilling and explosives, for example, generally account for almost half of conventional 3D seismic cost. These costs are eliminated with passive seismic. Furthermore, the recording station density required in this methodology is significantly less, meaning significantly smaller equipment inventory and crew size. A passive seismic crew will be normally five to 10 people, compared with a conventional crew that will number upwards of 50. Another important aspect of the technique is that it has the advantage of being environmentally friendly. The absence of explosives and heavy vehicle support for conventional sources permits activity in terrains that might otherwise be inaccessible, and almost eliminates any environmental issues. In the following sections we present a successful application of passive seismic tomography for hydrocarbon exploration in the area of Epirus, Greece.