| A Bird's-Eye View of the East African Rift System
By Fons Dekker The elaborately decorated wooden drums, covered with animal skins, are banged faster and faster. The brightly painted dancers move more and more quickly, coming nearer all the time, closer to the place where you're hiding in the dense vegetation at the edge of the village. Have they seen you, or is it just a coincidence that they are moving toward you? Suddenly the leader, whose painted face looks like a grotesque lion, leaps at you with his heavy spear raised high above his feathery headdress. The yellow light of the full moon flashes off the polished steel tip of his weapon. Black, fiery eyes look straight at you. You see the yellow of his eyes and smell his spicy breath. The rhythm reaches a feverish pitch. He throws! The spear glides towards you in slow motion and, as the tip enters the soft skin of your upper arm, you scream! You grab the spear and pull it out. Your hand is covered with blood.
You wake up. Another mosquito lies squashed in your hand, surrounded by the blood it just sucked out of your upper arm. You're dripping sweat. Welcome to fieldwork in the African jungle!
For those who have done their share of geological fieldwork in the tropics, it's always wonderful to go there. It's even more wonderful to come back. The climate is exhausting for most of us, transforming hills into mountains when one climbs them on foot, making field days seem endless. Insects are a major concern. Not only can they be extremely irritating, they also transmit numerous diseases that will haunt you for years, or even kill you. Malaria is no picnic. Then there are lots of unfamiliar animals and, last but not least, homo sapiens. Dr. Richard Leaky has studied some of the oldest human remains in East Africa, and we'd prefer not to add our much more recent remains to his collection. Humans are much more unpredictable than animals, especially when they are engaged in tribal warfare.
And so we gladly embrace remote sensing as a technology that can reduce our field time by more than 90 percent, while showing us much more of the terrain than we ever could hope to see in the field. The images provide us with the continuity we can never see in the jungle. They fill the gaps in our knowledge and permit us to pre-examine our areas of interest. A field party can now be designed around specific points of interest - or controversy - as recognized from the imagery. By interpreting this imagery before we set foot in the field, we can create a very detailed geological map, albeit with numerous question marks. And these question marks become field targets. Rather than stumbling through the area, hoping to find locations with meaningful geological information, we can select locations that may well hold the key to our investigation.
In a new book published by the American Association of Petroleum Geologists (AAPG), Geoscience of Rift Systems - Evolution of East Africa, Dr. Christopher Morley has managed to open a treasure chest of information, one that was mostly hidden within the archives of oil and gas exploration companies operating in various parts of the East African rift system. Many outstanding articles in this book shed new light on aspects of the rift system by using the traditional approach through seismic and field geology. But the most interesting part is the tremendous impact that satellite imagery has had on some of the author's interpretations. In the past, most fault patterns were detected by very expensive seismic programs, taking years to acquire, process and interpret that data. We now see relatively inexpensive satellite imagery providing fantastic details of the surface expression of fault systems. Of course, it is not possible to project all of the surface patterns down to the "basement," but these same surface patterns provide excellent guidance for any well trained interpreter. Whether it is a field geologist looking at outcroppings, a geophysicist working with seismic, gravity and magnetic data, or a geologist mapping the area using elaborate software programs and computers, they all benefit from the "real world" pictures provided by satellite images.
In the oil and gas industry it is often said, "If you put four geologists or geophysicists together to come up with the proper structural model for a certain area, they will provide at least five solutions." This is because they work with isolated bits of information, especially in poorly explored areas like the rift system. Seismic lines in most areas are relatively far apart, leaving huge gaps between the lines. Wells in new areas are widely spaced, and outcroppings may be scarce. This leaves much room for interpretation, and the data soon resembles chicken wire: narrow zones of information surrounding very large data gaps. Satellite imagery provides continuity between data points and lines at the surface, limiting the number of possible interpretations and likely even suggesting alternative models not directly evident from other data sources.
By no means do the authors suggest that a complete interpretation can be obtained from satellite imagery alone. Although many subsurface features reach the surface in one way or another, numerous other structural components may remain hidden. These can be seen from analysis of gravity, magnetic and seismic data. On the other hand, we have seen that a thorough understanding of structural geology will allow many an interpreter to "map" structural features on satellite imagery that are not visible to the general interpreter's eye. When interpreting satellite imagery for geological purposes, there is no substitute for experience, especially field experience. At school, one cannot learn all the subtle differences in topographic expression that are caused by structural phenomena across a variety of climatic and bedrock conditions.
Chapter 11 (Le Turdu et al., 1999) deals specifically with the use of SPOT multi-spectral imagery to unravel the structural and stratigraphic secrets of parts of the rift system in the Baringo and Bogoria Basins, and in the Magadi Basin in Kenya. The area is covered by Pleistocene lavas and sedimentary deposits over a Precambrian basement, and vegetation is mostly limited. Both structural data and lithological information was derived from the imagery.
The data consists of three stereo pairs used to created DEMs and ortho-imagery to eliminate distortion. From these, the final color composites were created. The resulting imagery was then studied in 2D and 3D, using VUE3D software from ISTAR for perspective viewing.
The results are nothing short of amazing. As shown in the various figures, the ability to view the data from any compass orientation - and from any elevation - allows the interpreter to optimize the analysis to a degree impossible to attain in the real world. One cannot go out into the field and create these viewpoints short of sitting in a helicopter for weeks at a time, reviewing every square kilometer of terrain, and photographing every structural feature from any possible angle, furthermore hoping that the weather will stay clear long enough, and that the sun is in the right position. With 3D technology all of this can be done from the comfort of the lab, without any of the inherent risks or costs of extensive helicopter surveys, and with the certainty that all of the main structural features are disclosed in the imagery.
Of course, one cannot see slickensides from the SPOT data. These fault plane features - which are striations that indicate the direction of movement to the trained eye of the field geologist - and numerous other important field-specific structural phenomena, can only be identified and measured on location by an expert in the field. But, as one can see from Chapter 11, remote-sensing data goes a long way in creating a three-dimensional structural picture of the rift system. In turn this allows scientists to explain many of the observations made in the field and upon seismic examination, observations that were hard to explain in isolation but which become understandable once a satellite imagery model was created.
I have had the privilege of working as a remote-sensing consultant in several rift areas in Africa, and around the world. In many cases the client would come to me with lots of skepticism, wondering how I could possibly map the rift system from the imagery alone, especially in areas where either an impenetrable jungle or a thick sand desert covered the exploration prospect. Most of the time the imagery would reveal a wealth of information to the trained eye, information that was very helpful to the client and, quite often, unexpected.
Oil industry clients are understandably secretive, providing as little information as possible to the outsider. Case studies of successful exploration interpretations in rift systems are therefore few and far between. In this book, the authors provide a prime example of how it can be done, and the amount of information that is possible to obtain. This is information that is crucial to the exploration effort, from the focused layout of seismic lines to the detailed planning of a field party, saving hundreds of thousands - if not millions - of exploration dollars while getting a much better overall picture. References cited:
C.K. Morley ed., 1999, Geoscience of Rift Systems-Evolution of East Africa: AAPG Studies in Geology No. 44.
Le Turdu, C., J.-J. Tiercelin, J.P. Richert, J. Rolet, J.-P. Xavier, R.W. Renault, K.E. Lezzar, and C. Coussement, 1999, "Influence of Preexisting Oblique Discontinuities on the Geometry and Evolution of Extensional Fault Patterns: Evidence from the Kenya Rift Using SPOT Imagery", in C.K. Morley ed., Geoscience of Rift Systems-Evolution of East Africa: AAPG Studies in Geology No. 44, p. 173-191. About the Author:
Fons Dekker is president of Dekker Remote Sensing Corporation in Calgary, Alberta, Canada, a company that provides geological remote sensing support for the oil and gas industry. He can be reached at 403-289-5441, or via e-mail at [email protected]. Back |
