the risk information in an easy-to-understand format. This is best seen by example in the case studies that follow.
Geomorphology in an Arid Terrain In the Egyptian Western Desert, almost 700 km [435 mi] west of Cairo, Apache Egypt planned a seismic study in the Ghazalat basin, which contains plateaus and steep escarpments. Prior to running the survey, WesternGeco included satellite remote sensing as part of a multiphysics, near-surface characterization to establish the risk for logistics and acquisition.8
The near surface comprises two formations, the Moghra and the overlying Marmarica (right). The lowest portion of the Marmarica formation has
alternating hard limestone and soft
gypsiferous marl layers, which transitions into a massive limestone in the upper part of the Marmarica formation. The underlying Moghra formation consists of an alternating sequence of sandstone and claystone layers. Both formations outcrop in the Ghazalat prospect area. A DEM image of the area was available from ASTER satellite data with lateral and vertical resolutions of 30 m, sharpened to about 17 m [56 ft] using the higher-resolution pan band. About 10% of the study area is in the Qattara Depression about 80 m [260 ft] below sea level, bordered by an escarpment of 100 to 120 m [330 to 390 ft] that reaches a plateau at a height of 50 to 60 m [165 to 195 ft]. The plateau makes up about 50% of the study area, with elevations greater than 200 m [660 ft] above sea level in the north (below right).
In addition to the large escarpment bounding the depression, other escarpments are present. These were determined using an eight-direction edge-detection algorithm.9
The escarpments can
be overlain on the terrain-height map to obtain a topographic classification map.
7 . The mud wave is also known as a Stoneley-Scholte wave, or j ust a Scholte wave.
8 . Laake A and Z aghloul A: “ Estimation of Static Corrections from Geologic and Remote-Sensing D ata,” The Leading Edge2 8 , no. 2 ( February 2 0 0 9 ) : 1 9 2 – 1 9 6 .
Cutts A and Laake A: “ An Analysis of the Near Surface Using Remote Sensing for the Prediction of Logistics and
D ata Q uality Risk,” paper presented at the 4 th North African/ Mediterranean Petroleum and Geosciences Con- ference and Exhibition, Tunis, Tunisia, March 2 – 4 , 2 0 0 9 .
For information on data quality characteriz ation in arid regions: Laake A, Strobbia C and Cutts A: “ Integrated Approach to 3 D Near Surface Characteriz ation in D esert Regions,” First Break2 6 ( November 2 0 0 8 ) : 1 0 9 – 1 1 2 .
9 . The method used is called a Sobel edge-detection algorithm. It is often used in north-south and east-west directions, but because of the complicated lobes of the mesas and other features, the eight-direction method used here provided smoother, continuous lines for the escarpments.
05k m 0
mi 5 – 8 0
Elevation, m 1 0 0
0 2 0 0
> Topographic map of Ghaz alat showing escarpments. A digital elevation model ( D EM) shows a part of the Q attara D epression ( blue, bottom right) bounded by a steep and tall escarpment. A broad plateau with mesas
( green) makes up about half the study area, bounded on the north by highlands ( yellow to brown) . An edge-detection algorithm determined the locations of escarpments ( black) .
Western Desert
0 0
k m mi 200 200 50 E G Y P T 100 250 L imestone M E D I T E R R A N E A N S E A
Ghaz alat study area
200 Cairo 150
gypsiferous marl
Soft
Alternating sandstone and claystone
0 > Ghaz alat geography and geology. Ghaz alat is in Egypt’s Western
D esert, bordering on the Q attara D epression ( map) . The area comprises mesas and tablelands to the south ( photograph) and heights to the north. The formations are layers of limestone, sandstone, and claystone and marl ( right) .
– 50
L imestone Sandstone
Claystone and marl
Winter 2008/2009
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t t a
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N ile River
Mogrha
Marmarica
Elevation, m Formation
D escription Lithology
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