Bands can also be compared by ratio or by difference of their gray scales. Some of the common combinations were not appropriate in the Ghazalat study because they specifically include bands sensitive to vegetation, and this is a dry desert area. Although several of the combina tions discriminated between the sandstone of the plateau and the limestone of the highlands, the combination of a thermal and two SWIR bands had the best differentiation between two limestone types. This RGB image was sharpened by using a multiband difference that includes responses in two of the visible bands to show texture within the limestone and sand - stone. The resulting image highlights the claypans and details of the layers on the escarpments (previous page, top).
A different way to classify lithology uses separate criteria specific for each rock type. In the Ghazalat area, several band ratios were evaluated to distinguish two types of limestone, two types of sandstone, marl, loess and sand, and sabkha or clay (previous page, bottom). This map helped guide a field validation of the data. Traverses through the area by foot and off-road vehicle confirmed the interpretation obtained by remote sensing.
With lithology and topography determined, an estimate of risk for a seismic study can be quantified (right). Logistics risks are associated with access and movement. The steep escarpments and terrain edges limit vehicle access. The limestone highlands have rough topography and sharp edges, making maneu - vering difficult but not impossible. The clay and sabkha areas also limit access because there is a danger of falling through the top crust into soft sediments. In contrast, the sandstone areas, for the most part, have no access limitations. Other risks are associated with the quality of seismic signals. The escarpments, including those at formation boundaries, present topographic scattering risks. The rough surface of the limestones increases the risk of point-loading problems with the vibrator pads. The two limestone formations have different levels of this risk, with the western limestone having less. The soft clay and sabkha have an increased risk of signal attenuation and resonance.
Acoustic velocities in the lithological units can be modeled to estimate source and receiver static corrections. In Ghazalat, this yields a good comparison with the lower-resolution estimate obtained from picking the first break in refraction statics. The risks were verified in a
Logistics Risk Map
05 0
km mi No access Access limitation 5
Maneuver limitation Low risk
Surface-Velocity Risk Map
05 0
km mi 5
Severe risk of scatter from escarpments Moderate risk of scatter from rugged surfaces
Risk of attenuation Low risk
> Risk maps for the Ghazalat area. The logistics risks include access and maneuvering risks for vehicles (top). The sandstone areas are generally low risk (pale blue, which is a mix of white coding for low-risk areas and the geographic background information), but the highlands present maneuverability difficulties (red). Soft surfaces, such as the large sabkha in the depression, limit access (blue). Large escarpments (black) are impossible for the trucks to access. In the surface-velocity risk map (bottom), escarpments also pose severe risks for seismic-signal scattering (black). The rough surfaces of the highland limestones result in moderate scattering risk (red); these areas also have increased risk for point loading of a vibrator baseplate. Claypans and sabkhas have high signal-attenuation risk (blue). Sandstone areas generally have low surface-velocity risk.
Winter 2008/2009
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