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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004 
4. ANALYSIS OF GEOLOGICAL STRUCTURES 
IN IMAGERY 
In order to understand earthquakes, one must know about faults. 
Knowing how major mountains sequences and the continents 
have occurred is closely associated with the comprehension of 
faulting and folding. Understanding plate-tectonic theory 
requires a knowledge of structural geology. In areas of active 
tectonics, the location of geological structures is important in the 
selection of suitable sites for houses, schools, hospitals, dams, 
bridges, factories, and nuclear power stations. Additionally, 
understanding structural geology is useful for solving the 
problem of finding natural resources. For instance, petroleum 
sites can be predicted based on the presence of certain geological 
structures. 
Structural geological interpretation of faulted landforms is of 
importance to land users. Active faults pose natural hazards, and 
dormant faults profoundly affect excavation, tunneling, and the 
geometry of mineral deposits and petroleum accumulations. 
Faults are fractures that displace the rocks on either side of the 
fault. 
4.1 Strike and dip 
The strike 1s the azimuth of the horizontal line formed by the 
intersection of an inclined plane, such as a bedding plane, with a 
horizontal plane. The direction of dip is the azimuth in which the 
angle of dip is measured, usually perpendicular to the strike. The 
angle of dip is a vertical angle measured downward from the 
horizontal plane to an inclined plane. 
Strike and dip information can be interpreted in carbonate and 
clastic terrains and in some volcanic terrains where flow surfaces 
are well expressed. The image signature of dipping layers 
depends upon the relationships among the dip direction, look 
direction and look angle. Dip slopes have bright signatures and 
anti-dip scarps in shadows have dark signatures. 
4.2 Faults 
A fault is a fracture in bedrock along which movement has taken 
place. Movements along faults cause earthquakes. For instance, 
the continuous movement of the Earth's crustal plates can 
squeeze, stretch or break rock strata, deforming them and 
producing faults and folds. Faults tend to occur in hard, rigid 
rocks, which are more likely to break rather than bend. Faults 
can be classified into the three main categories. They are dip-slip 
faults, strikc-slip faults, and oblique-slip faults. McGeary (1996 
and Strahler and Strahler (1994) provide a detailed discussion of 
these issues, and the reader is referred there for more 
information. 
Thrust or reverse faults form in compressive environments 
where the maximum principal compressive stress is horizontal 
and minimum compressive stress is vertical. These faults are 
difficult to interpret from Landsat TM and even from radar 
lemote sensing images. This is because the planes of most thrust 
faults are parallel or nearly parallel to the bedding planes of 
associated strata. Most thrust faults do not cause the discordant 
geometric relationships that are associated with many normal 
and strike-slip faults. In addition, thrust faults are recognized in 
the field by anomalous rock relationships, such as older beds 
Over younger beds, repetition of belts, and omission of beds. 
These relationships are difficult to recognize on images without 
the aid of field data. 
A strike-slip fault is a fault in which movement is parallel to the 
strike of the fault surface. Strike-slip faults are also compressive 
faults (both maximum and minimum compressive stresses are 
horizontal), but instead of rocks overriding each other, the fault 
displaces rocks horizontally. Fault planes are vertical or dip very 
steeply, and the traces of strike-slip faults tend to be straight and 
extend for long distances. Rocks on both sides of the fault tend 
to be strongly deformed in the vicinity of the fault, and structures 
on either side of the fault appear to be dragged into or along the 
fault. Movement along these faults produces many of the most 
devastating earthquakes, as in Turkey in 1999. 
4.3 Folds 
In folded structures, a fold is a bend in a rock layer caused by 
compression. Folds occur in elastic rocks, which tend to bend 
rather than break. Two main types of folds are anticlines (up 
folds) and synclines (down folds). Folds can change in size from 
a few millimeters long to folded mountain ranges hundreds of 
kilometers long. Folds may be recognized by attitudes of beds, 
outcrop patterns, and topographic expression. McGeary (1996) 
and Strahler and Strahler (1994) provide an extensive discussion 
of this topic. Folds cannot be seen in our study image. 
5. RESULTS AND DISCUSSION 
Remote sensing analysis has often taken a somewhat haphazard 
approach. The experienced analyst tries a range of tools and 
looks to see what works best, i.e., which result looks ‘right 
‘Right’ is often based on long experience and considerable 
expertise, but tends to be based on the individual analyst's 
skills. This is a consequence of attempting to deduce the nature 
of a complex mix of variables in reality from a small set of 
reflectance values in the image. 
Because of the need for the analyst's expertise in rapid 
recognition of complex image patterns, no fully automated 
system is likely to be effective or correct. One objective of this 
study has been to test some of the limits of the analytical 
process using a rules-based approach. If we can present a 
number of analytical scenarios to the analyst for consideration, 
then proceed to further analysis based on choices made by the 
analyst, we can make the process more effective and efficient. 
This method of automation keeps the people in the analysis 
loop, but tries to shift much of the low-level work to the 
computer, while leaving the high-level recognition work to the 
human expert. This system is one of shared cognitive 
responsibility (Turk, 1990, 1992). 
Implementing the rules base in Idrisi was fairly straightforward, 
as the software allows integration of imagery and cell-based 
GIS data, such as DEMs. Idrisi has a macro capability, which 
allows the development of routines that will run a complex 
analysis based on rules and other instruction. It would be 
possible to implement this methodology in a number of other 
packages such as the GRASS GIS. 
The results of this analysis show that the Sierra Nevada area in 
the study image has an homogeneous, fine texture-scaled 
dendritic stream drainage pattern. This means the area most 
likely has clastic sedimentary rocks. The image of the area also 
does not indicate a significant fault line according to the stream 
drainage patterns, such as trellis and rectangular stream 
drainage patterns. However, looking at the upper right corner of 
the image, a break line in the stream drainage pattern shows 
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