Aerial Photography and Remote Sensing – Study Outline

geology

You should be able to:

1. list some of the characteristics of different rock types visible in aerial images

2. use these characteristics to make an educated guess about the rock type in an image

3. recognize a landscape in which folding has had a major role in shaping the landscape

4. describe the difference between a fault and a fracture or joint

5. describe some of the features of a fault on an aerial image and identify a fault on an image

6. describe and recognize some of the features of a glaciated landscape

A. there are several clues to the geologic fields of petrology and stratigraphy, identifying rocks and their age in relation to other rocks, in aerial images

1. drainage pattern are one of the most easily identified patterns in an image

a. several basic types tell something about the underlying rock

i. dendritic (tree-like) patterns form in uniform, unfolded rocks

ii. trellised (parallel main streams with short tributaries and right angles) form between parallel ridges of resistant rock, usually indicating strongly folded landscapes

iii. rectangular (many right angles and straight lines) shows a strongly fractured rock

b. drainage density, number of channels in a given area, is controlled in part by how porous the rock is. Porous rock means less runoff due to water soaking in, and fewer stream channels

2. rock color can be subtle, but is a major clue to rock type in a geologic area

3. joints are locations where a break in a rock or a change in its chemistry allows increased weathering. Joints are visible as dips, cracks, changes in vegetation or stream channels. Things to consider are

a. prevalence – are there many joints or few

b. pattern – are the joints parallel?, are there several differently oriented joints? do joints cross each other?

4. spectra – for digital image processing, the amount of light reflected in many different bands may create a unique signature for a rock type.

5. examples of signatures of some rock types

a. sedimentary rocks

i. sandstone

- parallel joints

- often resistant to weathering

- permeable

- vegetation

- dry climate, no vegetation due to lack of water

- moist climate, often trees because soil in nutrient poor and excessively drained

- light color

ii. shale

- impermeable

- low erosion resistance

- fine textured drainage

- dark color

iii. limestone

- massive

- resistant to weathering in dry climates

- dissolves to form karst topography of sinkholes and caves

b. igneous rocks

i. granite

- massive

- few joints, joints random

- impermeable

6. digital image processing such as PCA and ratios can be used to highlight and visualize these differences

B. land forms – geomorphology (the study of the shape of the surface of the earth) and structural geology (the study of the deformation of rock by folding or faulting) can often benefit from remote sensing that allows large features to be viewed from above.

1. some are obvious such as the patterns of ridges and valleys that result from highly eroded folds and faults

2. some are more subtle such as changes in the average orientation of rock joints

3. many geomorphology processes have visible signatures in aerial imagery

a. the work of wind, streams, waves and glaciers can all be seen by a good image interpreter

b. that interpretation requires a good understanding of the underlying geological principles

C. structural geology

1. folding

a. both upfolding and down folding (synclines and anticlines) create characteristic patterns of parallel or oblong ridges

b. the location of resistant layers within the fold and the length of time that it has weathered create the details of the pattern

2. faults – the evidence for faults is often more visible from the air than on the ground

a. strike slip faults where the two sides are displaced horizontally are most obvious

b. thrust and normal faults, where the displacement is vertical show up as sudden changes in topography or weathering pattern of the rock

3. lineament – is a more general term for anything that appears as a straight line in an aerial image. Searching for lineaments and classifiying them according to frequency and direction, without trying to interpret their cause can lead to both economic and geologic discoveries