Basics of Photogeology

The interpretation of Aerial Photographs for geological purposes is termed Photogeology. This a form of Remote Sensing. The primary objective of Photogeology is, to identify geological structures and rock types of an area and to prepare a geological map of the area. Photogeology is usually employed before field geological work in order to get a general idea of the geological features of the area. The observations and inferences obtained from Photogeology are then confirmed by using field data.

The analysis of aerial photographs in photogeology is done using the same elements of interpretation. "Tone" is used in the identification of rock type. Generally lighter tones indicate rock types like Quartzite. Tone is also used in the identification of the density of vegetation which in turn provides clues about the underlying rock. Usually dense vegetation can be seen in area where the rock Khondalite is present and in areas where Quartzite is abundant, very little or no vegetation is present.

By using a stereoscope to view the three dimensional topography of the area structural features such as ridges, valleys and slopes can be identified. Due to differences in hardness and the extent of weathering, certain rock types show characteristic structural features. For example, Marble and Hornblendebiotite Gniess are found at the bottom of valleys because they are highly prone to weathering. Rocks that are much harder and are resistant to weathering like Quartzite, Granite and Granitic Gneiss usually form ridges.

geological features in an aerial photograph

When viewing outcrops from above, for example in a ridge like structure, by looking at the texture on either side of the ridge, the direction of dip and strike can be inferred. Since the side where multiple layers of rock are exposed undergoes differential weathering that side become ragged and rough. This side is termed the escarpment face. The side on which a single layer of rock is exposed undergoes uniform weathering and therefore is relatively smooth. This side is the Dip slope. Therefore, generally the side of a ridge that has a relatively smooth texture indicates the direction of dip.

cross section of a ridge

The drainage pattern and shapes of marshes lakes etc also provide clues about the geological structure of the area. A drainage pattern like shown in the first diagram indicates a homogeneous and relatively flat rock while a pattern like in the second diagram indicates a highly jointed rock. In addition to this, the displacement of any feature along a line indicates a rock fault.

Once all possible features are identified form the aerial photograph, a geological map of the area is plotted. This map is referred and then the data is confirmed by using field data.

 

Short 'n' Sweet Masterpiece

Producing a good short film is a challenge. Due to the limited time available to convey the message, it is essential to strike the right balance between providing enough plot details and allowing for a smooth gradual flow of the story. The short film "The Secret Number" directed by Colin Levy does this beautifully. It has a brilliant screenplay that rivals feature length Hollywood productions and although only 13 minutes long it feels like you watched a full movie at the end of it.

Here is the full short film, enjoy!

Official site : http://secretnumber.colinlevy.com/

Here is the plot summary of the movie in case you missed a few points during the first viewing. 

A psychiatrist meets with a patient of his who is a mathematician and claims to have found a "secret" integer between 3 and 4. He calls this integer "bleem". The psychiatrist dismisses this claim saying that it violates common sense. Although the mathematician insists that this number lies in another dimension which is not perceived by us, the psychiatrist still refuses to believe that he is sane and instructs the attendant at the mental institution to lock up the mathematician. That night the psychiatrist dreams of an accident that occurred when he was a child where an old man was hit and killed by the car he was travelling in. The following morning he goes to the mental institution and finds out that the mathematician is not in his cell. It is also seen that the mathematician has performed loads of calculations in search of the secret number. It is implied that the mathematician was successful in finding this secret number and in doing so he gained access to a hidden dimension which allowed him to travel through time. In the final scene it is seen that the old man hit by the car when the psychiatrist was a child was this mathematician, and the pen he used to do the calculations on the wall and some jellybeans which were with him are seen fallen on the road.

Stereoscopic Analysis of Aerial Photographs

Aerial Photographs were the primary source of visual information used in Remote Sensing before satellite imagery became widely available. Stereoscopic analysis of aerial photographs were widely used for reconnaissance purposes during the world war. Apart from this, interpretation of aerial photographs have proven to be useful in various field of study such  as geology, surveying, mine planning and development and even environmental studies.

aerial photographs, stereoscopes and a map.

A typical aerial photograph usually contains, information such as the flight line, time of capture, altitude and the photo number. The flight line and photo number becomes useful when locating the photographed area on a map. The scale of the photo is also an important detail. The scale can be obtained by dividing focal length of the camera by the vertical distance from the imaged ground, to the camera. Aerial photographs can be panchromatic(black and white), colour, or in any other band of the electromagnetic spectrum. These details should be selected depending on the scope and objective of the image analysis.

In the process of interpreting aerial images, several interpretative elements are used in order to make the task of identifying and interpreting objects in aerial photographs easier and systematic. These elements of interpretation are,

Shape - The general outline or form of the object. regular and irregular shapes can help identify natural and artificial object.

Tone/Colour - It refers to the relative brightness or colour of an image. While in colour images, objects can be identified by it associated natural colour alone, in panchromatic images identification is slightly more difficult with varying shades of grey.

Size - Refers to the physical extent or the dimensions of the object. It is often useful to assess sizes of objects relative to each other.

Texture - Refers to the frequency of tonal change in an area of an image. can be described as smooth, coarse etc. For example; water appears smooth while a forest canopy shows a rough texture.

Shadow - shadows provide valuable information about the height of an object while also being a tool of identification of certain features in monotonous areas. For example, shadows can help identify lamp posts, water towers which would otherwise be misinterpreted because of its plan view. Shadows can also help identify sand dunes in a desert.

Pattern - Pattern is the spacial arrangement of objects on the landscape. can be describes as regular, ordered, irregular symmetric etc. Can help distinguish agriculture from natural vegetation.

Association - refers to the fact that certain features and activities are always related to the presence of certain other features and activities. This association of one object to another helps the process of identification and also can provide additional information of the area under analysis. For example, a populated area indicates the presence of roads and other infrastructure and an area with fuel tanks, chimneys etc could imply that the area under observation is a power station.

Identifying most of these features using two dimensional images is a very difficult task and can lead to confusion.The identification of the objects in a photograph using the above elements becomes very easy in three dimensions. In order to view aerial images in 3D, a modified version of the simple lens stereoscope is used. 

stereoscope

 This stereoscope which is used in aerial photograph interpretation, consists of a set of mirrors that reflect the aerial photographs laid on the table into the two eyepieces. The photographs used here are overlapping each other and by viewing a two slightly displaced images through each eye, the 3D effect is created.

While proving clues about an objects height, a three dimensional view of an area can also avoid confusion created by similar textures and tones. For example, mountainous terrain without significant vegetation can appear as a flat terrain if not for the stereoscopic view. It also helps to follow the flow path of streams and rivers where subtle tone changes or other disturbances have hindered a direct view.

Geochemical Exploration

Geochemical Exploration is defined as, "The systematic measurement of one or more chemical properties of naturally occurring materials to identify abnormal chemical patterns or geochemical anomalies related to mineralization". In simple words it means, the identification of mineral deposits by the use of chemical methods. Although geochemical exploration alone is insufficient to detect and pinpoint ore reserves, it is used in conjunction with geologic and geophysical exploration methods. However when the effectiveness of geologic or geophysical methods become limited due to certain reasons, geochemical prospecting becomes a useful tool to confirm the presence of mineral deposits.

The term "Mineral deposit" is defined as an abnormal concentration of a naturally occurring material(Note that "economic viablity of extraction" is not included in the definition anymore). Since an abnormal concentration of any naturally occurring material is bound to leave behind traces, the objective of geochemical exploration is to identify these clues left by the mineral deposit and to use them to locate it.

Geochemical exploration usually includes the measurement of concentrations of group elements, trace elements, and single elements in,
- Soil, rock, Sediment
- Water, (sea/river)
- Plant material
- Glacial Debris
to determine geochemical anomalies that might indicate a mineralization. Before carrying out the investigation it is essential to have an idea about the behaviour and characteristics of the chemicals involved, the exploration methods to be used and the analytical techniques to be used. Once these factors are considered, proper planning and preparation will help optimize the exploration.

Depending upon the ways of sampling and analysis several methods of Geochemical exploration or surveys can be defined.

Stream sediment surveys - this is of two types, stream sediment panned concentrate surveys and stream sediment surveys. this method is used when heavy minerals and chemicals that bond with clay are involved.

Soil Geochemical surveys - used when looking for surface mineralizations especially in sedimentary environments.

Hydro Geochemical surveys - in analyses for highly soluble and mobile chemical elements.

Bedrock Geochemical surveys - This is usually done during  the last stages of the exploration in order to confirm the mineralization.

Biogeochemical surveys - Although rarely used, in certain cases of mineralization, plants that grow in the area show certain characteristics that point to the mineralization - in the case of copper the leaves of a certain plant that grows in the vicinity turns to a brownish colour due to the absorption of copper.

Considering the conditions and expected methods of mineralization, a suitable geochemical exploration methods can be adopted if required along with geologic and geophysical methods in order to identify areas of mineralization.