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.

Lens Stereoscopy

3D viewing is widely used in cinemas today. Technologies such as, Active Shutter, Polarization and Interference filter method are used for 3D projection. In all these methods the basic idea is to provide two different images to each eye like we naturally perceive the world. Then the brain does the rest of the work and combines those images to perceive depth. Although 3D vision is most common in movies at present, in the past it was used in Remote Sensing for military purposes. The instrument used for this purpose is the Stereoscope.

the stereoscope with an image

The Stereoscope consists of two lenses resting on a stand that focus on the two separate images of the stereoscopic image sheet. When looking through the two lenses, each eye sees a slightly different image that is meant for that eye and this results in the perception of depth - the image appears 3 dimensional. This is very useful in analyzing aerial photographs because it allows viewing the ground terrain in 3D.

test image for stereoscopic vision

Shown above is a test image that is used to check stereoscopic vision. The relative heights at which the symbols appear to be seen can be checked against the actual order of heights which are provided overleaf.

Stereoscope with a test image

While in modern remote sensing, stereoscopes have been replaced by more advanced computerized equipment that not only provide 3D views but also help manipulate images in 3D, the stereoscope helps understand the basics of stereoscopic vision. Lens Stereoscopy can also be considered as the foundation of all modern stereoscopic equipment.

Stepping into Field Geology

While Geology is a field of study that takes place both outdoors in the field and indoors in laboratories, Field Geology is a primary constituent of Geology. Most of the activities that are involved in Geology like sample collecting, mapping and recording takes place in the field. When moving into field geology for the first time, several important points need to be considered.

The first thing that needs to be considered is, Safety in the field. Mostly this means to be aware and protect one's self against threats from nature and wild animals. For this purpose a thorough knowledge about the surroundings, proper equipment and first aid facilities are essential. In the case of snake bites it is useful to know the closest hospital or medical center equipped with anti-venom serums. In addition to the above, proper usage of tools like the hammer and chisel will reduce the likelihood of an accident.

Once the safety is taken care of, the right tools and equipment related to field work must be made available. The common tools used in field geology are, Geological Compass(Brunton pocket transit or Silva compass), Hand lens(10X), Hammer and chisel, waterproof field book, mineral testing kit and sample bags. In addition to these basic tools, additional tools such as camera, GPS, binoculars  gold pans, metal detectors and Geiger  counters may be used for specific requirements.

The Geological Compass is arguably the most useful tool at the disposal at the Geologist or Engineer. In addition to the obvious purpose of locating the direction of north and calculating the bearing, a geological compass has the following functions.
- Finding the dip and Strike of geological features such  as foliation, joints bedding planes etc.
- Measuring slope angles
- approximating heights of objects
- a level
When using a compass, it is essential to keep away from vehicles, power lines and any other magnetized objects. This means that the Hammer and chisel also needs to be kept aside when using the compass. With the emergence of smartphones with magnetic sensors and accelorometers, applications that perform similar to a geological compass have been developed. One such application that can be used for traversing, calculating Dip and Strike along with a host of other useful functions is, Rocklogger which is available for the Android mobile platform.

Another important aspect of Field Geology is Sample Collection. When collecting rock samples, an approximate sample size of 3"x3"x3" or 4"x4"x4" is preferred. Weathered rock samples are generally avoided unless it is a specific requirement. Once collected, sharp edges should be trimmed off before bagging the sample to avoid tearing the bag. Cloth bags made out of a cotton fabric are usually used for this purpose. Samples should also be numbered systematically. While there is no hard and fast rule for this, a meaningful numbering system incorporating information such as collection year, area, and collectors initials is used to avoid confusion.

Field photography is also a requirement in certain cases. When taking a picture always a scale should be used. For small objects, a pen or another small object with a known size must be in the picture. In the case of photographing macro structures, wetting the surface of the rock helps to bring out the fine details on the surface. Care should be taken to avoid shadows to fall on a part of the rock when  photographing because it could lead to misinterpretation.

Brunton Compass - image from wikipedia

An Introduction to Remote Sensing

Remote Sensing, like the name implies, means observing and gathering information of an object from afar. In this sense we humans perform remote sensing all the time with our eyes and ears, where eyes and ears are our remote sensing equipment. However when moving away from the basics and expanding the scope of observation, many factors become involved and remote sensing becomes a broad and detailed area of study. Incorporating all these ideas, a general definition for remote sensing can be given as,

"The art and science of obtaining information about an object without being in direct contact with the object"

Remote sensing is extensively used in observing the earth and the environment for various purposes. The most common application of remote sensing is done in mine development, disaster mitigation and large scale environmental studies. Depending on the application, the sensors and the type of radiation used, the method of recording and processing and the analysis of the data will differ.

The major components in remote sensing are,
- an energy source for illumination :- usually the sun or an artificial source of radiation.
- medium of travel for energy :- the atmosphere.
- sensor :- orbits in a satellite eg. Landsat, Quickbird
- receiver on earth :- gathers the data transmitted from the satellite.
- equipment for interpretation and analysis of data :- remote sensing software.

All remote sensing systems have four types of resolution. They are,
- Spatial resolution :- refers to the pixel resolution of the image produced.
- Spectral resolution :- refers to the range of EM radiation that can be detected. A broader range increases the scope of analysis.
- Temporal resolution :- refers to the orbital period of the satellite carrying the sensor. Faster the period, higher the resolution.
- Radiometric resolution :- refers to the differences of the intensity of radiation that can be distinguished by the sensor. Given as a bit range. A higher bit range can distinguish smaller variations.

Once all the above details are decided according to the application, remote sensing is performed. Then a "reflectance pattern" is produced from the gathered data. By comparing these patterns with the spectral signatures of known objects, the type and extent of the sensed objects on the earth can be inferred. In addition to various applications on earth, remote sensing is also used in other planets to gather data about surface conditions etc without landing on the planet.

Remote sensing in Mars. image form wikipedia