Importance of Geological Fieldwork in Research
Geology is a field science. Even though much work is done in the laboratory and all data (including field data) are analyzed in the office, geological samples and information must initially be obtained from the context in which they occur in nature. This necessitates geological field work. Earth scientists use a number of field (including laboratory and numerical modeling) methods to decipher Earth history and understand the processes that occur on and beneath the Earth’s surface. In typical geological investigations, geologists use primary field data related to resource studies, petrology, stratigraphy, and structural geology. In many cases, geologists also study modern soils, rivers, landscapes, and glaciers; investigate past and current life and biogeochemical pathways, and use geophysical methods to investigate the subsurface.
The field instruments that would accompany most fully equipped field geologists on an expedition of mapping and sample collection include existing topographic/geological maps, the Brunton compass, hand-held GPS receivers, and tape measures. The geologist would typically also carry along a field notebook, hand lens, hammer, acid bottle, knife, shovels or trowels, sample bags, pens and pencils, aerial photographs and satellite imagery, maps and literature, camping equipment, and a camera.
Since any observation made in the field, or sample collected, has an important spatial dimension, perhaps the most important equipment of a field geologist is the GPS receiver.
In the modern era, these materials could also be supplemented by laptop/tablet PCs with real-time GPS tracking, digital cameras, and – depending upon the objectives of the survey – portable geophysical equipment (including a gravimeter, altimeter, magnetic susceptibility meter, GM counter, pH meter, conductivity meter etc.). Occasionally, an investigator may also need to carry power tools like a handheld cutter or drill.
Depending upon the objectives of the study, there are numerous specialized methods of geological field investigations. However, there are certain basic procedures that any field geologist will have to adopt. These basic field methods in geology may be considered in four main categories:
1. Obtaining and marking samples and describing and measuring where they came from in an outcrop;
2. Measuring and recording orientation (i.e., altitude) of strata or other planar features;
3. Measuring dimensions (height and width); and
4. Constructing geological maps.
Obtaining and marking samples and describing and measuring where they originate in an outcrop requires observational skills and patience for recording all information that might be obtained at one outcrop. Typically, the thickness of strata at an outcrop is recorded in a notebook where the layers are drawn to scale and described as to rock type, grain size, fossil content, color, sedimentary structures, and other attributes. Thickness of strata is measured using a tape measure or a Jacob's staff, which is a long stick made for sighting intervals of equal stratigraphic thickness (usually 5 ft, or 1.5 m). In the field notebook, detail is entered about GPS locations and altitude of samples, and where photographs of the rocks are made. Samples are marked with an arrow indicating 'up' direction and labeled with a number which relates to the notebook number for the outcrop plus a number relating to feet or meters above the base of the concerned stratigraphic horizon. The same process is followed at each locale. Later, this information is compiled into a measured and described section for each outcrop, which may be used for correlation between outcrops. In terrains where igneous and metamorphic rocks occur, it may not be necessary to record the up direction of the sample or its location above the base of the stratigraphic unit.
Measuring and recording the attitude of strata or other structural features is another important field activity that is necessary for understanding geological structures and for making geological maps. Strike, dip direction, and dip magnitude of rock layers and other planar geological features (e.g., schistosity) are obtained in as many places as possible within a study area in order to understand completely all the geological structures of an area. Analysis of geological structures can help geologists interpret the conditions of deformation of rocks in an area. Generally, the geologist tries to obtain as many orientation or attitude measurements as possible in the field area being studied.
Measuring dimensions (length, height and area) of a feature or area of interest, is an important aspect of many geological studies. Until recently this was done as an estimate by making readings with a Brunton compass, plane table and alidade, and employing trigonometric relationships to compute the height or width. Modern field geology methods, in conjunction with virtual globes like Google Earth, NASA World Wind and Bhuvan allow a user to enter the GPS coordinates and calculate the length or area to within a few square meters of accuracy. Elevation differences between two points may also be measured with an accuracy of +_ 7.5 m using ASTER DEMs.
Constructing geological maps (or adding content to existing geological maps) is the most important field activity that an investigator has to perform. Geological maps are made by using a base map (SOI topographic map on a 1:50,000 scale) and/or subset of high spatial resolution satellite imagery to record the observed rock type. Topographic maps give a valuable input in geological mapping because elevation information is important in interpreting physical relationships between rock formations. Contacts between litho units, their trends, strike and dip, structural and other attributes at different locales in the study area are plotted in their correct spatial relationships in topographic maps or imageries. An informed assessment is made of all these attributes in intervening areas that have not been directly observed, and contacts are drawn on the basis of observation and assessment. Sometimes, high resolution images of the study area will prove to be very helpful in revealing geological/structural information when examined in conjunction with limited field observations.
Other types of geological field work include reconnaissance studies of areas where detailed mapping is yet to be done, geological sample analysis conducted on-site at drilling sites, geophysical studies where the objective is to collect data such as the variation of gravity or magnetic characteristics, etc.; surface and groundwater studies where the emphasis is upon water availability, quality, and the relationship of its occurrence with geological features; study of economic resources where mines and excavations are studied and areas explored for the value of potential mineralization; engineering geology field work where studies assess the impact of human disturbance upon rock and soil stability; and many others.
Diversity of Geological Fieldwork:
Given the diversity of interests, geological field work varies depending on the task at hand. Typical fieldwork could consist of:
1. Geological mapping
a. Structural mapping: the locations of the major rock units and the faults and folds that led to their placement there.
c. Surficial mapping: the locations of soils and surficial deposits
2. Surveying of topographic features
a. Creation of topographic maps
b. Work to understand change across landscapes, including:
Patterns of erosion and deposition
River channel change through migration and avulsion
3. Subsurface mapping through geophysical methods
a. These methods include:
Shallow seismic surveys
Electrical resistivity tomography
(These methods are used for hydrocarbon exploration, finding groundwater, locating buried archaeological artifacts
4. High-resolution stratigraphy
Measuring and describing stratigraphic sections on the surface
Well drilling and logging
5. Biogeochemistry and geomicrobiology
Collecting samples to:
Determine biochemical pathways
Identify new species of organisms. These organisms may help to show:
Identify new chemical compounds
(These studies are aimed at understanding early life on Earth and how it functioned and metabolized, and finding important compounds for use in pharmaceuticals).
6. Paleontology: excavation of fossil material
For research into past life and evolution
For museums and education
7. Collection of samples for geochronology and thermochronology
8. Glaciology: measurement of characteristics of glaciers and their motion
For general tips before setting out for the field please visit: http://www.horne28.freeserve.co.uk/fieldwk.htm
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)