Geologic Mapping: Guide for Professionals and Students

Geologic mapping is a crucial process in the field of geology, which involves creating a detailed representation of the Earth’s surface based on the study of its rocks, minerals, and physical features. This article will provide a comprehensive overview of geologic mapping, its importance, and the various techniques and tools used in the process.

What is Geologic Mapping?

Geologic mapping is the process of creating a map that displays the distribution of rocks, minerals, and other geological features across the Earth’s surface. This map serves as a valuable resource for scientists, engineers, and planners, as it provides essential information about the Earth’s subsurface structure, natural resources, and potential hazards.

A geological maping is the basic professional work normally done by the geologists. A geological survey is a systematic investigation of the geology of an area. It reflects the geology and structure beneath a given piece of land. Surveys are conducted for the purpose of preparing a geological map. Any geological surveying method employs several techniques including the traditional traverses, walk-over surveys, studying the exposures, outcrops and landforms. Geological surveys also adopt some intrusive methods, like hand augering and machine drilled boreholes. Geological surveys also use the geophysical techniques and remote sensing methods, such as aerial photography and satellite imagery.

Geological surveys are normally undertaken by private agencies, state government departs of mines and geology, and national geological survey organizations. They maintain the geological inventory of various formations, mineral deposits and resources. They keep all records for the advancement of knowledge of geosciences for the benefit of the nation. Geological mapping are parts of a geological survey. It involves certain procedures. This lesson highlights the methods and procedures of geological mapping.

Kinds of Geological Surveys: A geological survey can be undertaken using a number of methods depending on the size of a region and the amount of information that are required. Different types of methods are involved in eological surveys. The first method is Remote Sensing. It is used in some geological mapping works. This is done using satellite remote sensing methods. While most of these methods rely on geophysical rather than pure geological data, the use of this method can give a broad scale view of surface geological structures such as folding, faulting, igneous intrusions etc. The next method is the Airphoto interpretation method. This can give a broad overview of the geological relationships of an area with no detailed knowledge of the mineral composition or fabric of the rocks. The third method is the Outcrop surveying method. This is normally done by geologists by conducting traverses along the fields and mapping the outcropping rock types. The last part is the Geology interpretation surveys. These are more detailed outcrop surveys, where the geological boundaries are established and interpreted in a small area.

Geological Maps and Mapping: Geological methods rely on the identification of rocks and minerals. They are done for getting an understanding of the environment in which they are formed. Geological surveys aim to find what rock types occur at or close to the surface and how these rock types are related to each other. The surveys orient to know about the disposition of rocks and minerals, their boundaries, ages, and structures. At the end of the surveys, geologists prepare the geological maps. The geological maps are prepared for three major purposes. They are: For smaller scale reconnaissance works

• For large scale underground mining
• Engineering site

Objectives of Geological Field Mapping

There are several reasons based on which a geological field mapping is carried out. They are all entailed in collecting variable amounts of field data. These basic reason is to delineate the natural mineral and other resources.  Mineral and oil exploration proceeds always in this way.

Geological mapping is usually the first task in any reconnaissance study. Geophysical investigations are carried out to answer the question of the extent of the system under the subsurface. Geochemical investigations are also used to estimate parameters such as the temperature of the system. Exploitation of all mineral resources requires the appreciation of basic geology and optimum utilization of a potential area. This requires the mapping of the resource. In addition, the understanding of the spatial distribution and deformation of rock units, at the surface, is critical in order to develop a 3-dimensional model of the subsurface geology.

Geological mapping: Geological mapping is done to obtain and provide basic knowledge about the prevailing field conditions, not only through direct observations but also by collecting and analyzing rock, mineral and sediment samples. A geologist conducts field surveys and prepare accurate geological maps by collecting samples and measuring the geometrical aspect of outcrops. There is no substitute for a geological map. Geological mapping is normally done in a project mode with people in a team, a set of special equipment, and a topographic base map. Careful observations are done during the geological mapping.

Remote Sensing for Geological mapping: Today, the availability of aerial photography and remote sensing from satellite imagery, and the computer capability for storage, recovery, and evaluation of data are used for geologic mapping and other purposes. These methods have almost replaced many old methods of geologic data collection, plotting, and interpretation. The Remote sensing technology and satellite products provide the fast access to all geospatial data. Also, a greater and finer resolution of data and images are readily available in planimetric and 3-D mode at any desired scale and time. These data can be integrated with Geographic Information Systems (GIS) for vertical and horizontal comparison. Maps can be combined with layers of information on topography, minerals, water, energy, and the environment. These technological advances have increased the usefulness of and public access to geologic maps.

Geological observations: The three basic reasons why geological field work is carried out include exploitation of natural resources, as a requirement of the government and for academic purposes. Good geological mapping should be executed in three phases; planning, data collection and reporting. The data collection phase involves detailed observations. All geological observations are marked on the base maps for future compilation and interpretation. Base maps are used to locate the positions of people objects and structures in the field.

Geochemical Methods: Geochemical methods involve the measurement of the chemistry of the rock, soil, stream sediments or plants to determine abnormal chemical patterns which may point to areas of mineralisation. When a mineral deposit forms, the concentration of the ore “metals” and a number of other elements in the surrounding rocks is usually higher than normal. These patterns are known as primary chemical halos. When a mineral deposit is exposed to surface processes, such as weathering and erosion, these elements become further distributed in the soil, groundwater, stream sediments or plants and this pattern is called a secondary chemical halo.

Secondary halos aid in the search for deposits as they normally cover a greater area and therefore the chance of a chemical survey selecting a sample from these areas is greater than from a primary halo area. Different elements have different “mobility” in the environment based on their readiness to dissolve in water, their density, their ability to form compounds with other elements and the acidity (pH) of the environment. Subsequently, the secondary halo may not contain the “metal” for which a geochemical survey is searching but other “marker” elements. These are commonly employed during geological surveys.

Geophysical surveys: Geophysical prospecting method are employed when there are no exposures and the entire region is covered with soils and regoliths. These are indirect method of finding out the hidden rock types and structures.

Geophysical survey refers to the systematic collection of geophysical data for geospatial studies. Geophysical surveys are conducted using a great variety of sensing instruments. The data are collected from above or below the Earth’s surface. Sometimes the data are collected from aerial, orbital, or marine platforms. Geophysical surveys have many applications in Earth science, archaeology, mineral and energy exploration, oceanography, and engineering. Geophysical surveys are part of the geological surveys.

Base maps and other Maps: During the preliminary phase, all existing data and maps of the area of study are to be collected and analysed. All suitable maps available like the physical, political, relief, road, physical, and topographic maps are to be seen first. These are to be carried to the field as it is possible that details in one may not be present in another. Most importantly, for a geological fieldwork, a handy base map is expected to be used as a reference. Depending on the areal extent of the field and the detail required, the scale of the map is to be chosen as it is an important aspect to be considered.

Geological investigations: Geological investigations normally start with base maps, run through the field areas and end in laboratory analysis of samples. The ultimate aim is to explain the geology and structure of the area. The approach is highly practical. The first geologic map was prepared, in the world,  to solve a practical

problem involving the distribution of different types of rocks at and near the Earth’s surface. Most building materials, except wood, are from various specific rocks and rock products.

Geological investigations are the basic needs for a country. A geologic map graphically communicates important information about the distribution of rocks and unconsolidated materials at and near the Earth’s surface. All are done by the geologists.

Basic skills of a Geologist: Geologists spend most of their time in the field and also in the laboratories. Their mind should be conservative and sympathetic to country-side environment and people of rural areas. The flowing are areas of concern to a geologist:

1. Field
2. Safety
3. Ancillary skills
4. Courage to carryout independent

Field Behaviour: When conducting surveys in some enclosed areas the gates should not be left open, people should avoid climbing dry stone, walls should not leave litter or disturb communities of plants and animals. While collecting specimens do not strip or spoil sites, where

type fossils or rare minerals occur. People should take only what is needed. It is necessary to ask permission to enter a land from land owners or anthorities.

Safety: Any geologist must be physically and mentally fit to work in the field. Even in a rugged area, mountainous terrain, poor weather, difficult climate and challenging situations .Geological works are prone to have physical hazards which are to handled with care and safety. Experience is the best teacher but commonsense is a good substitute during field mapping.

Ancillary skill: A geologist should be able to draw ,swim, drive ride and record in any type of conditions. He should be able to ride horses, in some areas, and walk slopes, they should clamber up and conduct surveys on cliffs and escarpments.

Independent mapping ability: Every geological mapping project is time-consume and mind depressing, especially when carry out alone .no matter how many hours are spend the compilation of all little information collected from the field ,will certainly make an outstanding contribution to the knowledge of geology of the area. People should not lose heart at the begging. They should not be frustrated.

Field Equipment and tools: Geological mapping requires a lot of small ,small field equipment and tools are

1. Hammer and chisels
2. Compasses ,clinometers and camera
3. Hard lenses and Tapes
4. Map cases and Field note books
5. Scales and Protractors
6. Acid Bottles and Hand gloves
7. GPS, pedometers and altimeters
8. Stereo net and

In addition to these pencil, erasers and a jack- knife are needed in the field.

Hammers and Chisels: Geologist need a hammer and some chisels .These are used to break the rock and get samples.1 Kg hammer is the most use full one in a resistive hard rock, Hammer should be fitted with a good wooden or fiber glass handle or a steel shaft. Normally a 45cm chisel with 2.5cm cutting edge is used in the field. Chipping a rock samples should be done carefully. People should were safety glasses.

Compasses and clinometers: Geologist use Brunton compass or a clinometers compass is an instrument helpful to detect the directions using a magnetic needle that swings freely on a pivot, is a horizontal plane. One end of this needle always points to the magnetic north, The compass is normally made of brass or an alloy ,so that it is not susceptible to any magnetic influence the circular dial of the compass is graduated into 360 degrees, In normal equipment this graduation is in clock wise direction, In Brunton compass ,it is drawn in reverse direction, The east –west cardinal points are juxtapose din order to enable the user to get direct bearing of readings.

Parts of a Brunton compass: The Brunton compass is composed of a non-magnetic cylindrical body as the base in which a graduated (360 degree) scale is fixed in circular form. At the center

of this circle, a pivot holds the magnetic needle which is marked with red point to denote the north direction. The compass is attached to a mirror lid with a hinge. There is an axial line marked in the mirror, At the opposite end of the mirror, there is a sig arm equipped with a slit and   an Peep sight. The compass is also containing a Bulls eye level to denote horizontality of the equipment. There is a clinometers to denote the vertical drop provision which helps to detect the angle of inclination of the axis of the Brunton compass oriented along the axial line, which can indicate the direction of elongation and measurement.

Method of using the Brunton compass:

In the Brunton compass, the sighting arm and the mirror containing the axial line are raised perpendicular to the cylindrical body of the main equipment. The folding sight on the top of the mirror and the peep site in the opposite sighting arm are used together to align the direction is which the bearing are decided to be determined. All object , axial line and peep slit should align in the same line to determine the direction of the object. This helps to decide the forward measurement (as forward bearing) or is marking the reverse direction detection (as backward bearing ) works.

Method of measuring the dip: Brunton compass is useful in determining the dip direction of any dipping strata. The clinometers kept inside the case is a free-fall type unit denoting the vertical position always, By orienting the axis of the Brunton along the dipping strata, the dip angle between the vertical plane and direction of the strata could be determined.

Clinometer: Clinometer is a simplified model of a Brunton compass containing the very essential parts for determining the dip of the strata inclination of joints, angle of slope of an embankment and or determining the direction and location of objects through forward or backward bearing .All readings are made with reference to the north direction. In all geological mapping ,a high degree of accuracy is recording the directions or bearing is essential. Dip ,strike and direction are the three major measurements made using the Clinometer.

Hand lenses and Tapes:

Every geologist must have a hand lens with a magnification of between 7 and 10 times. A short roll-up steel measuring tape is always needed during the field work .It should help in measuring everything from grain size to bed thickness. Sometimes a 10m or a 30 m linen tape may be of much use for small surveys.

Other tools: At the time of conducting a geological survey and carrying out geological mapping there is a need to use map cases, field notebook, scales, Protractors, stereonets, pedometers and hand gloves. Acid bottles are also carried to test and distinguish carbonate minerals or limestones.10% HCL is the normal or acid used in the field, kept in tiny dropping bottles.

Sample bags and safety clothing

Sample bags which best suit for keeping all the geological samples like a canvas in fabric is an essential item during surveys. Plastic bags may be used where the sample is soft, disintegrated or wet. Safety clothing is a necessity. These include sturdy shoes, clothes that are tough in fabric preferably jeans or khaki, hat and sunglasses. Safety glasses and gloves are important especially when hammering rock samples.

First aid kit: Safety in the field is always a priority; nevertheless accidents cannot be ruled out. There is need to be prepared with a first aid kit. It is crucial that at least one person in the field crew should have been  trained in basic first aid techniques.

GPS:A global positioning system is needed for conduction geological surveys, It helps in detecting the latitude, longitude, altitude and other. Geospatial parameters with precision and accuracy, A GPS needs a line-of-sight to several satellites for getting the way-points. Grid references of toposheets are accurately indicated with other benchmarks for locating the outcrops during geological mapping works.

Method of surface geological mapping: Geological mapping is the process of making observations of geology and structure in the field  and recording them on a base map and reproduce. It is the form of a geological map. The information recorded must be factual and thorough based on objective examination of rocks and exposures. There are several methods adopted during this process as traversing. Following contacts and exposure mapping is the methodology adopted. This involves mapping of poorly exposed region with indications of rocks from soils, vegetational guides, topography and geomorphology and structure. Contours are the major lines of trace involved during surface geological mapping.

Surface mapping methods: Mapping superficial Deposits, Drilling ,logging and geophysical measurements ,Underground mapping, pitting, trenching, augering and loaming are some of the subsurface mapping method adopted during surface geological mapping.

What to map during geological mapping:

The features recorded are the following:

‐ Rock types and Contacts

‐ Shape of the rock bodies

‐ Note on the sequence and relative ages

‐ Note on the primary porosity and permeability

‐ Note on the weathering and their patterns

‐ Note on the depositional or magmatic flow features

‐ Structures including

1. Folding – dip, strike, deformation, orientation of grains
2. Joints – attitude, size, open or closed
3. Faults – look for slickensides, fault gouge, breccia and their visible

Traversing :Traversing is a method of making observation of outcrops along a path across a country. It is a method to be repeated in roughly parallel alignment with the earlier traverse. This method helps in covering the ground in detail. Traverses are pre-determined rout contacts and other geological features are extrapolated between the traverses and top map of the geology of the area is prepared. The major traversing methods adopted during geological mapping are:

1. Cross-section traverses
2. Stream and ridge traverses and
3. Road

Cross-section traverse: These are done in structurally complex geological terrain by plotting a set of cross sections and compiling them. Streams and ridges traverses give excellent semi- continuous exposures and in places where the slopes are partly covered by colluvium. Locating a position on streams is relatively an easy method from the shape and direction of bends, position of islands, waterfalls and streams junction. In a dense and thick mountainous forest, the only way to locate on the base map is to find out the way along the river courses.

Ridge Traverses: Ridges and the spars are excellent traverse locations during geological mapping. They can be indentified easily on a map or aerial photograph. Exposures are normal clear in ridges .Since, ridges are more erosion-resistant rocks, they tend to follow and indicate the strike directions clearly.

Road Traverses: A very rapid reconnaissance of an unmapped area can be made along tracks and roads ad by following paths between them. Road cuttings in mountainous exposures for mapping. Several stratigraphic successions can be easily mapped and traced along such road cuttings.

Following contacts and Exposures:

A primary purpose of mapping the geology of an area is trace the contacts between different rock formations, groups and types and to show them on a map where they occur. One way of doing this is to follow the contact on the ground and trace it on the map. In some places contacts may be visible and easy to trace. In some places ,the contact may not be continuously exposed or the contacts may be beneath superficial deposits. In such cases ,the stratum contours are considered for tracing ,using geometric alignments and visual interpolations. When a contact is concealed by alluvium or scree, it should be shown by a dotted line.

Exposure mapping: Mapping by exposures is the major method of geological mapping .It is done at the scales of 1:10000 and larger. Exposure mapping shows the factual evidence on which interpretations are made .It shows what has been seen and inferred. A form line map is prepared based on the interpretation of the form of geological structure.

Poorly exposed Regions: In some places, rocks are poorly exposed .They are mostly covered and hidden by vegetation. They may show poor exposures due to coverage by weathered regoliths of all rocks. Mica schist forms the poorest exposure but show all evidence along the footpaths. In some places ,when trees are uprooted due to storm events, rocks are exposed for observations. In addition, road cuttings, railway cutting and in other man made /animal–made excavations, rock types are expected to be exposed for mapping.

Soil as an Indicator: Soil, if they are not transported. reflect the parent rocks existing beneath their sandy soils are indicative of rocks containing more quarts, clayey soils are from Kankan and weathered Dolerites and other basic rocks tend produce distinctive red-brown soils. More to acidic igneous rocks form lighter colored soils in which mica may be visible .Any soil depend on parent rock ,climate age and other factors. Their association with specific rocks may help in mapping the rock bodies.

Vegetational guides: Plants are good indicators of some elements present in the rocks beneath them. Some are typical around lime stones, on some acid rocks and on serpentinous rocks. Flora are capable of indicating the varieties of rocks ,in an indirect way ,when they are natural. Some plants can indicate metallic ores too .there are many copper indicative plants, uranium indicative plants and even gold indicative plants.

Topography and geomorphology: Geomorphology is the science of landscapes. Geologist always look into the rock and relief exposed in various places .In many places ,resistive rock bodies stand well above the land surfaces. Remnants, residual hills, pediment zones, flood plain deposits, alluvial cones and fans ,drumlins, dunes, boulder clays, and other fluvio-marine deposits show typical relief feature as landforms. Volcanic rocks always show very unique exposures. Typical drainage patterns exist in different rock types and relief zones. Landslides bring down a major land mass and expose the basement rocks.

Pitting, Trenching , Augering and Loaming:

When it is essential to investigate the rock lying beneath any overburden, pits and trenches are made to study them. Many contacts could be best indentified from trenches. In many cases, identifiable fragments of weathered rocks can  be from shallow anger-hole drilling method loaming is a method of mapping in poorly exposes and deeply weathered regions.

Test Drilling: Test drilling are commonly employed to locate formation at depths. They are also done to confirm their presence when there is a gap in other information, and also when there is a need to find out the details of structure and their geometry. There are two kinds of drilling methods adopted as percussion and rotary drills. In percussion drills, rocks are fully crushed and powdered or drilled using a ring like drill bit to obtain a core sample. In rotary drills, only crushed products will come up for identification.

Types of Drilling Methods: Drilling is used to obtain very detailed information about rock types, mineral content, rock fabric and the relationships between rock layers close to the surface and at depth. Drilling is only used in areas that have been selected as “targets” from geological, geophysical and/or geochemical methods.

Four drilling methods are widely used, each depending on the type of information required and/or the rock types being drilled. Air-rotary drilling-this is a quick and economical method of producing a sample. This method is used in soft rock materials to about 25m deep, or to drill through the top layer of decomposed rock and soil to get a fresh rock sample from the bedrock below.           Air-percussion drilling-this method is used for penetrating hard rock types to a depth of around 300m. As in air-rotary drilling, rock chips are brought to the surface by the returning air. Mud-rotary drilling method is used for drilling through soft rocks, sand and clay layers especially in the search for coal, oil or gas. This method is used for holes up to 3km deep and can be done from ships or offshore platforms especially in the search for oil and/or gas. Diamond core drilling method is employed using industrial diamonds for drilling through rock layers. This provides a “core” of rock being left in the centre of the pipe. This core is recovered and gives information not only about the rock types, but also about the relationships between the rock layers in detail.

Preparing a Geological map:A geological map is a spatial representation showing the distribution of rock units and structures across a region. It is drawn on a plane surface. A map showing the occurrence of structural features across a region, the distribution of rock units, and their type and age relationship is termed a geological map.

A geological map is expected to show all the rock types of a region, their structures, geological formations, geothermal manifestations, age relationships, distribution of mineral ore deposits and fossils. All these features are to be super imposed over a topographic map or a base map. The amount of detail shown in a map depends largely on the scale and a smaller scale will naturally disclose finer detail.

Reporting: Geologic reports and maps prepared to assist in public decision making. Geologic maps and reports can be used in regional-scale environmental and resource management planning documents to assist geologists, engineers and land-use planners in making decisions that affect public health and safety, critical environmental habitats, water quality, uses of public lands, and help identify areas where more detailed geologic studies are needed. It is often said that a report is as good as its data collected from a field work. It is necessary that there is a need to collect very clear and accurate data. Nothing can be overemphasized. Ultimately when all possible available data are collected, then they are taken back to the aboratory for sorting, interpretation and analysis. This phase is the most challenging phase. Any wrong analysis or misinterpretation of data can lead to an inaccurate report and in consequence misinformation.

Interpreting geological maps: Once a geological map is complete, then the most important task is interpreting it correctly. A map is basically a visual summary of an entire report and the two should complement each other. When the interpretation process is at hand, it is crucial to have group discussions with all those involved in the actual fieldwork. It is also deemed important to have discussions with those who are experienced in the field of geology as well as other related scientific fields of the area.

Quality and Interpretations: Basically, the quality of a geologic map will depend upon the accuracy and the precision of the field work. The interpretation of a geological map depends on adequate training, interest and the techniques used. It is necessary to visualise the scenarios that might have been involved during the formation processes of geological features displayed in maps while analysing the geological maps. The ability to form a three- dimensional image from a two dimensional map, is in real sense, a major part of the geologic map interpretations.

Uses of Geological Maps: A Geologic map helps to protect groundwater resources. The geologic maps are also used for habitat prediction. Geologic maps are used to evaluate the mineral resources. Geologic maps are also used to delineates the landslide prone areas. Geologic map guides to delineate the earthquake-prone areas and help in damage prediction. Geologic map delineates volcanic hazards and aids in the mitigation of earthquake damage, cyclone damage, tsunami damage,etc. Geologic maps show the locations of exploring sand and gravel resources. Geologic maps identify the economic resources and mining areas. A Geologic map guides transportation planning.

Importance of Geologic Mapping:

– Natural resource exploration: Geologic mapping is essential for the exploration of natural resources, such as minerals, oil, and gas. It helps identify areas with high concentrations of these resources and guides drilling operations.

– Environmental management: Geologic mapping plays a vital role in environmental management, as it helps identify areas with high potential for natural disasters, such as earthquakes, landslides, and flooding. This information is crucial for the development of mitigation measures and the planning of infrastructure.

– Infrastructure planning: Geologic mapping is crucial for the planning and construction of infrastructure, such as roads, railways, and pipelines. It helps identify areas with high potential for geological hazards and guides the selection of safe and cost-effective routes.

– Disaster risk reduction: By providing information about the distribution of geological features, geologic mapping helps reduce the risk of natural disasters, such as earthquakes, landslides, and floods. This information is vital for the development of disaster-resistant communities and the implementation of effective disaster management strategy.