MICROGRAVITY SURVEYS AND ITS
APPLICATIONS
Course Seminar: GS-663
Under Dr. M. Radhakrishna
Professor, Department of Earth Sciences, IITB
•Presented by: Abhinav Poonia (Roll No. => 173060011)
• M.Tech Geo exploration 1st year
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This Contain Introduction: What is “Microgravity survey” ? its Applications , Data Acquisition and Importance.
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MICROGRAVITY SURVEYS AND ITS
APPLICATIONS
Course Seminar : GS-
Under Dr. M. Radhakrishna
Professor, Department of Earth Sciences, IITB
- (^) Presented by : Abhinav Poonia (Roll No. => 173060011)
- (^) M.Tech Geo exploration 1
st
year
OVERVIEW
- (^) Introduction: What is “ Microgravity survey ”
- (^) Applications
- (^) Data Acquisition
- (^) Importance
APPLICATIONS
- The microgravity method measures very small variations in the earth's gravity field. Depths to bedrock, approximate fill thickness, locations of faults, buried river channels, fissures, and solution cavities may be determined by measuring the earth's gravity field at numerous stations along a traverse, and correcting the gravity data for elevation, tidal effects, topography, latitude, and instrument drift.
- (^) Some of the various fields of application of Microgravity surveys are as follows-
- AQUIFER MONITORING
- ARCHAEOLOGICAL SURVEYS
- CAVE & SHALLOW HOLE DETECTION
- COAL MINE WORKING DETECTION
- ENVIRONMENT STUDIES
- EVAPORITE DISSOLUTION MAPPING
- GEOLOGICAL MODELLING
- HARD ROCK MINING
- LANDFILL STUDIES
Aquifer Monitoring :
- (^) Microgravity has been proven to be of value in monitoring the depletion of underground aquifers, not only for water supplies but also for geothermal reservoirs.
- (^) It can also be used to determine and monitor mass redistribution due to fluid extraction and re-injection Considerable work has been conducted in the USA regarding this.
- (^) The aquifer can be monitored over time and the data can be applied to hydrological models.
Cave & Shallow Hole Detection
- (^) Microgravity is the primary tool used for the location of shallow (<50m) caves and swallow holes.
- (^) Microgravity is the only surface geophysical method that will provide the location of such karstic features regardless of their shape or fill provided a sufficient density contrast exists.
- (^) Under correct conditions, a microgravity survey can successfully detect and delineate natural cavities.
- (^) Cavities constitute a considerable hazard to development and redevelopment and any migration to the surface may seriously damage property and services. Such features need to be positively identified prior to any engineering design and construction.
Coal Mine Working Detection:-
- (^) Microgravity has been highly successful in the detection and location of abandoned shafts in a wide range of mining environments.
- (^) The same principles for detecting natural cavities can be applied to locating and delineating man made underground workings.
- (^) Nowadays microgravity survey is recognised as the prime method for establishing the presence of shafts, adits and workings.
- (^) If conducted properly Microgravity surveys are capable of detecting workings to a depth as much as 100m when data is acquired carefully and the information undergoes stringent analysis.
Geological Modelling:-
- (^) The technique is most suitable for studies of bedrock at depths in excess of 50m.
- (^) microgravity can also be utilised in the mapping of near surface faulting.
- (^) More recent applications are the mapping of pot holes within bedrock.
- (^) Microgravity has successfully been applied to a number of hard rock mining environments specifically in South Africa, Namibia and Australia
- (^) The applications vary from structural mapping, old mine working detection and delineation of sinkholes caused be dewatering.
- (^) The two last examples have serious safety implications and not only affect the operational side of mining but also the economics of operating in such environments. Hard Rock Mining:-
.
- (^) Gravity anomalies arising from natural or manmade subsurface features such as voids and cavities are superimposed on much larger variations due to height, latitude and regional geological variations.
- (^) In order to isolate the subtle signal of interest, careful data acquisition and processing is required.
- (^) Micro-gravity surveys are particularly sensitive to the data collection methodology which must be carefully tailored to the site and environmental conditions, as well as being appropriate to the survey target.
DATA ACQUISITION
DATA PROCESSING &
PRESENTATION
- Data recorded on site should include accurate position and height measurements for each recording station.
- Corrections are made for variations in latitude elevation, local terrain and regional geology.
- The result is a residual microgravity map from which both qualitative and quantitative interpretations may be derived.
- An accurate micro-gravity anomaly map can be used to calculate the location, depth density contrast and therefore volume of the subsurface feature.
ADVANTAGES residual effects of a microgravity investigation as there may be with other void detections methods
- (^) Microgravity can be used to detect different aspects in a soil profile. Microgravity can often be used to determine the depths and size of the voids. The level of detection also allows for smaller voids to be detected as well, but only with intense analysis.
- (^) DISADVANTAGES
- (^) The process can become very slow as the corrections and records required at each point can become tedious, and the analysis of data can become very tedious and time consuming.
- (^) Any vibrations can invalidate the data. The levels of change recorded by the sensor could be shielded by