- o Gravity Overview (these are all hot linked to the headings below)
- o Gravity Survey Applications
- o Survey Design and Specification
- o Types of Gravity Surveys
- o Helicopter Borne
- o Vehicle Borne
- o Traversing on Foot
- o Microgravity
- o 4D Microgravity
- o Gravity Meters
Gravity surveying uses gravity meters to measure very small variations in the earth’s gravity field. Observations are corrected for instrument drift, earth tide, latitude, station height and the effect of surrounding terrain. Other lesser impacts on gravity observations include atmospheric pressure, ocean loading, polar motion, the height of the water table and the amount of soil moisture. As appropriate to the objectives, precision and accuracy of a survey, all of these effects may be removed using rigorous observational procedures, standardized models and computations.
Once the various corrections have been applied to gravity observations, the reduced data is essentially a reflection of variations in the gravity field directly related to changes in rock density and structural architecture.
Sun shaded image of bouguer corrected gravity data acquired by IMT for the Geological Survey of Queensland – part of the 100,000 km² Normanton – Westmoreland survey
Gravity Survey Applications
Reduced gravity data can be input to sophisticated 3D modelling software such as ModelVision and Encom PA (as used by IMT Geophysics associates) where it may be used to develop interactive models of geological and or mineral deposit architecture at depth. The accuracy of such models may be improved by incorporating magnetic data in the analysis.
Modern gravity meters such as the Scintrex CG5’s, owned and operated by IMT Geophysics, are capable of routinely measuring a change in the earth’s gravity field through an elevation shift of as little as 5mms. To realize the full potential of such instruments, it is important that station elevations be precisely and accurately measured.
GPS surveying and absolute positioning techniques have revolutionized gravity surveying by enabling the rapid, low cost coordination and leveling of gravity stations to accuracies in the range of 10-50mm depending on survey methodology. Other digital survey instruments such as Total Stations and Digital Levels facilitate the acquisition of more precise survey data down to millimeter levels.
Observation of secondary GPS and gravity reference station, Normanton-Westmorland QLD gravity survey
Regional and semi-regional geological mapping for applications in:-
- o Reconnaissance minerals exploration to define structural architecture, locate greenstone belts, BIF’s, caldera systems and volcanic, hypabyssal and plutonic intrusions.
- o Basin studies for petroleum, coal, and gas exploration to define basin architecture, depth to basement, intra-basin structures, diapirs and economic constraints.
- o Groundwater & Environmental Studies
- o Geothermal exploration.
- o Geodesy
Detailed prospect scale surveys for applications in mapping density contrasts associated with:-
- o Structure – faults and shear zones, brecciation, kimberlitic pipes and diatremes etc
- o Lithology – geometry and thickness.
- o Alteration and alteration assemblages (e.g., propylitic, skarns, greissens)
- o Direct detection of density contrasts associated with mineralized bodies.
- o Ore body geometry and mass estimation
- o Coal outcrops, mining hazards (sills, dykes), structural controls etc
- o Time lapse waterflood surveillance of gas cap water injection in oil production,
- o Time Lapse dewatering and degassing of coals
- o Archaeology
- o Engineering applications (tunnel planning and design)
- o Cavity detection – existing tunnels, mine workings, karsts etc
- o Environmental – buried objects
SURVEY DESIGN AND SPECIFICATION
The configuration, precision and accuracy of a gravity survey will depend on the objective of the survey. The survey design may be constrained by budget, ground conditions, access and timing considerations. Depending on your requirements, IMT Geophysics can work with you to design, specify and cost a survey that matches your survey objectives and constraints.
As part of this process, our associate geophysicist can use ModelVision software to model the desired target and any known local geological attributes of the proposed survey area to determine the required data sensitivity, distribution and spatial extent to ensure detection of the target body.
TYPES OF GRAVITY SURVEYS
Gravity survey methods vary to suit the required data spacing, accuracy, and physical constraints such as topography, vegetation and climatic conditions. Regardless of the style of survey you may need, IMT Geophysics has the resources, expertise and experience to complete your survey quickly and efficiently within specification and budget.
The main styles of gravity survey we offer may be summarized as:-
This style of survey is generally used for regional and semi-regional surveys. A typical field crew comprises a technician and helicopter pilot equipped with a gravity meter and GPS survey instrumentation. The technician is transported from point to point using a helicopter guided by an in-flight GPS navigation system. In high production scenarios, the technician and pilot will be rotated during the day.
Helicopter assisted gravity surveying.
Helicopter gravity surveys require careful planning by experienced operators with a thorough understanding of the technology involved, potential problems, risks and risk management. Properly planned and operated, they are a rapid, low cost non-destructive method of establishing a regional or semi-regional geological framework. They are especially useful in areas with deep regolith cover.
Helicopter borne gravity surveying for Geological Survey of NSW – Riverina Gravity Survey
This style of survey is generally used for semi-regional to detailed surveys. Data may be acquired along roads, tracks and fences or within a grid structure at regular intervals. For safety reasons, the field crew will normally comprise a technician and a driver/operator working from a 4WD vehicle. In some circumstances, the vehicles used may be Quad Bikes but their use is subject to stringent safe operating rules. Instrumentation is similar to that for heliborne work but the survey methods used lean more towards real time short occupation GPS techniques.
Base tie during a vehicle borne 1 x 1 km regional gravity survey of north eastern Tasmania.
Data intervals typically range from as much as 1km for road traverse work down to 25m for grid based work.
Traversing on Foot
There are many circumstances in which topography, vegetation and other restraints or data spacing make it difficult or impossible to effectively use vehicle or helicopter transport for routine gravity observations. Close data spacing can make it more effective to walk the grid lines. In such circumstances, the crew will conduct traverses on foot carrying all their instruments and accessories from point to point.
The crew comprises a gravity technician and a survey technician. Depending on tree cover and topography, the crew may use post process rapid static survey techniques if there are difficulties in maintaining lock to Real Time Kinematic (RTK) corrections. Where survey observations can’t be taken at the gravity station because of GPS signal blockage, we use reflector-less directional laser ranging instruments to accurately coordinate and level gravity stations from nearby GPS measurement points.
Traversing on Foot. Close spacing gravity survey where crew carry all equipment. Southern Highlands, NSW.
Microgravity surveys use precise gravity observation techniques and digital levelling to obtain very accurate, close spaced gravity data which is in turn mostly used for the detection of voids and cavities or other small, localized density contrasts. This type of survey usually focuses on small scale targets in the shallow (tens of metres) subsurface.
Applications include the detection of mine workings and subsidence areas (geohazards); tunnels and karsts, buried valleys and paleochannels, hydrogeology and archaeology. Microgravity surveys map lateral changes in the density of subsurface soil, rock and fluid which is in turn influenced by the size and density of the target.
This non-destructive technique is especially useful in urban areas where cultural features can limit the use of other techniques such as seismic and resistivity. Data intervals vary from as much as 100m down to 2m depending on the size of the target and the anticipated density contrast. High resolution surveys require gravity observations to be made at precision of 0.01μ/s², an accuracy of 0.05μ/s² and elevations accurate to better than 5mm.
Base Line Observations – 4D Microgravity survey to monitor dewatering of coals, QLD.
IMT utilises Scintrex CG5 and LaCoste & Romberg Model G Land Gravity Meters as appropriate to the requirements of the work being undertaken. The adjustment of meters is regularly checked and the meters are tested on Absolute Gravity Ranges before each survey is undertaken as a check of calibration.
Observation of Scintrex CG-5 gravity meter in the course of helicopter borne gravity surveying – South Gasgoyne Region W.A.