Gravity is a useful geophysical tool for mapping subsurface differences in rock density for the exploration, geotechnical or hydrogeology industry sectors. Gravity surveying is a non-evasive, non-destructive method that is relatively fast to acquire which makes this method an attractive geophysical option in order to construct large regional or more refined investigation grids.
Measurements in gals (commonly milligal, mgal, for regional gravity) of gravity are acquired using a gravimeter. Gravity readings/stations are acquired generally along profile lines or in a grid. the gravity station data are acquired at regular time intervals, repeat visits to a fixed, known gravity station are also carried out to assist with connecting the gravity investigation area to the known local/regional gravity stations. This repeat readings are also taken at a base station location and the readings taken at the base station assist with instrumental drift corrections.
Gravity readings taken at a station location are influenced by a number of anomalies made up of both regional and local affects. These regional and local affects are corrected for, other corrections include latitude, Bouguer correction (mass effect) plus free-air elevation. The residual gravity data that remains is generally presented in the form of a contour map and can be modelled in 2D (or 2.5D) with interpreted densities of geology to fit a model curve to the residual gravity result. An example image of a gravity data contour map can be viewed in Figure 1.
Figure 1. An example of a residual Bouguer gravity map (image source. Cunion, E., (2009). “Comparison of ground TEM and VTEM responses over kimberlites in the Kalahari of Botswana”, Exploration Geophysics, 40, 308-319)
Gravity investigations can be carried out in a number of environments which would generally be detrimental to many geophysical technologies i.e. areas of electromagnetic and electrical noise (e.g. inside buildings, in urban areas). Draig Geoscience can provide gravity investigations customised to the requirements of the application. Gravity data can also be correlated with Ground Magnetics, airborne electromagnetics, seismic reflection and other geophysical data. Where site access is appropriate, gravity equipment can be carried by hand or quad bike and consequently the data can be collected very quickly.
Limitations of the gravity technique include:
- A strong density contrast is required between modelled geological layers/bodies
- The gravity meter is an extremely sensitive piece of equipment and can influenced by seismic noise and ground vibrations, so acquisition should be carried out under seismically quiet conditions (i.e. away from vehicle traffic, wind, rain etc.)
- Gravity measurements are also influenced by topography, depending on objectives topographic corrections may be required
Gravity surveying could be used together with a complimentary geophysical technology (e.g. airborne gravity, aero-mag, ground-mag, Seismic Reflection), conventional mapping and drilling information.
The Earth’s magnetic field interacts with rock, or other subsurface materials, which causes variations in the Earth’s magnetic field received at the Earth’s surface. The magnetic susceptibility of the geology has a large influence on the induced magnetic field (i.e. a high-susceptibility body will produce a stronger induced field than a low susceptibility body).
Ground Mag measures the physical parameter described as the Total Magnetic Intensity (TMI) and this is measured in units of nano-Tesla (nT) using a magnetometer. Steel and other ferrous metals in the vicinity of a magnetometer
can also distort the recorded data (so metal objects should be removed from clothing when recording). The strength of the TMI field is dependent on location (geographically) and varies with distance from the Earth’s centre (elevation) and with time. These factors are removed during the data processing phase. Reduction to the pole (RTP) is a transformation of the observed data with simulates the magnetic field distribution which could be observed if the inducing field were vertical (i.e. at the geomagnetic pole). The RTP transformation places magnetic highs directly over their causative bodies, and simplifies qualitative interpretation of data from moderate to low geomagnetic latitudes. The instabilities in the RTP process begin to become significant for geomagnetic latitudes <20 degrees, and generates strongest artefacts in the direction of magnetic declination (Rajagopalan, 2003). Further processing steps can also be performed on the data i.e. First Vertical Derivative (1VD), Analytical Signal (Ansig), to enhance the techniques visualisation of certain features. The processed magnetic maps (TMI, RTP) can be further modelled in 2D, by modelling the data response along a profile line with a geologic body of an inferred magnetic susceptibility. The model can be further refined with known drilling results and other available geophysical data. An example of a ground-mag contour map can be viewed in Figure 1.
Figure 1. An example of a Total Magnetic Intensity (TMI) map (image source. Cunion, E., (2009). “Comparison of ground TEM and VTEM responses over kimberlites in the Kalahari of Botswana”, Exploration Geophysics, 40, 308-319)
Ground-mag is a useful geophysical technology, especially in mineral exploration programmes. It is logis2tically straightforward to deploy, and provides detailed mapping of targeted zones very rapidly. Ground-mag is generally acquired on foot, but magnetic data can also be collected in an airborne (aero-mag) or marine environment (marine-mag).
Limitations of the ground-mag technique include:
- The magnetic method only responds to variations in the magnetic properties of the Earth
- Highly magnetic geologic or modern materials may obscure subtle features of interest
Ground-mag surveying could be used together with a complimentary geophysical technology (e.g. aero-mag, Gravity), conventional mapping, drilling, and indeed any other available source of information.
The fluxgate magnetometry tool operates on the two fundamental assumptions. First, the foundation pile contains imbedded ferromagnetic material, such as reinforcement bars. Second, such ferromagnetic material generates an observable, interpretable induced magnetic field relative to the background earth’s geomagnetic field (Jo et al., 2003). An example illustration of a downhole fluxgate magnetometer set-up is shown in Figure 1.
Figure 1. An example image of a 3-component downhole fluxgate magnetometer set-up (Image source. Jo et al. (2003). “A borehole magnetic logging tool for estimating unknown foundation depths”. Presented at the 3rd International Conference on Applied Geophysics, Dec 8-12)
If the surrounding soils are generally non-ferromagnetic (a smaller induced magnetic field) then a relatively strong induced magnetic field can be observed from the ferromagnetic steel rebar in response to the earth’s magnetic field. The base of a foundation pile containing ferromagnetic material (e.g. steel) should be observed as a strong response in the data and the base of the foundation pile can be estimated. An example of the magnetic response calibrated against a known foundation pile depth is shown in Figure 2.
Figure 2. Example of the vertical (black line) and horizontal (red line) magnetic response from a ferromagnetic foundation pile. The Vertical Derivative of the vertical component (green line) provides an indication of the steel foot of the foundation pile (Image source: Jo et al., 2003).
Image Reference: Jo, C.H., Cha, Y.H., & Choi, J.H. (2003). “A Borehole Magnetic Logging Tool for Estimating Unknown Foundation Depths”. Presented at the 3rd International Conference on Applied Geophysics, Orlando, USA. December 8-12, 2003.
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