Gravity Surveys

The gravity field at the surface of the Earth is influenced by density variations in the underlying rocks. Rock densities range from less than 2.0 for soft sediments and coals to more than 3.0 for mafic and ultramafic rocks. Many ore minerals, particularly metal sulfides and oxides, are very much denser than the minerals that make up the bulk of most rocks, and orebodies are thus often denser than their surroundings. However, the actual effects are tiny, generally amounting even in the case of large massive sulfide deposits to less than 1 part per million of the Earth’s total field (i.e. 1 mgal, or 10−6 m/s).

Gravity surveys measure lateral changes in the density of subsurface rocks. The instrument used, called a gravimeter, is in effect an extremely sensitive weighing machine. By weighing a standard mass at a series of survey stations, the gravimeter detects minute changes in gravity caused by crustal density differences. Maps of gravity variation can hence be used to map subsurface distribution of rocks and structures, including the anomalous density distributions that might be associated with concealed ore.

Given the small differences differences of the gravity field brought about by even large orebodies, gravity meters are required to be extremely sensitive, a requirement which to some extent conflicts with the need for them also to be rugged and field-worthy. Gravity meters measure only gravity differences and are subject to drift, so that surveys involve repeated references to base stations. Manual instruments are relatively difficult to read, with even experienced observers needing about a minute for each reading, while the automatic instruments now becoming popular require a similar time to stabilize. Because of the slow rate of coverage, gravity surveys are more often used to follow up anomalies detected by other methods than to obtain systematic coverage of large areas.

A very wide range of geological situations give rise to zones of anomalous mass that produce significant gravity anomalies. On a small scale, buried relief on a bedrock surface, such as a buried valley, can give rise to measurable anomalies. On a larger scale, small negative anomalies are associated with salt domes. On a larger scale still, major gravity anomalies are generated by granite plutons or sedimentary basins. Interpretation of gravity anomalies allows an assessment to be made of the probable depth and shape of the causative body.

To provide usable data, raw gravity measurements need to be corrected. The first correction (for short-term drift in the instrument) is provided by regular reading of a base station in much the same manner as a magnetic survey. The second correction compensates for the broad scale variations in the earth’s gravitational field – this correction is only significant in regional surveys. The third correction, much the most important one, corrects for differences in gravity caused by variation in the elevation of the survey station above a datum, usually sea level. To make this correction, stations need to be leveled with great precision – in the case of a very broad regional survey to at least one meter; in the case of a detailed survey aimed at direct ore location, to correspondingly greater accuracies, down to centimetre scale.

The costs involved in the very accurate surveying necessary for altitude correction has, until recently, generally restricted the use of gravity surveys in mineral exploration to low-density, broad-scale, regional coverage. However, differential GPS (DGPS) surveying now allows rapid and relatively cheap levelling of stations and has made detailed gravity surveys comparable in cost to that of ground-magnetic surveys.

A good example of the successful use of a gravity survey as an aid in ore discovery is the location of the high-grade Hishikari epithermal gold deposit of Japan (Izawa et al., 1990). Here, a detailed gravity survey was used to define a buried mineralized structure in an area of known mineralization. The key to the successful use of the technique in this case was the high degree of understanding of the local geology and mineralization, which was used in the design and interpretation of the survey. Gravity surveys (along with regional aeromagnetic data) also played 150 9 Geophysical and Geochemical Methods a significant part in the discovery of the giant deeply buried Olympic Dam (Rutter and Esdale, 1985) and Prominent Hill (Belperio et al., 2007) IOCG deposits of South Australia.

Modelling of the gravity response for the likely range of size, depth, and SG of targets is an important reality check before the technique is used for direct exploration. In some Australian regolith settings, undefined geometries of variable density material in the regolith can produce ambiguous gravity results and spurious anomalies.

Applications of Gravimetric Surveys and their Requirements

Gravimetric surveys serve a variety of scientific and economic objectives. Quite often they are employed as the sole, preferred method of solving a particular problem, at other times they are used in combination with other methods, based on other physical properties. These objectives include: regional geological mapping; petroleum exploration; mineral exploration; geotechnical studies; archaeological studies; groundwater and environmental studies; tectonic studies; volcanology and geothermal studies. What follows is a brief description of these applications, particularly the precision of gravity measurements that may be required for each applibation, and other survey specifications.

REGIONAL GEOLOGICAL MAPPING: Gravimetric surveys for regional geological mapping characteristically demand measurements on a 5–10 km grid. The ultimate accuracy of individual regional gravity data is usually limited by the accuracy of the elevation control on the station, commonly 1–2 m, i.e. equivalent to ±0.2 to 0.4 mGals in gravity.

The purpose of such regional gravimetric surveys is to provide geological information on the distribution of major rock units, and their tectonics, and information about the Earth's crust. The dimensions of features resolvable by such surveys are usually in excess of about 20 km. Gravimeters with a reading resolution of 0.01 mGal and accuracy of the same order are more than adequate for this purpose, since the limitation on the elevation control is usually the limiting factor on the ultimate accuracy of the gravity data so determined.

PETROLEUM EXPLORATION: The objectives of gravimetric surveys in petroleum exploration include the mapping of sedimentary basins and their tectonic features. Stations for petroleum exploration surveys will be laid out on grid centres whose separation is predicated on the dimensions of the targeted structures. If the general shape of a large sedimentary basin is the objective of the survey, then the stations may be 2 to 5 km apart. If detail is required to accurately define a salt dome or a fault structure, then stations may be 100m to 500m apart. ]Similarly, the measurement accuracy required of the gravity measurement will be higher for such detailed surveys than for regional surveys. Gravimeters with a reading resolution of 1 µGal and an accuracy of about 5 µGals are preferred for detailed petroleum surveys. Sometimes subtle changes in gravity, e.g. of the order of 10 µGals, may have to be resolved, reflecting a relatively minor structural feature or facies change in the subsurface. For the same objective, it will be necessary to determine the relative elevation of each gravity station to within ±2cm.

MINERAL EXPLORATION: Gravity surveys are employed in mineral exploration, to provide the basic geological information about possible host rocks and their controlling tectonic features as well as to provide a direct indication of the presence of mineral deposits. The latter possibility may occur when the densities of the target minerals, or mineral deposit of economic interest, are significantly different from their host rocks. For example, deposits of iron, chrome, base metals and barite may fall in this category, being of relatively high specific gravity, as well as salt and coal, being of relatively low specific gravity compared to the usual range of density in most rock types.

Since the dimensions of mineral exploration ore deposit targets are usually of the order of a few hundred metres, the gravimeter stations for mineral exploration are often only 25–30m apart, on lines that may be only 100m apart. High precision in the gravity measurements and in the station elevation control is generally useful and sometimes, absolutely necessary, for such surveys (i.e. reading resolution of 1 µGal, relative accuracy of 5 µGals and relative elevation accuracy of ±2 cm).

GEOTECHNICAL AND ARCHAEOLOGICAL STUDIES: Geotechnical studies to which gravimetric surveys may be applied include: the mapping of subsurface cavities in karst areas or old mining camps, whether open or filled with water or clay; the mapping of overburden variations, particularly in urbanized areas; the mapping of tectonic features, i.e. faults and major shear zones, and the mapping of railway roadbeds to locate cavities or loose sections.

Archaeological studies are often concerned with the same objectives, namely the mapping of subsurface voids and overburden variations.

For all such purposes, it is imperative to be able to achieve highly accurate gravity data. Grid stations as close as 25–50m (or even at 3m intervals for detail) may be employed for such purposes.

GROUNDWATER AND ENVIRONMENTAL STUDIES: These are similar in specifications to those just listed above, but their objectives differ. They may help to map aquifers which are formationally or structurally controlled, or to determine the extent of old landfill sites on which documentation is lacking.

TECTONIC STUDIES: In regions of major, active, tectonic movement, the buildup of stress may be reflected in a warping of the ground surface over long periods of time. These stresses may, ultimately, lead to sudden release in the form of an earthquake. Very carefully controlled gravity measurements, carried out periodically, e.g. at one year intervals, on a grid of permanent stations in the area, can provide a sensitive measurement of such ground warping. The gravity values on the grid stations are measured relative to one or more stations which are deemed to be stable, with 1 µGal resolution and better than 5 µGal accuracy, on a stable pillar at each station.

In this manner, tectonically derived changes, in relative land elevations, of less than 3 cm may be determined.

VOLCANOLOGY AND GEOTHERMAL STUDIES: Periodic gravity measurements on a grid of permanent stations can provide useful information on changes which are taking place, with time, in respect of the upwelling of lava within a volcano, and therefore, provide a forewarning of harmful volcanic eruptions.

Likewise, in the case of geothermal fields which are under exploitation for energy production, periodic gravity measurements, at permanent stations, will provide useful information on changes in the level of water in the geothermal reservoir, and therefore information on the longevity of the geothermal resource.

In both types of applications, high precision gravity measurements are required, for maximum sensitivity.

 

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