GRAVITY METHOD OF MINERAL EXPLORATION



GRAVITY METHOD


Introduction
Geophysical prospecting, except in its simplest forms, involves specialised techniques and gives consistent results to a geophycist having thorough understanding of the principles and construction of the instruments and of the mathematics of interpreting the results.
The principal methods used in geophysical exploration are:
·       Magmatic methods      -    for magnetic susceptibility in
      rocks.
·       Electrical methods      -    for electrical conductivity and   
     resistivity in rocks.
·       Gravity method           -   for variation in density of
                                             rocks in the earth.
·       Siesmic method          -   for elasticity of rocks.


Gravity measurements define anomalous density within the Earth; in most cases, ground-based gravimeters are used to precisely measure variations in the gravity field at different points. Gravity anomalies are computed by subtracting a regional field from the measured field, which result in gravitational anomalies that correlate with source body density variations. Positive gravity anomalies are associated with shallow high density bodies, whereas gravity lows are associated with shallow low density bodies. Thus, deposits of high-density chromite, hematite, and barite yield gravity highs, whereas deposits of low-density halite, weathered kimberlite, and diatomaceous earth yield gravity lows. The gravity method also enables a prediction of the total anomalous mass (ore tonnage) responsible for an anomaly. Gravity and magnetic (discussed below) methods detect only lateral contrasts in density or magnetization, respectively. In contrast, electrical and seismic methods can detect vertical, as well as lateral, contrasts of resistivity and velocity or reflectivity. Applications of gravity to mineral deposit environmental considerations includes identification of lithologies, structures, and, at times, ore bodies themselves .Small anomalous bodies, such as underground workings, are not easily detected by gravity surveys unless they are at shallow depth.
The Gravity Geophysical Method
For human exploration of the solar system, instruments must meet criteria of low mass, low volume, low power demand, safe operation, and ruggedness and reliability (Meyer et al., 1995; Hoffman, 1997; Budden, 1999). Tools used for planetary exploration will need to address fundamental scientific questions and identify precious resources, such as water.
The primary goal of studying detailed gravity data is to provide a better understanding of the subsurface geology. The gravity method is a relatively cheap, non-invasive, non-destructive remote sensing method that has already been tested on the lunar surface. It is also passive – that is, no energy need be put into the ground in order to acquire data; thus, the method is well suited to a populated setting such as Taos, and a remote setting such as Mars. The small portable instrument also permits walking traverses – ideal, in view of the congested tourist traffic in Taos.
Measurements of gravity provide information about densities of rocks underground. There is a wide range in density among rock types, and therefore geologists can make inferences about the distribution of strata. In the Taos Valley, we are attempting to map subsurface faults. Because faults commonly juxtapose rocks of differing densities, the gravity method is an excellent exploration choice.

Gravity Survey - Measurements of the gravitational field at a series of different locations over an area of interest. The objective in exploration work is to associate variations with differences in the distribution of densities and hence rock types.

This is a generalized summary of the types of corrections that we have applied to the Taos gravity data:
The Gal (for Galieo) is the cgs unit for acceleration where one Gal equals 1 centimenter per second squared. Because variations in gravity are very small, units for gravity surveys are generally in milligals (mGal) where 1 mGal is one thousandth of 1cm/s2. Standard gravity is taken as the freefall accelleration of an object at sea level and at a latitude of 45.5° and is 9.80665 m/s2 (or equivalently 980.665 mGal).
Observed Gravity (gobs ) - Gravity readings observed at each gravity station after corrections have been applied for instrument drift and earth tides.
Latitude Correction (gn ) - Correction subtracted from gobs that accounts for Earth's elliptical shape and rotation. The gravity value that would be observed if Earth were a perfect (no geologic or topographic complexities), rotating ellipsoid is referred to as the normal gravity.
gn = 978031.85 (1.0 + 0.005278895 sin2(lat) + 0.000023462 sin4(lat)) (mGal)  where lat is latitude
Free Air Corrected Gravity (gfa ) - The free-air correction accounts for gravity variations caused by elevation differences in the observation locations. The form of the Free-Air gravity anomaly, gfa , is given by:
                         gfa = gobs - gn+ 0.3086h (mGal)
where h is the elevation (in meters) at which the         gravity station is above the datum (typically sea level).
Bouguer Slab Corrected Gravity (gb ) - The Bouguer correction is a first-order correction to account for the excess mass underlying observation points located at elevations higher than the elevation datum (sea level or the geoid). Conversely, it accounts for a mass deficiency at observation points located below the elevation datum. The form of the Bouguer gravity anomaly, gb, is given by:
                 gb = gobs - gn + 0.3086h - 0.04193r h (mGal)
where r is the average density of the rocks                  underlying the survey area.
Terrain Corrected Bouguer Gravity (gt ) - The Terrain correction accounts for variations in the observed gravitational acceleration caused by variations in topography near each observation point. Because of the assumptions made during the Bouguer Slab correction, the terrain correction is positive regardless of whether the local topography consists of a mountain or a valley. The form of the Terrain corrected, Bouguer gravity anomaly, gt , is given by:
gt = gobs - gn + 0.3086h - 0.04193r h + TC (mGal)
where TC is the value of the computed Terrain correction.
Assuming these corrections have accurately accounted for the variations in gravitational acceleration they were intended to account for, any remaining variations in the gravitational acceleration associated with the Terrain Corrected Bouguer Gravity can be assumed to be caused by geologic structure.





Application of the gravity method to iron ore exploration
The gravity method has played an increasingly important role in the search for new reserves of Fe ores since the development of highly portable gravimeters capable of a high degree of precision. This method has been used in the search for and study of direct shipping ores, but it has proven to be especially useful in the study of large tonnage, wide, near surface "taconite type" ore bodies that have been the primary concern of the Fe ore industry during the past decade. The gravity method was first applied to Fe ore exploration as a tool for detecting nonmagnetic ores, but advantages of this method over other exploration methods have also made it useful under certain geological conditions in the study of magnetic ores and regional structures favorable for the occurrence of Fe ore. However, the gravity method is restricted by several limitations that must be realized and understood if the application of the method is to be successful. In addition, the full utilization of the method is dependent on a complete understanding of the density relationships of ores and their contrast with the country rocks. This is made particularly difficult by the wide range of densities of Fe ores that can lead to the association of both positive and negative gravity anomalies with Fe ore bodies. The end result is that the amount and quality of information interpreted from the results of gravity surveying is a direct function of the auxiliary geological information available either through geological or other geophysical studies.

SUMMARY


Many geophysical methods commonly used in exploration have potential application to geoenvironmental investigations. Although these methods have mainly been used to identify pollutants and record their dispersion from mine areas, their application is not limited to studies of this sort. For instance, geophysical monitoring of pollutant activity, which requires significantly greater study, is another aspect of geoenvironmental investigations. Monitoring differs from detection chiefly in recurrent use of geophysical techniques. The effort required to extend application of geophysical techniques to naturally occurring pollutants related to mineralized, but unmined, rock or to other cultural concentrations of toxic or potentially toxic substances is minimal and could be of considerable assistance in meeting national needs for healthy environmental conditions.






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