GRAVITY METHOD - Field Geophysics - page 41

of the atmosphere, given by:
δ g = 0 . 874 − 0 . 000 099 h + 0 . 000 000 003 56 h 2 mGal
Provided this correction is included (and it is all too often overlooked),
the actual differences in theoretical gravity between the 1967 and 1980
formulae are usually smaller than the errors in absolute gravity values at
individual gravity stations. Since no changes in base station values are
required, the changeover has been widely regarded as not urgent and is
proceeding only slowly. The reluctance of many organisations to move to
the new standard is increased by the fact that further refinements are being
proposed almost yearly, although with negligible implications for practical
gravity processing.
Gravity measurements are useful because subsurface changes can produce
measurable deviations from the theoretical field. A major sedimentary basin
can reduce the gravity field by more than 100 mGal, while targets such
as massive ore bodies may produce anomalies of the order of a milliGal.
The effects of caves and artificial cavities such as mine workings, even
when very close to the surface, are usually even smaller. Gravity differences
may therefore have to be measured to accuracies of at least 0.01 mGal
(approximately one-hundred-millionth of the Earth's field), and this is the
sensitivity of most manual instruments. Automatic meters and the so-called
'micro-gravity meters' have readout precisions of a microGal (µGal), but
even their manufacturers do not claim them to be consistently accurate to
better than about 3 µGal.
Topographic effects may be much larger. Elevation alone produces a
gravity difference of nearly 2000 mGal between sea level and the summit of
Mt Everest.
2.1.2 Rock density
The SI unit of density is the kg m − 3 ,buttheMgm − 3 is widely used because
the values are, numerically, the same as those in the c.g.s. system, in which
water has unit density. The density ranges for some common materials are
shown in the first column of Table 1.2. Most crustal rocks have densities
of between 2.0 and 2.9 Mg m − 3 . A density of 2.67 Mg m − 3 was adopted
as standard for the upper crust in the early days of gravity work, and is
still widely used in modelling and in calculating elevation corrections for
standardised gravity maps.
2.2 Gravity Meters
For the past 70 years the vast majority of gravity measurements on land have
been made using meters with unstable ( astatic ) spring systems, and this