| Microgravity investigations are comparatively rarely used for searching of hidden ancient
targets (e.g., Eppelbaum, 2013). It is caused mainly by small geometric size of the desired
archaeological objects and various types of noise complicating the observed useful signal. At
the same time, development of modern generation of field gravimetric equipment allows to
register microGal (10-8 m/s2) anomalies that offer a new challenge in this direction.
Correspondingly, an accuracy of gravity variometers (gradientometers) is also sharply
increased.
How we can improve the interpretation effectiveness and reliability? Undoubtedly, it must
be a multi-stage process. I believe that we must begin since nonconventional methodologies
for reducing topographic effect and terrain correction computation.
Topographic effect reducing
The possibilities of reducing topographic effects by grouping the points of additional
gravimetric observations around the central point located on the survey network were
demonstrated in (Khesin et al., 1996). A group of 4 to 8 additional points is located above and
below along the relief approximately symmetrically and equidistant from the central point.
The topographic effect is reduced to the obtained difference between the gravity field in the
center of the group and its mean value for the whole group. Application of this methodology
in the gold-pyrite deposit Gyzyl-Bulakh (Lesser Caucasus, western Azerbaijan) indicated its
effectiveness.
Computation of terrain correction
Some geophysicists compare the new ideas in the field of terrain correction (TC) in
gravimetry with the “perpetuum mobile” invention. However, when we speak about very
detailed gravity observations, the problem of most optimal computation of surrounding relief
influence is of a great importance. Let us will consider two approaches applied earlier in ore
geophysics.
First approach
A first method was applied in the Gyzyl-Bulakh gold-pyrite deposit situated in the
Mekhmana ore region of the Lesser Caucasus (western Azerbaijan) under conditions of
rugged relief and complex geology. This deposit is well investigated by mining and drilling
operations and therefore was used as a reference field polygon for testing this approach. A
special scheme for obtaining the Bouguer anomalies has been employed to suppress the
terrain relief effects dampening the anomaly effects from the objects of prospecting. The
scheme is based on calculating the difference between the free-air anomaly and the gravity
field determined from a 3D model of a uniform medium with a real topography. 3-D
terrain relief model with an interval of its description of 80 km (the investigated 6
profiles of 800 m length are in the center of this interval) was employed to compute
(by the use of GSFC software (Khesin et al., 1996)) the gravitational effect of the
medium (Ïă = 2670 kg/m3). With applying such a scheme the Bouguer anomalies
were obtained with accuracy in two times higher than that of TC received by the
conventional methods. As a result, on the basis of the improved Bouguer gravity with the
precise TC data, the geological structure of the deposit was defined (Khesin et al.,
1996).
Second approach
Second approach was employed at the complex Katekh pyrite-polymetallic deposit,
which is located at the southern slope of the Greater Caucasus (northern Azerbaijan). The
main peculiarities of this area are very rugged topography of SW-NE trend, complex geology
and severe tectonics. Despite the availability of conventional δgB (TC far zones were
computed up to 200 km), for the enhanced calculation of surrounding terrain topography a
digital terrain relief model was created (Eppelbaum and Khesin, 2004). The SW-NE regional
topography trend in the area of the Katekh deposit occurrence was computed as a rectangular
digital terrain relief model (DTRM) of 20 km long and 600 m wide (our interpretation profile
with a length of 800 m was located in the geometrical center of the DTRM). As a whole,
about 1000 characteristic points were used to describe the DTRM (most frequently
points were focused in the center of the DTRM and more rarely – on the margins).
Thus, in the interactive 3D δgB modeling (by the use of GSFC software) was
computed effect not only from geological bodies occurring in this area, but also from
surrounding DTRM. In the issue of this scheme application, two new ore bodies were
discovered.
Quantitative analysis of gravity anomalies
The trivial formulas of quantitative analysis (based on simple relationships between the
gravity field intensity and geometrical parameters of the anomalous body) are widely
presented in the geophysical literature (e.g., Telford et al., 1993; Parasnis, 1997). However,
absence of reliable information about the normal gravity field in the studied areas strongly
limits practical application of these methods.
Gravity field intensity F is expressed as
F = - gradW,
(1)
where W is the gravity potential.
For anomalous magnetic field Ua we can write (when magnetic susceptibility ≈¤ 0.1 SI
unit) (Khesin et al., 1996):
Ua = - gradV,
(2)
where V represents the magnetic potential.
Let’s consider analytical expressions of some typical models employed in magnetic and
gravity fields (Table 1).
Table 1. Comparison of some analytical expressions for magnetic and gravity fields
Field Analytical expression
MagneticThin bed (TB)
z
Zv = 2I2b–2–-2
x + z
(3)
Point source (rod)
mz
Zv = –––-3/2
(x2 + z2)
(4)
Gravity Horizontal Circular Cylinder
(HCC)
–z––
δg = 2GÏă x2 + z2
(5)
Sphere
––z––-
δg = GM (x2 + z2)3/2
(6) |