Geology Reference
In-Depth Information
gu
1000
Free-air anomaly
0
-1000
Ridge crest
0
Crust
Low density zone
40
Mantle
80
km
0
1000 km
Fig. 6.25
Free-air anomaly profile across the mid-Atlantic ridge. (After Talwani
et al.
1965.)
On the medium scale, gravity anomalies can reveal the
subsurface form of igneous intrusions such as granite
batholiths and anorthosite massifs. For example, gravity
surveys in southwest England (Bott
et al.
1958) have
revealed a belt of large-amplitude, negative Bouguer
anomalies overlying a region of outcropping granites
(Fig. 6.27). Modelling of the gravity anomalies (Fig.
6.23) has led to the postulation of a continuous batholith
some 10-15 km thick underlying southwest England
(see e.g. Brooks
et al.
1983). Studies such as these have
provided important constraints on the mechanism of
emplacement, composition and origin of igneous
bodies. Similarly, gravity surveying has been extensively
used in the location of sedimentary basins, and their in-
terpreted structures have provided important informa-
tion on mechanisms of basin formation.
The gravity method was once extensively used by the
petroleum industry for the location of possible hydro-
carbon traps, but the subsequent vast improvement in
efficiency and technology of seismic surveying has led to
the demise of gravity surveying as a primary exploration
tool.
In commercial applications, gravity surveying is rarely
used in reconnaissance exploration. This is because the
method is relatively slow to execute, and therefore ex-
pensive, due to the necessity of accurately determined
elevations and the length of the reduction procedure.
Gravity methods do find application, however, as a
follow-up technique used on a target defined by
6.12 Applications of gravity surveying
Gravity studies are used extensively in the investiga-
tion of large- and medium-scale geological structures
(Paterson & Reeves 1985). Early marine surveys, per-
formed from submarines, indicated the existence of
large positive and negative gravity anomalies associated
with island arcs and oceanic trenches, respectively; sub-
sequent shipborne work has demonstrated their lateral
continuity and has shown that most of the major features
of the Earth's surface can be delineated by gravity sur-
veying. Gravity anomalies have also shown that most of
these major relief features are in isostatic equilibrium,
suggesting that the lithosphere is not capable of sustain-
ing significant loads and yields isostatically to any change
in surface loading. Figure 6.25 shows the near-zero free-
air anomalies over an ocean ridge which suggest that it
is in isostatic equilibrium. The gravity interpretation,
which is constrained by seismic refraction results, indi-
cates that this compensation takes the form of a zone of
mass deficiency in the underlying mantle. Its low seismic
velocity and the high heat flow at the surface suggest that
this is a region of partial melting and, perhaps, hydration.
Gravity surveying can also be used in the study of ancient
suture zones, which are interpreted as the sites of former
plate boundaries within the continental lithosphere.
These zones are often characterized by major linear
gravity anomalies resulting from the different crustal
sections juxtaposed across the sutures (Fig. 6.26).
