Geology Reference
In-Depth Information
+10
0
km
5
0
Fig. 1.1
The gravity anomaly over the
Grand Saline Salt Dome,Texas, USA
(contours in gravity units — see Chapter
6).The stippled area represents the
subcrop of the dome. (Redrawn from
Peters & Dugan 1945.)
N
0
Fig. 1.2
Magnetic anomalies over the
Grand Saline Salt Dome,Texas, USA
(contours in nT — see Chapter 7).The
stippled area represents the subcrop of the
dome. (Redrawn from Peters & Dugan
1945.)
N
0
km
5
any seismic energy incident on the boundary of a salt
body is partitioned into a refracted phase that is transmit-
ted through the salt and a reflected phase that travels back
through the surrounding sediments (Chapter 3). These
two seismic phases provide alternative means of locating
a concealed salt body.
For a series of seismic rays travelling from a single shot
point into a fan of seismic detectors (see Fig. 5.21), rays
transmitted through any intervening salt dome will
travel at a higher average velocity than in the surround-
ing medium and, hence, will arrive relatively early at the
recording site. By means of this 'fan-shooting' it is
possible to delineate sections of ground which are
associated with anomalously short travel times and
which may therefore be underlain by a salt body.
An alternative, and more effective, approach to the
seismic location of salt domes utilizes energy reflected
off the salt, as shown schematically in Fig. 1.3. A survey
