Geology Reference
In-Depth Information
Figure 11.7
Weight-drop contact switch. On impact the inertia of the bolt
compresses the spring and contact is made with the upper surface of the
weight.
drop is not absolutely straight. A crude but more reliable home-made device
that can be attached to any dropping weight is shown in Figure 11.7.
Time-break pulses may be strong enough to produce interference on other
channels (
cross-talk
; see Section 11.3.5). Trigger cables and circuits should
therefore be kept well away from data lines.
11.3 Detection of Seismic Waves
Land seismic detectors are known as
geophones
, marine detectors as
hydro-
phones
. Both convert mechanical energy into electrical signals. Geophones
are usually positioned by pushing a spike screwed to the casing firmly into
the ground, but it may be necessary to unscrew the spike and use some form
of adhesive pad or putty when working on bare rock. Hydrophones are
simply towed behind boats.
11.3.1 Geophones
A geophone consists of a coil wound on a high-permeability magnetic core
and suspended by leaf springs in the field of a permanent magnet (Figure
11.8). If the coil moves relative to the magnet, voltages are induced and
current will flow in any external circuit. The current is proportional to the
velocity of the coil through the magnetic field, so that ground movements
are recorded, not ground displacements. In most cases the coil is mounted so
that it is free to vibrate vertically, since this gives the maximum sensitivity to
P-waves rising steeply from subsurface interfaces, i.e. to reflected and crit-
ically refracted (but not direct) P-waves. These geophones, when normally
