104
The Last Resource
stratum to another—some of the energy is reected back to the
surface. These ‘echoes’ are picked up by ‘geophones’ to give
recordings which, with suitable processing, can be built up into a
picture of the various layers and structures beneath the surface.
Seismic surveying on the land is a laborious process involving
carrying large amounts of equipment from one place to another,
drilling holes for the explosive used to create the shock waves and
perhaps skirting round obstacles such as rivers, lakes and objecting
land—owners. At sea the job is much easier. The seismic charges are
detonated just beneath the surface, movement from place to place is
easier and, by paying out the cables carrying the geophones at the
same
rate as the forward Speed of the boat, the cable remains
stationary during the recording of the echoes although the recording
craft can be making a steady speed all day (see gure 9). Small
wonder then that the cost per mile of seismic surveying at sea is only
about a quarter of that on the land and that sixty miles or more can
be covered a day.
ln marine seismic systems a wide variety of energy sources other
than conventional explosive charges have been used, including rapid
discharges of compressed air, gas ‘poppers’ created by an explosion
of a gas/air mixture, exploding wires caused by a sudden release of
electrical energy into a thin wire, ‘sparkers’ where an open spark
passes through the water, and ‘boomers’ in which an electrical force
propels an aluminium plate rapidly against the water. All produce a
shock wave which travels through the water and into the sea—oor.
The development of such sonar proling devices has been one of
the major developments in geophysical exploration over the past
decade or so. They have been employed in anything from mapping
sediments to surveys of the English Channel as part of investigations
into building the Channel Tunnel.
One of the earliest uses of sonar, however, was related to work of a
different nature but which has also come to play a part in mineral
surveying—deep—sea photography. Once the technical problems of
building undersea cameras and providing them with appropriate
light sources had been solved, there arose the question of controlling
their position relative to the bottom. The solution was to use a sonar
‘pinger’ which transmitted a direct signal giving the depth of the
camera and a reected signal giving the depth of the sea oor. In this
way the height of the camera could be controlled and the camera
prevented from being held too high or, conversely, from burying
The Last Resource
stratum to another—some of the energy is reected back to the
surface. These ‘echoes’ are picked up by ‘geophones’ to give
recordings which, with suitable processing, can be built up into a
picture of the various layers and structures beneath the surface.
Seismic surveying on the land is a laborious process involving
carrying large amounts of equipment from one place to another,
drilling holes for the explosive used to create the shock waves and
perhaps skirting round obstacles such as rivers, lakes and objecting
land—owners. At sea the job is much easier. The seismic charges are
detonated just beneath the surface, movement from place to place is
easier and, by paying out the cables carrying the geophones at the
same
rate as the forward Speed of the boat, the cable remains
stationary during the recording of the echoes although the recording
craft can be making a steady speed all day (see gure 9). Small
wonder then that the cost per mile of seismic surveying at sea is only
about a quarter of that on the land and that sixty miles or more can
be covered a day.
ln marine seismic systems a wide variety of energy sources other
than conventional explosive charges have been used, including rapid
discharges of compressed air, gas ‘poppers’ created by an explosion
of a gas/air mixture, exploding wires caused by a sudden release of
electrical energy into a thin wire, ‘sparkers’ where an open spark
passes through the water, and ‘boomers’ in which an electrical force
propels an aluminium plate rapidly against the water. All produce a
shock wave which travels through the water and into the sea—oor.
The development of such sonar proling devices has been one of
the major developments in geophysical exploration over the past
decade or so. They have been employed in anything from mapping
sediments to surveys of the English Channel as part of investigations
into building the Channel Tunnel.
One of the earliest uses of sonar, however, was related to work of a
different nature but which has also come to play a part in mineral
surveying—deep—sea photography. Once the technical problems of
building undersea cameras and providing them with appropriate
light sources had been solved, there arose the question of controlling
their position relative to the bottom. The solution was to use a sonar
‘pinger’ which transmitted a direct signal giving the depth of the
camera and a reected signal giving the depth of the sea oor. In this
way the height of the camera could be controlled and the camera
prevented from being held too high or, conversely, from burying