Gerard Dunford gave us an extremely interesting and informative
illustrated talk about Geological /Geophysical Applications of Seismic
Data and it’s interpretation.
Seismic waves are waves of energy that travel through the Earth . They
can result from an earthquake, volcanic activity, movement of magma,
landslides, or a large man-made explosion that produces low-frequency
acoustic energy.
Gerard told us how his job was to interpret this data to find oil reserves.
There are two main types of wave P (Primary) compressional waves that
travel longitudinally and slower moving S (Secondary) transverse waves
that displace the ground at right angles to the direction of travel. P waves
can travel through any type of material whereas S waves stop at a liquid
boundary.
For exploration on land, waves can be created manually by vibroseis
trucks, by banging steel plates onto the ground or by explosives. Marine
exploration makes use of ships dragging an array of 48 or 96 compressed
air guns behind them, These air gun waves penetrate deep into the sea bed
reflecting back information that is picked up by hydrophones on the
surface.
The rays bend as they pass through rock layers (strata) of differing
densities that may be more or less porous and thus capable of holding oil
in the cavities. Both wave velocities and rock densities need to be known.
These records are then stacked and migrated using powerful computers to
give a detailed and accurate model of the times taken in milliseconds for
the waves to return to the surface. Modern digital colour display is used to
record the amplitude of the seismic waves. This can then be translated into
a map of the strata.
Oil is found trapped in the porous rocks as a result of several geological
structures. Anticlines, fault lines, salt domes and stratigraphic traps where
layers of permeable rocks meet an impermeable seam.
Modern 3D data provides far more accurate mapping that is represented in
the form of cubic models. Exploration using electric tools lowered down
existing boreholes to record the properties of the strata making up the sides
of the drilling wall can be extended outwards and away from the borehole
using modern survey methods to get excellent matches of data to map new
potential oil traps.
A very promising source of oil is in turbidites. These rocks formed by
loose sediments slipping down the continental shelf margins into the deep
water beyond. These sandstones may be found to be oil rich if organic
matter is trapped in anaerobic conditions and converted into oil.
Sandstone channels created by these turbidity currents can be identified by
amplitude cut off and mapped across large areas from 1 km wide to 50+
km long with a lobe shaped fan at the end where the bulk of the slip
material comes to rest.
Gerard finished by telling us:
1. The difference between earthquake foci (the actual points of movement
– the cause) as opposed to epicentres (the point on the surface above the
trigger point).
2. How earthquakes and volcanoes are found at plate boundaries whether
subduction zones or areas of slip, collision or spreading. (Pacific ring of
Fire) for example.
3. How the differing properties of P and S waves have informed us of the
composition of the interior of the earth.
A fascinating talk which was very well presented.
Sue