Introduction - 3-D Seismic Interpretation - page 15

has not been carried out or has been unsuccessful, a single isolated impedance interface
may give rise to a complicated reflection signal, with several loops of roughly the same
amplitude. In this case, polarity is not a meaningful idea, and a sketch of the response
of an isolated interface should accompany seismic displays. A more detailed discussion
of these issues has been given by Simm & White ( 2002 ) .
There is no uniform convention in the industry regarding units of distance. Both
feet/inches and kilometres/metres/centimetres units are commonly employed, and are
often freely mixed (e.g. horizontal distances in metres and vertical distances in feet).
This is easy to cope with using the conversions in section 1.7 . In this topic, both systems
are used depending on the source of data under discussion. In the real world, units are
almost invariably annotated on displays, so confusion should be minimal. Much more
confusion is generated by inadequately documented displays of well data; depths may
be as measured (measured depth, the distance along hole from a fixed reference point,
e.g. the derrick floor of the drilling rig), or relative to a geographical datum (usually
sea-level for marine data), or may have been corrected for well deviation to give vertical
depths (again, relative to derrick floor, sea-level, etc.). Close inspection of displays of
well logs is often needed to establish what the depth reference actually is. (A similar
problem arises with onshore seismic data, where zero time will usually correspond to
a datum plane at some particular elevation, which may not however be documented
in a way that is easily retrievable.) Another possible source of confusion in horizontal
positioning of well and seismic data arises from the use of different map projection
systems. Many different systems are in use, and even within a given projection system
there are different possible choices for projection parameters. This can easily cause
problems in relating wells to seismic survey data. Since new well locations are usually
chosen from a seismic survey grid, at the intersection of a particular inline and crossline,
it is obviously critical to be able to translate this intersection into a point on the ground
where the rig will actually be placed. One of the problems of 3-D seismic is that
interpretation is often carried out on a more or less self-contained workstation volume;
such volumes often have a complicated history of reprocessing by different people at
different times, and it may not be easy to check whether the real-world coordinates
assigned to this volume are correct. If there is any doubt at all about the coordinate
systems being used, the services of a specialised surveyor are needed.

1.7 Unit conversions

Conversions are stated to four significant figures where not exact.

Length: 1 inch = 2 . 540 cm
1 foot = 12 inches = 30 . 48 cm
1 metre = 100 cm = 3 . 281 ft