Geology Reference
In-Depth Information
hydrophones are made up into hydrophone streamers by
distributing them along an oil-filled plastic tube. The
tube is arranged to have neutral buoyancy and is manu-
factured from materials with an acoustic impedance
close to that of water to ensure good transmission of
seismic energy to the hydrophone elements. Since
piezoelectric elements are also sensitive to accelerations,
hydrophones are often composed of two elements
mounted back to back and connected in series so that the
effects of accelerations of the streamer as it is towed
through the water are cancelled out in the hydrophone
outputs. The response of each element to pressure
change is, however, unaffected and the seismic signal is
fully preserved.
Arrays
of geophones or hydrophones may be con-
nected together into linear or areal arrays containing tens
or even hundreds of transducers whose individual out-
puts are summed. Such arrays provide detectors with a
directional response that facilitates the enhancement of
signal and the suppression of certain types of noise as dis-
cussed further in Chapter 4.
ing many transducers on the surface, and measuring the
small changes of arrival time as the waves move across
them.Typically, this number might be 24 for a small en-
gineering survey, to several thousand for a large hydro-
carbon exploaration survey.
The electrical signals from the transducers must be
recorded in real time. Before the availability of portable,
powerful, computer systems this was a fundamental
problem. Before the 1960s the majority of seismograms
were recorded as wiggly traces written directly to paper
or photographic film charts.The seismic computor was a
human geophysicist with a slide rule.While direct paper
recording is still used for some very specialist applica-
tions, virtually all seismic data are now recorded by digi-
tizing the analogue transducer output, and storing the
series of digital samples in some computer format. It is a
little surprising to realize that recording a seismic source
is technically more demanding than recording a classical
orchestra.The dynamic range of signals and the required
accuracy of amplitude recording are both more stringent
in the seismic case. An amplitude ratio of one million is
equivalent to a dynamic range of 120 dB. A maximum
dynamic range for geophones of about 140 dB and an
inherent minimum noise level in seismic amplifiers of
about 1 mV effectively limits the maximum dynamic
range of a seismic recording to 120 dB.
Seismic signals from the transducer must be amplified,
filtered if necessary, digitized then stored with appropri-
ate index information. International standards produced
by the Society of Exploration Geophysicists (SEG 1997)
are used for the format of seismic data storage.Virtually
all seismic data, from small engineering surveys to litho-
spheric studies, are now recorded on computer systems
in these formats. The physical nature of the computer
media used is continuously being upgraded from
magnetic tape, to magnetic cartridge and CDROM.The
data volumes produced can be impressive. A seismic
acquisition ship working on the continental shelf can
easily record around 40 gigabytes of data per 24-hour
day. This generates a data storage problem (about
60 CDROMs), but more importantly a huge data cata-
loguing and data processing task.
The high capacity of modern computer systems
for data recording and processing has allowed experi-
mentation with more data-intensive survey methods.
It is becoming commonplace to record three-
component surveys, with three geophones at each
survey station recording the east-west and north-south
components of motion as well as the vertical.This triples
the data volume, but does allow investigation of S-waves
3.8.3 Seismic recording systems
Recording a seismogram is a very difficult technical
operation from at least three key aspects:
1.
The recording must be timed accurately relative to
the seismic source.
2.
Seismograms must be recorded with multiple trans-
ducers simultaneously, so that the speed and direction of
travel of seismic waves can be interpreted.
3.
The electrical signals must be stored for future use.
The least difficult of these problems is the timing. For
nearly all seismic surveys, times need to be accurate
to better than one thousandth of a second (one milli-
second). For very small-scale surveys the requirement
may be for better than 0.1 ms. In fact, with modern
electronics, measuring such short time intervals is not
difficult. Usually the biggest uncertainty is in deciding
how to measure the instant when the seismic source
started the wave. Even in a simple case, as for a sledge-
hammer hitting the ground, is the correct instant when
the hammer first hits the ground, or when it stops
compressing the ground and a seismic wave radiates out-
ward? The first is easy to measure, the second is probably
more important, and they are usually separated by more
than 1 ms.
In order to determine the subsurface path of the seis-
mic energy, the direction from which the wave arrives at
the surface must be determined.This is achieved by hav-
