May
2015
Oilfield Technology
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7
below the earth noise. If this is possible, the sensor
specmanship becomes irrelevant.
There is no point in having a sensor that is quieter
than the background and if such a sensor did exist, it
would be impossible to measure, at least on earth. At
that point the engineers can focus on less glamorous
issues like making it cheaper, less power‑hungry or
more durable.
The second issue with digital sensors is the
fact that the sensor is the system from a technical
specification perspective. Once data is recorded with
a digital sensor it would seem that the only task of
the system is to transfer those numbers to media as
efficiently as possible. From this point of view the
digital sensor very neatly reduces the components of
seismic recording technology to just two elements.
Or does it?
Therecordingsystemis important
System research and development in the previous
20 years have focused on a single primary goal:
recording as many channels as possible in near
real time. In parallel with this effort, development of practical and
effective cableless node based systems also occurred. The growth
of channel count has been explosive and the cost, weight and
deployment flexibility of systems have all improved dramatically.
It is difficult to imagine what a processor would have said 15 years
ago if they were handed 1 Petabyte of data with 60 000 channels per
record and asked to process it. Not only was it not possible, it was
unimaginable at the time. In 2015, it is not only possible but also
increasingly practical.
This growth is driven by four factors: 1) demand for high
fold wide azimuth recording; 2) the advent of 3C recording and
processing; 3) demand for higher density shooting, and; 4) high
productivity vibroseis methods. Most geophysicists no longer worry
about issues like system filters, system dynamic range, or system
timing issues since available systems have very well understood
properties with regard to these technical issues. Instrument
technology has taken a backseat to operational issues and
deployment strategies, since those are the key factors in the practical
application of high channel count acquisition.
To engineer a system capable of recording 250 000 channels in
real time, with or without connecting cables is a huge task that has
yielded unlimited possibilities in project design and implementation.
The effort to record broader bandwidth data is fully dependent on
this expanded capability; denser shooting is now practical along with
creative sampling strategies. All this can be achieved with incredible
efficiency and productivity. However, issues like dynamic range
and system filters can become limiting factors to the broadband
technology chain, if source technology and particularly sensor
technology continue to advance.
Theweak linkandother issues
So where is the weak link in the technology chain? The least
understood element is the source technology. The strategy on the
source side historically has focused on the ‘bigger hammer’ concept.
Even though great strides have been made in recent years, there is a
long way to go in understanding how low a frequency is needed, and
how high a frequency can be expected in return for the effort. Modern
commercial sources have demonstrated an effective sweep from
1.5 Hz to as high as 400 Hz in some conditions while maintaining
control of phase and ground coupling. Significant strides have also
been made in reducing harmonic distortion and other unwanted
mechanical noise. The sensors and the recording systems are poised
to handle the challenge of these broader band sources if the sources
can get enough energy into the ground.
Virtually any geometry is possible at high densities. A uniform
grid of sensors, 15 000 ft
2
at 55 ft group interval requires about
75 000 live channels. More modest design parameters can cover
vast regions to enable multiple fleets of vibroseis sources to
shoot DS4 strategies. The DS4 vibroseis method requires very
large static receiver patches and open terrain to utilise multiple
fleets of vibroseis sources sweeping in overlapping times (Bouska
2009 and Chengfu 2014). On the sensor side of the equation, the
development of a more sensitive analogue sensor is possible
but that sensor output must be married to an A/D converter with
the same dynamic range and noise characteristics. The seismic
equipment industry has benefited greatly from the availability of
commercial A/D converters specifically designed to meet current
needs and current specifications. However, they are not designed
to meet future specifications, making the continued development
of the digital sensor potentially more appealing. If a digital sensor
is introduced with a noise floor below earth noise, then companies
will use it. That is, if it is not too expensive, if it does not use too
much power and if it is durable enough to last for more than a
couple of years. The work of engineers and seismic equipment
development teams is far from over.
References
1.
Wei, Z., ‘Reducing Harmonic Distortion on Vibrators ‑ Stiffening the Vibrator
Baseplate’, 70
th
EAGE Conference & Exhibition – Rome, Italy, (2008).
2.
Wei, Z., Phillips, T., ‘Analysis of vibrator performance at low frequencies’, First
Break Vol.29 (July 2011).
3.
Brune, R., Yates, M., Liner, C., Bell, L., ‘Evaluating Vibrator Spacing Including Mutual
Admittance Interaction Effects’, SEG Annual Meeting – Houston Texas, (2013).
4.
ten Kroode, F., Bergler, S., Corsten, C., de Maag, J., W., Strijbos, F., Tijhof, H.,
‘Broadband seismic data – The importance of low frequencies’, Geophysics,
Vol. 78, No. 2, pp. WA3‑WA14 Online Publication Date: (March ‑ April, 2013).
5.
McNamara, D., Buland, R., Boaz, R., Weertman, B., Ahern, T., ‘Ambient Seismic
Noise’, US Geological Survey, (August 2005).
6.
Mahrooqi, S., ‘Case Study: Ensuring success‑UmmAs Samim sabkha design and
acquisition’, SEG Annual Meeting ‑ Denver (2014).
7.
Bouska, J., ‘Distance separated simultaneous sweeping: Efficient 3D vibroseis
acquisition in Oman’, 79
th
Annual International Meeting, SEG.
8.
Chengfu, H. et. al., ‘The Implementation and Application of High‑Production
Vibroseis Acquisition: A Case Study’, Annual International SEG meeting –
Denver, (2014).
Figure 9.
MEMs based single component digital sensor, INOVA AccuSeis.
