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Oilfield Technology
May
2015
objectives in the past 30 years have focused on getting higher
frequencies from seismic data. Higher frequency data directly
relates to data resolution and the details interpreters can derive
from seismic data. However, common vibroseis sweep parameters
in most parts of the world seldom exceed 100 Hz. Apparent
absorption of signal in the earth and noise levels at higher
frequencies are difficult challenges to overcome. But not all of
the limitations impacting higher frequencies are beyond control.
Sensor deployment strategies and improvement in vibroseis
stability help address these limitations.
On the low frequency side of the broadband equation, coherent
noise such as ground roll and source limitations tend to regulate
low cut filters to the 6 ‑ 8 Hz range. Many processing algorithms and
methods are inherently unfriendly to low frequency data. However,
newer energy sources capable of generating frequencies as low
as 1.5 Hz make the effort to gather low frequencies worthwhile
(Z. Wei – 2011).
Interpreters and
petrophysicists will
appreciate the benefits of
lower frequency data and the
positive impact on lithology
and inversion processes
(Kroode et. al. 2013). The
introduction of frequencies
between 1 ‑ 8 Hz to the
seismic data adds 3 octaves
and emphasises the
importance of low frequency
enhancement. These benefits
highlight the reason the
low frequency portion of
the spectrum has become a
focal point in the broadband
effort. Figure 1 illustrates the
change in the seismic wavelet
that is apparent when adding
low frequency octaves.
Theroleof seismic
equipment in
broadbandrecording
The elements of a seismic
recording system and their
contribution to broadband
technology can be defined
with three fundamental
contributors, – sources,
sensors and recording
systems. The goal is to
optimise each element, but
‘a chain is only as strong
as its weakest link’, as the
saying goes. When beginning
the task of recording better
seismic data, one must bear
in mind that a balance of
performance is important.
It does not make sense
to develop a broadband
source capable of generating
undistorted signals down
to 0.5 Hz if that same signal
cannot be recorded with
the available sensors and
systems. The uneven pace of
technology growth usually
mandates that some of the
fundamental contributors will
progress at different rates,
Figure 1.
The five zero phasewavelets shownall have the same high bandpass of 100Hz. From left to right the
low frequency octaves are added startingwith 16, 8, 4, 2, 1Hz respectively. The addition of low frequency octaves
significantly changes the side lobes on thewavelet.
Figure 2.
The processed seismic data shown is courtesy of PDOOman. The data illustrates the impact of low
frequency vibroseis sweeping. The leftportion of the plot is datawith a standard 8 ‑ 86Hz sweep, and the right
portion of the plot was recordedwitha 1.5 ‑ 86Hz sweep. The datahas been filteredwithabandpass filter 1.5 ‑ 6Hz.
(Mahrooqi 2014).
Figure 3.
This diagram illustrates the relationshipbetween survey design, systemcomponents and the downstream
steps like processing. Each contributes to the broadband recording.
