A Publication of Petroleum Geo-Services Vol. 6 No.
13 December 2006
Far-field Measurement Program to Extend
our NUCLEUS Source Modeling Capabilities
Introduction
The ability to accurately model the output pressure wave- Summary
field from a seismic source array is a key requirement in the
In July 2006 PGS conducted a new series of far-
planning and execution of marine seismic acquisition and
field signature measurements of single air guns and
processing. The most common applications include source
two-gun clusters in cooperation with the main
array design, analysis of the effects of gun drop-outs, and
vendors of marine seismic air guns; Bolt Technology
signature processing. In addition, firing time delay- and depth
Corporation and Sercel. The main objective of this
stability- analyses are occasionally performed. In survey
project was to perform measurements of new guns
design, the directivity-dependent signatures from source
modeling are used as input to seismic modeling algorithms.
for NUCLEUS source modelling calibrations.
NUCLEUS has been the industry standard for source A range of guns were tested, including the new
signature modeling for more than 15 years. The software Annular Port Gun (APG) from Bolt Technology, the
package is developed continuously and extended with new Mini G. GUN, the Mini GI GUN, and the 380 cu.in.
gun types and increased depth and volume ranges. Since its G. GUN, all from Sercel. In addition to
introduction in 1988, the NUCLEUS modeling has been measurements of the new air guns, the 150 cu.in.
calibrated and verified against real far-field measurements. In Bolt 1900LLXT and G. GUNS (already included in
total, more than 20,000 signatures from single air guns, NUCLEUS) were re-tested, to confirm previous
clusters and arrays have been measured by SERES (1985 - measurements, and to provide a set of reference
1994), and later by PGS. In addition to the far-field signatures for future tests.
measurements, a large set of laboratory near-field trials have
Based on the measurements from this test the
also been conducted to gain an understanding of the physics
NUCLEUS source modeling has already been
of bubbles generated by air guns.
extended to include new guns such as the Bolt APG
In July 2006 PGS conducted a new series of far-field and Sercel Mini G. GUNS. This new version of the
signature measurements of single air guns and two-gun software will be available to users early 2007.
clusters in cooperation with the main vendors of marine
seismic air guns; Bolt Technology Corporation and Sercel. The
main objective of this project was to perform measurements In total, more than 2000 shots were recorded for different
of new guns for NUCLEUS source signature modelling gun volumes and depths. The test was performed in
calibrations. A range of guns was tested, including the new Salhusfjorden, north of Bergen on the Norwegian west coast,
Annular Port Gun (APG) from Bolt Technology, the Mini G. onboard the research vessel Håkon Mosby (Figure 1), owned
GUN, the Mini GI GUN, and the 380 cu.in. G. GUN, all from by the University of Bergen.
Sercel. Continued on next page
TechLink December 2006 Page 2
Introduction
Continued from Page 1
Figure 2: A comparison between the measured and modeled
signatures for a 40 cu.in. Mini G. GUN at 6 m depth and a chamber
Figure 1: The research vessel Håkon Mosby, which is owned by the pressure of 2000 psi.
University of Bergen and operated by the Institute of Marine
Research.
The test site was carefully selected to provide optimal
conditions for the measurements. Located inside a fjord, it
was well protected against bad weather and high waves. The
water depth of more than 500 m made it possible to record a
signature of sufficient length to be uncontaminated by water
bottom reflections, even with a deeply deployed far-field
hydrophone.
The PGS personnel and vessel crew were joined by
personnel from both Bolt Technology and Sercel in the
operations. In addition to measurements of the new air guns,
Figure 3: A 2-gun Bolt Annular Port Gun cluster hanging from a gun
150 cu.in. Bolt 1900LLXT and G. GUNS (already included in frame, with the near-field hydrophone and pressure transducer
mounted.
NUCLEUS, and in use by PGS) were re-tested, to confirm
previous measurements and to provide a set of reference
stable signature was recorded. The guns were deployed from
signatures for future tests. Based on the measurements from
a special gun frame that allowed accurate positioning of the
this test, the NUCLEUS source modelling has already been
near-field hydrophone, and also allowed the installation of a
extended to include new guns such as the Bolt APG and
pressure transducer in the airline close to the gun (see Figure
Sercel Mini G. GUNS. This new version of the software will be
3). The shots were typically fired with a 20 second shot
available to users early in 2007. As an example, a comparison
interval to simulate a realistic acquisition scenario. For the
of the modelled and measured signature for a 40 cu.in Mini G.
deepest configurations this was increased to 30 seconds to
GUN can be seen in Figure 2.
allow the bubbles from the previous shot to rise to the surface
before a new shot was fired (see Figure 4).
Test Description
During the tests each airgun or airgun cluster was hung Throughout the measurements, the near- and far-field
from the ship’s cranes, approximately 5-8 m out from the traces were displayed “online” using the PGS geophysical
ship’s side. Two cranes were normally used, with one gun or acquisition system (gAS). Any gross errors, such as electrical
gun cluster hanging from each crane, shooting in flip-flop leakage, were detected using these displays. Immediately
mode. These two cranes were approximately 10 m apart, with after the completion of each test, the SEG-D file was
the far-field hydrophone deployed in between, at a depth of transferred to a laptop together with pressure readings for
170 m. For each combination of chamber volume, pressure every shot. These data were loaded into the Viper data QC
and depth, the guns were fired 25 times to ensure that a system, and displayed both in the time and frequency domain
Page 3 A Publication of Petroleum Geo-Services
The Mini G. and Mini GI GUNS were tested at 1.5 and 3
m depth, and at 2000 and 2500 psi. The other guns were
tested at 3, 6 and 9 m depth at 2000 psi, and at 6 m depth at
2500 psi. The guns listed above are currently being added to
the list of 18 different gun makes and models already available
in the latest version of the NUCLEUS software. Graphs
presenting peak amplitude, peak-to-bubble ratio, and bubble
period, as functions of gun volume can be seen in Figures 5 to
7, respectively. The results correspond well with experimental
and theoretical findings presented in (Vaage et al., 1983), and
Continued on next page
Figure 4: Air bubble rising to the surface during field testing.
to check that the recorded data were of good quality. The
spectra were also monitored to check that the notches agreed
with the set gun depth for each test. During the tests, the
temperature, salinity and the velocity of sound in the water
column from the surface to the depth of the far-field
hydrophone was measured several times. The velocity was
very stable, with a slight increase in the velocity near the
surface towards the end of the project. The mean velocity
Figure 5: Peak amplitude vs. chamber volume for different gun types.
from the surface to the depth of the far-field hydrophone was
1487 m/s, whereas the mean velocity from the surface to the
depth of gun was 1493 m/s, increasing to 1495 m/s towards
the end of the project, due to rising water temperature. In
addition, the atmospheric pressure and the wave height were
continuously measured and recorded during the project.
Sources and Range of Parameters Tested
The following single gun and cluster combinations were
included in the tests:
Air guns Chambers Second chamber 6: Peak-to-bubble ratio vs. chamber volume for different gun types.
(cu.in.) (cu.in.)
Mini G. GUN 12, 20, 24, 40, 60
(Sercel)
Mini GI GUN Generators: 15, Injectors: 15, 20,
(Sercel) 20, 30 30
G. GUN (Sercel) 150, 380, 2x380
1900LLXT (Bolt) 40, 90, 150, 250
APG (Bolt) 50, 100, 200,
300, 2x300 Figure 7: Bubble period vs. chamber volume for different test gun
types.
TechLink December 2006 Page 4
Sources and Range of Parameters Tested The Link to Seismic Modeling and Survey
Continued from Page 3
Design
the references therein. For the cluster configurations, the A key capacity of the NUCLEUS software suite is the
separation between the guns was 1 m for both Annular Port integration of source signature modeling, with state-of-the-art
Guns (Bolt) and the G. GUNS (Sercel). The APG guns were seismic modeling algorithms, for application within 3D survey
mounted as an inline cluster, and the G. GUNS as a parallel design. This unique feature allows the geophysicist to include
cluster. realistic source directivity effects into modeling studies. The
algorithms available include 1D recursive reflectivity modeling,
Source Modeling Research and Development
which is an ideal method for addressing the full wave-field
The source signature measurement program is part of an
interplay between signal and noise upon recorded gathers. It
ongoing effort within the Geoscience & Engineering division in
is regularly used for amplitude-based studies such as AVO
PGS to extend the capabilities of the NUCLEUS marine source
modeling, fluid substitution and converted wave studies. In
modeling application. The sophisticated modeling algorithms
addition, the source signature modeling is also integrated with
that are implemented in the software have evolved since the
several ray-tracing algorithms and applications, including
pioneering work of Ziolkowski (1970), and incorporate
NORSAR-2D and NORSAR-3D. This means that it is possible
interaction and thermodynamic effects, in addition to the
to extend the capabilities of these modules beyond kinematic
acquisition system responses. The current research includes,
illumination analyses in complex Earth models. It is also
amongst other activities, a close cooperation with leading
possible to combine the effects of source and receiver array
university researchers on developing the theory of bubble
directivity with attributes from the dynamic raytracing, such as
interaction in clusters. Significant efforts are also being
reflection and transmission losses, and absorption and
currently invested in the development of a new generation of
geometrical spreading, when generating synthetic data.
the NUCLEUS software application. These new developments
Finally, it is also possible by the use of the visco-elastic finite
are aimed at significantly improving user efficiency by allowing
difference module to generate very realistic synthetic data
automation of many source modeling related tasks and
that includes all effects of the full wave-field in complex 2D
procedures. As described in a previous edition of TechLink (Vol.
and 3D models, combined with the directivity-dependent
6, No. 10, September 2006), the NUCLEUS source signature
effects of the source and receiver array.
modeling has also recently been implemented as a key
element in PGS’s new Real-Time Source Signature Estimation, References
built within the onboard Viper data QC system. Ziolkowski, A., 1970. A method for calculating the output
pressure from an airgun, Geophys. J.R. Astr. Soc., 21, 137-161.
Vaage, S., Haugland, K., and Utheim, T., 1983. Signatures
from single airguns, Geophysical Prospecting, 31, 87-97.
C O N TA C T
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