A Publication of Petroleum Geo-Services
`
`Vol. 9 No. 12
`
`December 2009
`
`Acquisition Technology Snapshots
`
`Door Generator and Navigation System
`Several PGS Ramform vessels now have paravanes (or
`“doors”) equipped with generators to power navigation
`sensors. These navigation sensors, GPS receivers and
`acoustic ranging units, are used to enhance the streamer
`positioning in terms of accuracy and reliability (refer to
`Figure 1).
`
`Relative positions for towed seismic streamers are
`established using magnetic compasses placed along each
`streamer, acoustic networks covering either part or all of
`the streamer spread, or a combination of these methods.
`Absolute coordinates for each receiver location can only be
`established using one of more precisely known reference
`positions, for example, GPS sensors on head or tail buoys.
`Traditionally, acoustic networks have been used to link the
`streamer fronts (the forwardmost point on each streamer)
`to the vessel. This objective, however, is complicated by
`the wake of the vessel, which is not an ideal environment
`for acoustic ranging. Wider streamer spreads mean longer
`ranges, which compounds the problem.
`
`To overcome the difficulties described above, floats
`with surface and sub-surface navigation sensors have been
`towed between the streamer fronts, but this is logistically
`unattractive. An attractive alternative is to use the
`paravanes as platforms for the navigation equipment.
`Paravanes are used to maintain streamer separation, and
`represent the outermost component of any towed
`streamer spread. Vertical foils are kept in position by a float
`at the top (refer to Figure 2). The largest paravanes used in
`operation have foils with a height of 10 m, suspended
`below cylindrical floats more than 9 m in length. Tension in
`the “superwide” towing rope can exceed 20 tons.
`
`Electrical power is necessary if navigation sensors are
`mounted on the paravanes, which presents a technology
`challenge. Access to the paravane for repair and
`maintenance is difficult, both alongside the vessel (refer to
`Figure 3) and under tow. A “2 x 100%” solution was
`designed in order to improve the redundancy of the
`system. In other words, it was decided that each paravane
`
`Continued on next page
`
`Summary
`
`Two PGS technology innovations are described
`that are helping to improve the quality and
`efficiency of PGS towed streamer operations.
`
`PGS has installed GPS receivers and acoustic
`ranging units on the paravanes of several Ramform
`vessels. The resultant benefit is improved streamer
`positioning accuracy. A solution to the associated
`power requirements has been the development of
`a unique generator technology that is mounted on
`each paravane.
`
`On the source side, PGS has been developing
`an “air leak” model that provides an understanding
`of the physics of air leaks in air guns, and the
`associated effects upon source output. Threshold
`tables specific to each air gun in the air gun arrays
`can now be developed as a complement to
`traditional “drop out” tables used during towed
`streamer survey QC.
`
`Figure 1: Schematic illustration of the full acoustic network deployed
`behind a PGS Ramform vessel.
`
`PGS v. WESTERNGECO (IPR2014-00688)
`WESTERNGECO Exhibit 2053, pg. 1
`
`

`

`TechLink December 2009
`
`Page 2
`
`Door Generator and Navigation System
`Continued from Page 1
`
`would be equipped with two generators and navigation
`sensor systems.
`
`hydro-powered
`off-the-shelf
`available
`Unlike
`generators, the PGS paravane generators had to be
`custom-designed to deliver optimum performance at
`typical seismic towing speeds of 4 to 5 knots. This meant
`that the propeller’s torque and RPM had to be tailored to
`match the characteristics of the gear and motor at these
`relatively slow speeds.
`
`After the first prototype production generator was
`built, an in-sea tow testing delivered 210 Watts at 5 knots
`towing speed, which is typically 3-4 times more than off-
`the-shelf generators.
`
`Figure 2: Baro48 paravane. Note the personnel.
`
`Figure 3: Baro45 paravane alongside a Ramform vessel.
`
`Figure 4: Baro48 paravane on a PGS Ramform vessel a generator and
`acoustic pingers mounted.
`
`As reliability was an essential factor for the paravane
`generators, a 5 year design lifetime was established in the
`early development phase. The generator’s motor is a
`brushless type with high lifetime expectancy, and the
`connected gear is ruggedly designed. In order to reduce
`the risk of water intrusion into the generator house, the
`torque from the propeller is transferred through a magnetic
`coupling. This approach allows the gear and motor to be
`hermetically sealed, and also separates the gear and motor
`(dry side) from the propeller (wet side), thus enabling easy
`replacement in the field of either side of the coupling (refer
`to Figure 5).
`
`Figure 5: 3D schematic that illustrates a cross-section and the
`wet/dry sides of the PGS deflector generator.
`
`The system’s “combibox” is mounted in a recessed
`hole in the paravane’s float. The unit contains the charging
`electronics for each generator, and lithium-ion batteries to
`increase the reliability of the 24 Volt DC output current.
`Each generator is coupled to a separate charger, and
`battery pack as part of the “2 x 100%” design philosophy.
`
`The combibox contains a status surveillance system
`with a radio link to the mother vessel. The voltage from
`each generator, status for each battery pack, measured
`
`PGS v. WESTERNGECO (IPR2014-00688)
`WESTERNGECO Exhibit 2053, pg. 2
`
`

`

`Page 3
`
`A Publication of Petroleum Geo-Services
`
`abort or scratch any sail lines in which an air leak occurs.
`Unfortunately, this practice is indiscriminate, and takes no
`account of the geophysical effects of any particular air leak.
`The ability to distinguish between destructive and
`insignificant air
`leaks could save many hours of
`unnecessary downtime each year. A sensitive air leak
`detection tool was developed some years ago, and is
`already in use across the PGS fleet. This tool has been very
`successful in detecting even very small leaks, and
`identifying their position in the source array. Experience
`with this system demonstrated that it was important to
`quantify air leak effects, and to develop acceptability
`thresholds for survey execution.
`
`PGS has consequently had an on-going project to
`improve our understanding of air leaks. Two separate field
`tests at the PGS test barge facility in western Norway
`were conducted. A variety of different air leak scenarios
`were created, and then their effect upon the wavefield of a
`sub-array of air guns was measured. The ultimate aim was
`to develop an accurate air leak model by improved our
`understanding of the physics and nature of these events.
`
`An air leak causes a stream of bubbles to rise from the
`leak position, forming an aerated plume of water, which
`stretches rearwards as the source is towed forwards (refer
`to Figure 7). This has a significant effect on the source
`signature in the near-field, and also affects the far-field
`Continued on next page
`
`Figure 7: Example of an air leak plume behind a sub-array of air guns
`being towed through the water..
`
`Figure 6: PGS paravane generator combibox.
`
`combibox operational temperature, and values from a
`water intrusion sensor is sent via radio every four seconds.
`
`The navigation equipment installed is Seatrack RGPS
`pods and DigiRANGE acoustic units. There are two
`complete installations of these units on each paravane, in
`case one should fail. The two RGPS units are put in a small
`tower at the top of the float, as high as possible above the
`waterline. Conversely, depth below the water line is an
`advantage for the acoustic transducers (refer also to
`Figure 4). The zone between the paravane and the
`streamers is important, and will ideally be affected by
`minimal turbulence. Consequently, the transducers are
`placed on the inside of the aftmost (rear) foil, while the
`cans with the electronics necessary to trigger the
`acoustics, and the radio antennas all are placed together
`within the paravane float.
`
`Overall, two fixed points at the front of the outer
`streamers provides an excellent baseline for the acoustic
`network. Experience shows that magnetic compasses are
`no longer required to help an acoustic network solution to
`converge, vessel heading is no longer relevant to the
`acoustic network, and time-consuming gyro calibrations
`can be skipped.
`
`Geophysical Implications of Source Air Leaks
`An analysis of PGS’ marine operations reveals that air
`leaks from the seismic source continue to be a major
`component of total technical downtime. Air leaks can affect
`the source emission wavefield, so the current practice is to
`
`PGS v. WESTERNGECO (IPR2014-00688)
`WESTERNGECO Exhibit 2053, pg. 3
`
`

`

`TechLink December 2009
`
`Page 4
`
`Figure 8: Effect of an air leak on the far-field signature of a single air gun, and its
`associated amplitude spectrum. Upper: No leak. Lower: With an air leak.
`
`Figure 9: Air leak acceptability map for the 3090
`in3 source array at 6 m depth. A red colour
`indicates that the air leak is unacceptable at that
`gun position.
`
`Geophysical Implications of Source Air Leaks
`Continued from Page 3
`
`source signature in a very different way. In general, the
`guns immediately behind the leak are affected the most,
`so the position of the air leak in a full source array is a key
`parameter for determining the overall effect on the source
`wavefield. Figure 8 shows an example of a far-field
`signature measurement and its associated amplitude
`spectrum for a single air gun in the absence of an air leak
`(top), and in the presence of an air leak (bottom).
`
`An air leak produces a variety of physical effects on the
`source output. There is short period energy emission from
`the air leak bubbles themselves. The ghost notch in the
`amplitude spectrum of the overall air gun array is shifted to
`a lower frequency because of the increased travel time
`through the aerated water of the air leak plume. Also, the
`apparent magnitude of the ghost reflection is reduced,
`which can be attributed to absorption, scattering and
`multiple reflections in the air plume. In addition to these
`
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`© 2009 Petroleum Geo-Services. All Rights Reserved
`
`effects, the gun in the position of the air leak may show
`reduced output and a different bubble period because of
`reduced gun pressure in that position.
`
`The next step in the PGS project was to create an air
`leak model for use in conjunction with the PGS Nucleus air
`gun modeling package to predict the effects of air leaks. A
`variety of the criteria commonly used to assess the
`geophysical effect of air gun “drop outs” upon the
`signature and spectrum were tested, resulting in a “traffic
`light” acceptability map that can be produced for any PGS
`air gun array and any attribute criteria to show the gun
`positions at which air leaks have an unacceptable effect. It
`was decided that a very conservative approach should be
`taken when producing such maps, and that they would be
`calculated for an extremely large air leak in each scenario.
`Thus, a high confidence can be placed upon the resultant
`thresholds. An example is given in Figure 9, based upon
`the 3090 in3 source array and the “Shell” criteria set (peak-
`to-bubble ratio > 15.00, average spectral deviation < 1.20
`dB from ideal, and absolute maximum spectral deviation <
`1.70 dB from ideal).
`
`As observed in Figure 9, the acceptability map is a
`schematic of the three sub-arrays of the array, with gun
`sizes marked for each gun position. The sub-arrays are
`aligned in the vertical direction, with the front of the sub-
`array at the top of the plot. The PGS Nucleus package is
`being correspondingly enhanced to incorporate this new
`functionality.
`
`For Updates on PGS Technological Advances, visit
`www.pgs.com
`More TechLinks at www.pgs.com/techlink
`
`PGS v. WESTERNGECO (IPR2014-00688)
`WESTERNGECO Exhibit 2053, pg. 4
`
`