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Zero Velocity Update Primer
Marine Construction Land Seismic Background
In many active geographical regions of land seismic exploration the
canopy (trees/forestation) is such that RTK GPS cannot be used for the
precise installation of survey stakes. The signals from the satellite
segment of the GPS system cannot penetrate through to the receivers
being carried by the field surveyors.
Global Positioning System (GPS) is based on satellites providing range
data to a receiver at the location to be surveyed. We can reliably
position a point today in an open space with GPS. We can accurately
survey if we use Differential GPS - DGPS (1m to 2m). We can very
precisely survey if we use Real Time Kinematic GPS – RTK (5cm to 15cm).
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If we try and position using GPS
in areas where the signals are masked or disturbed such as the urban
setting seen right, we will degrade our positioning capability
significantly
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If we
were to try and position under trees (canopy) we can see that GPS will
fail as the signals from the satellites are being masked (attenuated)
by the trees.
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If we
combine the few GPS ranges we get with a good quality inertial unit we
can
maintain accurate survey grade positioning in these areas of poor
(stand alone) GPS performance.
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What we can also do is align the inertial unit to
an installed monument and then position throughout the day just with
the inertial unit. Alignment is maintained through the use of zero
velocity updates (zupt’s) while surveying under canopy. When
convenient, the unit is tied to a control point or survey monument and
all data is re-processed to minimize errors. |
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This,
although a very simplistic explanation, is how Zupt delivers very
productive land survey positioning for seismic exploration.
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Marine Construction
- A basic introduction
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Subsea precise positioning is achieved today
through the use of Long Baseline (LBL) acoustic positioning systems (LBL
provides relative accuracies of between 30cm to 3m depending on
frequency). In a similar manner to GPS, ranges are measured to
stations. In the case of LBL systems, multiple stations are deployed
onto the seabed to provide the local coverage needed. These stations
(transponders) are then calibrated (relatively [location with respect
to each other] and absolutely [Lat, Lon and Depth]). Once calibrated
the system is ready to be used for precise positioning. The type of
work these arrays are used for is pipeline installation, sub sea
manifold installation and general measurements during offshore oil and
gas field construction. Remotely operated vehicles (ROV’s) are used
today in the place of divers for deep water construction support. LBL
systems are often used to position ROV during these tasks.
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Below we start to look at the example of a Remotely Operated Vehicle (ROV)
involved in the installation of some subsea components (well-head,
manifold, jumper, etc.) The ROV will be positioned with respect to
this LBL array. To provide adequate coverage, positioning reliability
and accuracy, a number of (usually 6 or more) transponders will be
deployed and calibrated to form a LBL “array”. This “array” deployment
and calibration will consume several “spread” days. The cost of this
spread (vessel, ROV/personnel, survey equipment/personnel) will be in
the region of $55K/day to $90K/day.
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If we combine the measurements (observations) from just two
transponders with a good inertial unit, pressure transducer and a
velocity log we can provide the same positioning precision with much
greater reliability. The spread time consumed to deploy these two
beacons and calibrate them will be on the order of 30% to 40% of the
time taken to deploy and calibrate the complete LBL array shown above.
For each location Zupt’s services and equipment will shave between
$100K and $200K off the overall cost of the operation. These oerations
are ongoing throughout any deep water offshore field development. |
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