American Oil and Gas Reporter - June 2014 - 61

SpecialReport: Global Opportunities

South African Shoot Battles Elements
By Patrick Coole
and John Sheehan
WEYBRIDGE, ENGLAND-In 1497,
on rounding the Cape of Good Hope for
the second time, the famed Portuguese
sailor and explorer, Vasco da Gama, noted
a strong southward current near Port Elizabeth, which set back the flotilla three
days. This notorious force of nature is
the Agulhas Current.
More than 500 years later, a state-ofthe-art seismic survey operation began
acquiring a broadband 2-D survey offshore
South Africa, using the latest broadband
technology (Figure 1) with dual-sensor
towed streamers. The acquisition program
of this multiclient survey is a story of
battling both the strong current and stormy
weather, and of perseverance in order to
complete an ambitious and challenging
seismic acquisition project. The seismic
images produced by the survey are beginning to unlock the hydrocarbon potential in the deeper waters offshore South
Africa's eastern coast.
The Agulhas Current runs along the
eastern coast of South Africa and is the
western boundary current of the southwest
Indian Ocean (Figure 2). Formed by the
confluence of the East Madagascar,
Mozambique, and smaller part of the Agulhas Current itself, it is thought to be
the largest western boundary current in
the world at 100 sverdrups (a sverdrup,
or Sv, is the volumetric measure of the
rate of transport of oceanic currents, with
one Sv equivalent to 1 million cubic
meters per second) and flowing at 1.2
knots.
Comparable currents, such as the Brazil
(16.2 Sv) and Kuroshio (42 Sv) pale in
comparison. Strong currents are a major
cause for concern in any seismic acquisition because they cause feathering (the
deviation of a streamer away from the
towing direction) and, where strong
enough, can all but stop a ship in its
tracks-just as da Gama experienced in
his quest to find a passage to India.
Developing A Plan
The current was always going to be
the biggest factor affecting the seismic
acquisition process. The first stage in
mitigating risk was choosing the right
acquisition vessel.
Once the PGS Sanco Spirit was chosen,

the crew worked diligently to build its
understanding of the Agulhas Current
and its potential impact on seismic acquisition. A basic study was conducted
initially, looking at previous reports from
a 2012 2-D acquisition in Tugela (in the
north part of the survey) and public
domain literature, so that operations had
historical information from which to work
to start the project. The team also identified
a need for more up-to-date information
on the day-to-day changes of the current.
This is when a major breakthrough
occurred. To navigate the challenging
currents in this area, the survey crew decided to utilize software that the local
fishermen usually used. The software
provided satellite imagery of oceanic currents based on temperature differentials.
The nature of the currents, with eddies
forming and disappearing, requires daily
updates to better plan for acquisition.
Meter readings of the currents from the
acquisition vessel corroborated the software-generated models, allowing PGS to
actively predict the likely current patterns
and react accordingly.
Acquisition began in the southern mar-

gin off South Africa's East Coast. However,
the currents still were simply too strong
to allow for accurate acquisition. The decision was made that in order to better
understand the current, the vessel would
travel up the coast, shooting a major
north-south strike line and making current
measurements while sailing.
The vessel took some interesting
recordings, notably that the current was
strongest around the 1,000-meter bathymetry mark, especially when about 30 kilometers from the coastline. As the vessel
moved north of East London (Eastern
Cape Province), the current also decreased
(from five to four knots). At this point, it
was decided acquisition should move
south along the coast.
The time invested in understanding
and building a work plan around the currents paid off, despite the currents being
only one of the environmental challenges
encountered. Major storms occurred off
the coast of South Africa, which reduced
the amount of time the ship could spend
at sea. Fortunately, the time saved by the
team at the start of operations by mitigating
the impact of the Agulhas Current means

FIGURE 1

The multiclient 2-D dataset is shown in purple in this image of the southeastern African
margin.

JUNE 2014 61

American Oil and Gas Reporter - June 2014

Table of Contents for the Digital Edition of American Oil and Gas Reporter - June 2014