It also covers the following optional activities:

Transcription

1 2 PROJECT DESCRIPTION UPDATE 2.1 INTRODUCTION The Original EMPr evaluated the potential impacts of 2D seismic surveys in the Exploration Area. Two seismic surveys were subsequently carried out: one in 2011 and one in Both surveys were completed without incident to environmental or social receptors. The second Addendum Report covers the following activities which could be carried out in the First Renewal Period: 3D seismic survey. It also covers the following optional activities: additional 2D and 3D surveys; airborne geophysics survey; multibeam bathymetry survey; seabed heat flow measurements; seabed and water column sampling programmes; and Autonomous Underwater Vehicle (AUV) survey Activities Undertaken During Initial Exploration Right Period In 2011 Silver Wave appointed SeaBird Exploration to undertake a 2D seismic survey within the Exploration Area. The MV Northern Explorer conducted the survey from 25 May 2011 to 16 July The survey plan consisted of 94 survey transects of varying lengths covering a total distance of approximately 5,500 line km and an area of approximately 5,000 km 2. The MV Fiona was designated as the guard and supply vessel during the survey. Due to various factors, mainly poor weather conditions, only about 34 percent (approximately 1,900 line km) of the survey was completed. In 2012, after the transfer of operatorship, Impact Africa appointed PGS Exploration UK to undertake a 2D seismic survey within the Exploration Area. This survey was essentially a continuation of the 2D survey carried out in The MV Sanco Spirit conducted the 2012 survey from 22 February 2012 to 26 March The survey consisted of 42 survey transects of varying lengths, covering a total distance of approximately 3,100 line km. The MV Torsvik was designated as the guard and supply vessel during this survey. 5

2 2.2 3D SEISMIC SURVEY Overview Information was provided in the Original EMPr regarding seismic surveys in general, and there are no major differences between three-dimensional (3D) seismic surveys and two-dimensional (2D) seismic surveys. Typical activities and components of 3D seismic surveys are described in further detail within Section 2.2. Seismic surveys are undertaken to collect either 2D or 3D data. The 2D seismic surveys provide a vertical slice through the earth s crust along the survey track line and are typically applied to obtain regional data from widely spaced survey grids (tens of kilometres apart), although infill 2D seismic surveys on closer grids can be applied to provide more detail over specific areas of interest. The 3D seismic surveys are typically applied to potentially promising petroleum prospects to assist in fault line interpretation, distribution of sand bodies, estimates of oil and gas in place, and the location of exploration wells. The data is gathered as a 3D data set, which can be processed and displayed in a variety of ways. Continued improvements in satellite positioning and computer processing technology have produced higher quality 3D seismic imaging of subsurface geology. Two 2D seismic surveys have already been undertaken within the Exploration Area during the three-year Exploration Right Initial Period. EMEPSAL and Impact Africa are proposing to undertake a 3D seismic survey during the First Renewal Period. The proposed minimum work programme includes acquisition and processing (and/or licensing) of km 2 of 3D seismic data which is estimated to take between 45 and 100 days. EMEPSAL and Impact Africa may decide to acquire (and/or license) additional 3D seismic data up to a combined size of 5000 km 2. The entirety of two seasons (approximately 7 months or 3 and a half months per season) would likely be needed in order to acquire a 3D survey of this size. The location of the survey is not yet finalised but is likely to be located in the deeper water parts of the Exploration Area. EMEPSAL and Impact Africa may also elect to acquire (and/or license) up to 2000 line km of 2D seismic data. The lines for this survey could be located anywhere within the Exploration Area. A typical 3D seismic survey configuration is illustrated in Figure 2.1 and comprises the following components: Dual towed airgun arrays. Up to 12 or more hydrophone streamers spaced 50 to 100 m apart and between 3 m and 50 m (depending on survey technique) below the water surface. The streamer configuration can be up to 8 km long and up to 1.2 km wide. 6

3 On the tow-ship, a control and recording system co-ordinating the firing of shots, the recording of returned signals and accurate positioning for any of the above configurations. An alternative recording configuration might be considered to better obtain efficient coverage in harsh seas and strong currents. Such a configuration would employ a vessel towing the same dual-source and streamer configurations described above but the streamers would be about half the length (ie up to 4 to 5 km maximum). To record the longer offsets (to 8 km), a second vessel with only dual towed airguns arrays would sail approximately 4 km in front of the primary source / streamer vessel. The sources on the two vessels are fired almost simultaneously (< 2 seconds apart). Although there is more source firing within a given area, there are overall time savings in survey duration due to shorter line changes, less infill recording, and faster streamer deployments and retrievals. Figure 2.1 The Configuration of a Typical 3D Seismic Survey Operation HYDROPHONE STREAMERS Source: CCA, 2001 As data acquisition requires that the position of the seismic survey vessel(s) and the arrays be accurately known, seismic surveys utilise accurate navigation of the acoustic source over pre-determined survey transects. The source arrays and the hydrophone streamers need to be towed in a set configuration behind the seismic survey vessel. As a result, the seismic survey vessel has limited manoeuvrability while operating, and is unable to deviate significantly from the planned seismic lines. Ship tracks in a 3D seismic survey are typically about 300 m to 800 m apart. Because of the large dimension of the towed streamers, the seismic survey vessel has a wide turning circle (approximately 4 km to 6 km turning radius). 7

4 2.2.2 Seismic Survey Vessel and Equipment A seismic survey vessel travels along transects of a prescribed grid that is chosen to cross any known or suspected geological structure in the area. The acoustic source is fired approximately every 10 to 20 seconds. The sound waves are reflected by boundaries between sediments of different densities and velocities and returned signals are computer processed after being recorded by the hydrophone streamers. During surveying, seismic survey vessels travel at a speed of four to six knots. The seismic survey would involve towed airgun array(s), which provides the seismic source energy for the profiling process, and a seismic wave detector system, usually known as a hydrophone streamer (see Figure 2.2). The anticipated airgun and hydrophone arrays would be dependent on the vessel used. The acoustic source or airgun array (one array for 2D and two arrays for 3D) would be situated some 80 m to 150 m behind the vessel at a depth of 3 m to 15 m below the surface. A 2D seismic survey typically involves a single streamer, whereas 3D seismic surveys use multiple streamers (up to 12 streamers spaced 50 m to 100 m apart). The array streamers for a 2D seismic survey can be up to 12 km long, but for 3D seismic surveys, streamer lengths usually do not exceed 8 km. The streamer(s) would be towed at a depth of between 3 m and 50 m and would not be visible at the surface, except for the tail-buoy at the far end of the cable. A typical 3D seismic survey vessel is illustrated in Figure 2.2 below. Figure 2.2 Seismic Survey Vessel Conducting a 3D Seismic Survey Source : 8

5 2.2.3 Exclusion Zone While the need for an exclusion zone was discussed in the EMPr (May 2010), this related to a 2D survey, where a single streamer is towed. In contrast, because of the size of the 3D configuration, the exclusion zone for the 3D seismic survey is to be 5 nautical miles (approximately 9 km) from the exploration vessel and streamers. The streamers (up to 12) towed behind the exploration vessel are up to 8 km long (for 3D) and can be up to 1.2 km wide with a tail buoy marking the end of each streamer. The seismic survey vessel may therefore need to systematically turn and acquire survey lines in the form of a spiral race track as shown in Figure 2.3. Figure 2.3 Schematic Illustrating the Movement of a 3D Seismic Survey Vessel Source: ERM, AIRBORNE GEOPHYSICS SURVEY Whilst not included in the committed work programme for the year First Renewal Period, EMEPSAL and Impact Africa may in the future decide to acquire (and/or license) airborne gravity and magnetic data over parts, or all, of the Exploration Area. If data were to be acquired over the entire Exploration Area, this would take an estimated 70 days to complete. A smaller survey is more likely, for example in conjunction with (and as an extension of) a survey acquired in an adjacent exploration area. 9

6 Acquisition would be accomplished using a fixed-wing aircraft. To obtain data, the survey design would likely include flight altitudes of 300 m and relatively close parallel spaced lines, typically oriented at right angles to the main geological structure orientation. 2.4 MULTI-BEAM BATHYMETRY SURVEY Whilst not included in the committed work programme for the two-year First Renewal Period, EMEPSAL and Impact Africa may in the future decide to acquire multi-beam bathymetry data over parts of the Exploration Area. This survey would be conducted to produce a digital terrain model of the seafloor (Figure 2.4). If data were to be acquired over the entire Exploration Area, this would take an estimated 110 days to complete at a vessel speed of approximately 5 knots. A marine survey vessel would be equipped with a multi-beam echo sounder to obtain swath bathymetry and a sub-bottom profiler to image the seabed and the near surface geology. The multi-beam echo sounder provides depth sounding information on either side of the survey vessel s track across a swath width of approximately two times the water depth to produce a digital terrain model of the seafloor. The multi-beam echo sounder emits a fan of acoustic beams from a transducer at frequencies ranging from 10 khz to 200 khz and typically produces sound levels in the order of 207 db re 1μPa at 1 m. The sub-bottom profiler emits an acoustic pulse from a transducer at frequencies ranging from 3 khz to 40 khz and typically produces sound levels in the order of 206 db re 1μPa at 1 m. The operating frequencies of the acoustic equipment used in sonar surveys typically fall into the high frequency khz range. 10

7 Figure 2.4 The Multibeam Echo Sounder uses a Fan of Sound Beams to Construct a 3D Picture of the Seafloor Source: National Oceanographic and Atmospheric Administration (NOAA) National Ocean Service (2009) (accessed at Backscatter data is typically collected concurrently by the multi-beam echo sounder as it can measure several properties of the seafloor associated with hydrocarbon seeps including: hardness; roughness; and volumetric heterogeneity. One or more of these three properties can result in an increase in backscatter intensity recorded by the multi-beam system and aid in the identification of potential natural hydrocarbon seeps on the seafloor. The data acquired by these sonar techniques will be used to identify, prioritise, and target potential seabed sample and heat-flow measurement locations. Selected sites could then be sampled with precisely positioned piston cores. 2.5 SEABED HEAT FLOW MEASUREMENTS PROGRAMME Whilst not included in the committed work programme for the two-year First Renewal Period, EMEPSAL and Impact Africa may in the future decide to acquire heat-flow measurements. The heat flow measurements would be conducted using heat flow probes, which would measure both the in situ temperature and thermal conductivity of sediments up to 12 m below the seabed. The measurement probe typically consists of a 6 cm diameter solid alloy steel bar, which extends from the wire termination at the top through the 500 kg lead-fill weight stand, down to the tip of the heat flow probe. The outrigged thermistor string is attached parallel to the steel bar. The measurement device would be lowered from a survey vessel into the seabed, allowed to equilibrate for the measurement to be taken, and then recovered to the 11

8 surface. No environmental samples or other materials would be recovered with the heat flow probe. Acquisition of this data would be used to determine the thermal regime and calibrate thermal models to understand hydrocarbon system potential. Figure 2.5 Example of Heat Flow Probe Source: TDI-Brooks International ( It is anticipated that up to 90 measurements would be collected across the Exploration Area, which would take an estimated 50 days to complete. 2.6 SEABED AND WATER COLUMN SAMPLING PROGRAMMES Whilst not included in the committed work programme for the two-year First Renewal Period, EMEPSAL and Impact Africa may in the future decide to undertake a seabed and water column sampling programme. The decision to undertake the sampling programme might be taken if hydrocarbon seeps were identified on the seafloor during other exploration activities. The number of samples and the exact location(s) would be identified from the analysis of the 3D seismic and the multi-beam bathymetric survey results Water Column Sampling Water column properties may be measured or water samples may be collected for analysis of naturally occurring hydrocarbons and heavy and trace metals. 12

9 Additionally a Conductivity, Temperature, Depth (CTD) profiler may be deployed to measure additional parameters such as salinity, temperature, dissolved oxygen and turbidity. An estimated number of water sample stations has yet to be determined, however, for each station it is expected that three water depths will be sampled, including: near surface (approximately 1 m below surface); mid-water; and near bottom. Mid-water samples will likely be taken approximately 10 m below a measureable (distinct) thermocline. The depth of the thermocline will be determined using a single deployment of the CTD profiler. If a distinct thermocline does not exist, or cannot be established, the mid-water sample will be taken at a depth that is nominally half-way between the water s surface and the seabed. Although specific water sampling equipment selections are yet to be made, there are many discrete water sampling devices available. Niskin and Go- Flo are examples of commonly available discrete samplers suitable for water column sampling and can be deployed singularly, in chains separated at predefined depth intervals, or attached to intelligent depth actuated clusters (rosettes) Seabed Sampling Piston and box coring (or grab sampling) would be used to collect seabed samples for geochemical analysis. The number of samples will depend on amount of seepage or number of expulsion features identified and also depending on the observed geologic complexity. Piston coring is one of the more common methods used to collect seabed samples for geochemical analysis (see Figure 2.6). The piston coring operation is carried out by winching the tool over the side of the survey vessel and lowering the corer to just above the seabed (A). As the trigger weight hits the bottom (B), it releases the weight on the trigger arm and the trigger arm begins to rise. Once the trigger arm has risen through its full 1.2 m of travel (C), the corer is released to free-fall the 3 m distance to the bottom, forcing the core barrel to travel down over the piston into the sediment. When the corer hits the end of its 3 m slack loop, the piston starts up the core barrel (D) creating suction below the piston, and expelling the water out the top of the corer. When forward momentum of the core has stopped, a slow pullout on the winch is begun. The suction created by the core sample in the liner prevents movement of the piston to the top of the core barrel in response to tension on the core wire. This suction triggers the separation of the top and bottom sections of the piston (E). The bottom half of the piston remains in place over the sediment to maintain integrity of the sample, while the top half (attached to the coring wire) fetches up against the stop in the core head, allowing the 13

10 corer to be pulled out of the sediment. The entire assembly, including the sample, is retrieved on to the survey vessel. The recovered cores are visually examined at the surface for indications of hydrocarbons (gas hydrate, gas parting, or oil staining) and three sets of subsamples are retained for geochemical analysis in a laboratory. Any material having geologic or environmental interest would be preserved for further study. The remaining sediment would be returned to the seabed. Figure 2.6 Diagram Illustrating a Piston Core Operation at the Seabed Source: TDI-Brooks International ( An alternate to the piston corer is a box corer. A box corer (see Figure 2.7) is deployed from a survey vessel using an A -frame or a sliding beam with at least 3 m clearance and is lowered vertically to the seabed. At the seabed the instrument is triggered by a trip as the main coring stem passes through its frame. The stem has a weight of up to 800 kg to aid penetration into the seabed. While pulling the corer out of the sediment a spade swings underneath the sample to prevent loss. When the corer is brought back on board, the spade is located under the box. The recovered sample is completely enclosed after sampling, reducing the loss of finer materials during recovery. Stainless steel doors, kept open during the deployment, to reduce any bow-wave effect during sampling, are triggered on sampling and remain tightly closed, sealing the sampled water from that of the water column. On recovery, the sample can be processed directly through the large access doors or via the removal of the box completely, together with its cutting blade. 14

11 Another means to collect sediment samples it a grab sampler. Grab sampling is a simple process of bringing up surface sediments from the seafloor. This method, however, cannot be used to characterise different sedimentary layers since it is unable to penetrate the sediment at any depth and as such a mixture of surface sediments is produced. Once the grab sampler is launched, the jaws open and it descends to the seafloor. A spring closes the jaws, trapping sediments or loose substrate. The grab sampler is then brought up to the surface and the contents removed and studied. For all sampling techniques, water depth, date, time and WGS 84 latitude and longitude are recorded for each sample. Figure 2.7 Example of a Box Corer Source: Hannes Grobe, Alfred Wegener Institute for Polar and Marine Research, 2006 (Accessed via Wikipedia, 29 April 2014) The number of seabed samples that would be collected has not yet been determined. Each individual piston core would have a disturbance area and volume of 0.01 m² and 0.07 m³ respectively. Each individual box core would have a disturbance area and volume of 0.05 m² and up to 0.3 m³, respectively. If implemented, it is anticipated that the seabed sampling programme would take an estimated 110 days to complete and no more than 5 m 3 of sediments would be taken for analysis. 2.7 AUTONOMOUS UNDERWATER VEHICLE (AUV) SURVEY Whilst not included in the committed work programme for the two-year First Renewal Period, EMEPSAL and Impact Africa may in the future decide to undertake an Autonomous Underwater Vehicle (AUV) survey. An AUV is a 15

12 self-propelled underwater robotic device controlled and piloted by an onboard computer. AUVs constitute part of a larger group of undersea systems known as unmanned underwater vehicles, a classification that includes nonautonomous, underwater Remotely Operated Vehicles (ROVs), controlled and powered from the surface by an operator/pilot via an umbilical or using remote control. AUVs are used in oil and gas exploration to make a detailed map of the sea floor and the shallow subsurface to help exploration activities be undertaken in a safe, cost effective manner and help minimise disruption to the environment. In this regard, the AUV also allows survey companies to conduct precise surveys of exploration areas where traditional surveys would be less effective or too costly. In this regard, AUV work may be undertaken in addition to or in substitution for some of the other activities listed (eg multibeam bathymetry and water column sampling). In addition, AUVs can be equipped with specialised sensors to collect and monitor seabed and water column data where necessary. If carried out, the survey would be use a commercially-available AUV equipped with a multi-beam echo sounder, side scan sonar, sub bottom profiler and geochemical sensors to locate, characterise and map hydrocarbon seeps over the Exploration Area to help enhance the chance of exploration success. See an example in Figure 2.4 Figure 2.8 Deepwater Seafloor Mapping using an AUV Source: Kongsberg Maritime ( 16