While seismic surveys are often used by the oil and gas industries to find reserves, scientists can use the same techniques to look for air bubbles, which can signify the presence of hydrothermal vents. Geophysical surveys are complex operations that require skilled specialists to operate the equipment.
Seismic surveys work by having a sound source generate energy, commonly in the form of airwaves, which travel towards the seabed and through different layers. Eventually this energy is reflected back up towards a listening device where the waves are recorded over a predetermined time period using hydrophones. Seismic surveys performed by the NOC are 2-D and consist of the following.
- A research vessel
- An array of air guns (sound source)
- A towed streamer (listening device) ending in a tail buoy.
The 3000m acoustic streamer that the NOC uses is a ‘Solid Sentinel Active Section’, which has a polyurethane outer skin surrounding a foam-filled inner with data wires and strain members to take the tension running through the middle. Each section is 150m in length, containing eight hydrophones in 12.5m compartments, with two communication coils per section. Sections are connected end to end with a maximum of five sections before requiring a module to boost and transmit the data. 2-D surveys commonly run 12km streamers and 3-D commonly eight times 6km.
Digicourse birds are connected to a communication coil every 300m to help maintain streamer depth and transmit compass data down the streamer to the acquisition system. This data can be used in processing to define the streamer shape, drift angle and streamer depth all to help locate the position of the reflection on the sea floor.
The tail buoy is used as an identifying mark for the end of the streamer- helps other vessels avoid the underwater obstruction, as well as for positional data.
How seismic surveys work
Before a survey can even begin the operators must perform a marine mammal observation. If the observation period passes, the test work can begin.
Positioning of the vessel, the source, the streamer and the tail buoy is extremely important. The greater the accuracy of the equipment the greater the accuracy of the data. Vessels use differential GPS (DGPS), which uses base stations with a known position to calculate what the travel time of the GPS signal should be to what it actually is. It then sends this correction to the vessel to be applied to the GPS signal sent to it to ensure greater positional accuracy of the at-sea equipment.
The gun source(s) and tail buoy can also accommodate GPS modules to help improve the accuracy. Commonly these would be used as referential GPS (RGPS) where the range and bearing of the units are applied to the overall positioning geometry of the streamer.
The data that is produced is a time slice of the ocean floor at the point of reflection for a given time period. The point of reflection is called Common Mid Point (CMP), or layback value, and is calculated as the distance from the vessel reference point to the centre of source plus half the distance from the centre of source to the first receiver group on the streamer. This data is then processed to produce a time slice per CMP showing the ocean floor and the layers beneath.
Conducting a survey
To run a seismic survey there is a lot of hardware and software systems required to capture the real time data and to ensure its quality. There should be at least one person to oversee the sound source software and data, one person for the compass data and positional data of all the systems, one person for streamer data acquisition and one for quality control of the real time data including networking and data transfers.
One of the major problems with seismic is the synchronisation of all the systems operating at real time. One-way to overcome this is to route all the data and networking through a central system that monitors and controls all the IO for the streamer the guns and the vessel. Allowing one central system, referred to as a Navigation system, to be in control of synchronisation ensures the triggering and time stamping for each shot is all done centrally and not by different GPS systems that may vary slightly in time and position causing undue errors and deviations.
As seismic software and hardware progresses so will the manner in which data is captured. There are new developments and alternatives to traditional seismic such as magnetic and gravity seismic surveys. The seismic industry is fast paced and ever changing and with over 70% of the ocean still unexplored there is a lot of opportunity for new discovery.