At present all countries have shifted the focus of oil and gas exploration to Ocean. As the main means of oil exploration in shallow sea areas, OBC (Ocean Bottom Cable) seismic exploration and OBN(Ocean Bottom Node) seismic exploration have been rapidly promoted and applied in seismic exploration for shallow sea by relying their flexible layout characteristics. However, OBC and OBN have much difference in some situations. This FAQ is talking about what’s the difference on OBC and OBN Seismic Exploration?
Basically, OBC places an underwater geophone (hydrophone) on the seabed, and then generates seismic waves which will be received by underwater geophones(hydrophones), and finally transmit the data received from geophones to the data acquisition system through OBC cables. At present, the mainstream data acquisition system for OBC is Sercel’s 408UL, 428XL and Inova’s G3I.
OBN is a multi-component seismometer located on the seafloor which could independently collect and record seismic signals. Actually, OBN are independent “geophones” that do not need cables to transmit data and are flexible to deploy on the seafloor, which makes OBN more accurate and higher quality on data acquisition. Nowadays the commonly used OBN systems are the Z series from Fairfield and the OBX series from Geospace.
OBC and OBN seismic exploration are all laid “geophones” on the seafloor to collect seismic data. A complete OBC and OBN field acquisition operation includes three main processes: cable laying, cable positioning and seismic data acquisition. Cable laying put OBC or OBN cable with geophones installed to the seabed in accordance with the designed position. Cable positioning mainly confirms the specific spatial location of the geophone or individual node on the cable and generally adopts the way of acoustic positioning or first arrival positioning. The acquisition of seismic data is to generate seismic waves in the ocean by artificial means, so as to collect seismic data accordingly.
OBC seismic exploration requires that the navigation process cannot be interrupted during the cable laying operation. The whole laying process is a continuous process. When there are obstacles on the laying survey line, the navigation route should be planned in advance to ensure the continuity of the cable.
Since each node on OBN seismic exploration is an independent acquisition device, the connecting cable between nodes is only used as a physical connection, not as a data transmission. When there is no special terrain or obstacle in the working area and the water depth is no more than 100m, TMS(Tether Management System) is often used to carry out the cable laying operation.
In this case, the OBN cable releasing operation is basically the same as the OBC cable laying operation. When there are special terrain or obstacles in the working area, the characteristics of the nodes themselves make it easier to lay the cables. In this case, no cables are used to connect the nodes and the navigation system directs the DP ship to arrive at the designated position, then a single node will be laid on the designed point to complete the cable laying operation.
ROVs (Remotely Operated Vehicles) are often used to place nodes in complex subsea terrain with a depth more than 100m.When the navigation software directs the ship, it also monitors the ROV in real time to ensure that the nodes are laid on the designed points and the cable laying can be completed.
The purpose of OBC and OBN cable positioning operation is to obtain the actual spatial position of the underwater geophone (hydrophone). The commonly used methods are acoustic positioning and first arrival positioning. First-arrival wave positioning adopts picking up the first-arrival signal after the event, and there is no difference between OBC and OBN operations.
The commonly used acoustic Positioning methods are Long Baseline Positioning (LBL) and Ultra short Baseline Positioning (USBL). OBC seismic exploration cable laying is a continuous process, so LBL is usually used for submarine cable Positioning. When OBN seismic exploration carries out node laying, USBL is commonly used to obtain the spatial location of nodes in real time, so as to ensure that nodes are laid on the designed points in deep water or complex terrain areas.
In OBC seismic exploration, seismic data acquisition mainly relies on navigation and synchronizing the source and the instrument. In the specified record length, no other air gun source is allowed to excite, otherwise, the current excitation will interfere or the next excitation will be incomplete, thus affecting the quality of seismic data. OBN seismic exploration is a real-time process, which can also be separated from the overlapping seismic wave data, so the air gun source can be continuously excited in a very short time and the data collection efficiency is higher.
Nowadays, submarine seismic exploration has entered an era that is dominated by high technology. OBC seismic exploration adopts double geophone reception technology commonly and wide azimuth acquisition technology has become a normal method. At the same time, with the development of OBN technology, multi-source simultaneous acquisition technology, broadband, wide azimuth and high-density acquisition technology have been promoted. Permanent and semi-permanent submarine node equipment and efficient& rapid multi-phase seismic monitoring make the offshore time delay (4D) seismic technology is developing rapidly.