The development of seismic acquisition systems for land is divided into two types of systems, traditional cable-based systems and cableless systems. Cableless systems may be subdivided further by communication strategies and form factors. The hybrid system combines the strengths of cable systems and cableless systems into one seamless system that minimizes the weaknesses of each architecture and enhances the over-capability of the system.

The cable-based system is the industry standard for recording seismic data with an architecture that still dominates global usage. Capable of working in a diverse set of environments, the primary benefits of a cable-based system include detailed quality control (QC) of the data and secure collection. Data stream to a central computer and are recorded in near-real time with the shooting.

The drawback of this method is the heavy and cumbersome cables. Cables expose the operation of the system to unwanted human contact as well as impact on animals and other environmental factors. Cables also constrain the acquisition designs to standard receiver station spacing and geometry configurations. Cables add a substantial amount of weight to the system, which increases the burden on deployment crews.

The cableless system architecture typically uses a storage and harvest strategy. Individual nodes record and store data for harvest after the shooting is complete. The lack of cables means that system weight is reduced, and there are no constraining factors like cable lengths and take-outs to limit the receiver station interval and final form of the recording geometry. However, the benefits gained by the cableless architecture come at the cost of limited QC and the risk of data stored in individual nodes that remain in the field until harvest.

Hybrid system

A hybrid system combines the two architectures into a single system where operators can mix cableless recording with cable-based recording to effectively adopt the strengths of both systems and minimize the weaknesses. This provides many operational advantages. Regions with difficult access can be reserved for cableless recording, while the cable-based system can be deployed in less sensitive areas. Another added benefit is that the cableless system adds very small additional operational cost since the nodes can be added to the project at any time and removed at any time with no impact to the shooting strategy. However, the benefits extend beyond the operational considerations. The flexibility of the hybrid system creates the opportunities for hybrid geometry strategies such as variable density recording, which allows geophysicists to modify sampling strategies to achieve specific sampling goals.

System concept

By design, most cableless recording nodes are continuous recording instruments. They synchronize their timing using precise 2-ms pulses of the GPS satellites and record data continuously. The data are later parsed into records when synchronized with the source timing, and the remainder of the continuous data is discarded. The continuous recording nature of the cableless system makes it a nearly perfect candidate to function as a slave system.

The old concept of master/slave systems was originally introduced to build up channel counts by combining multiple systems. Very high channel counts of modern systems have eliminated this need, but the concept still applies when mixing system types for added flexibility and benefits.

A cable-based system can be deployed, and the continuously recording cableless system can be mixed in with the cabled system in nearly any fashion to suit nearly any requirement. The cable-based approach deals with templates. A template is a subset of sensor stations that record for any specific source point.

Cableless systems require no notion of templates since they are always recording when deployed. This means that a cable-based system that is augmented with cableless nodes can be used in a normal way with only limited consideration for the channels added by the cableless nodes. A well-implemented hybrid system will combine the traces from the two systems after shooting and harvest is complete to form complete trace gathers of raw data ready for processing.

Geophysical advantages

The operational aspects of a very flexible system make possible high-density or otherwise large seismic spread recording in nearly any environment such as urban areas or mountainous areas where the use of cable-based systems is quite complicated. However, some of the greatest strengths of this approach can be realized in the opportunity for creative sampling strategies.

Survey geometries are often designed to image single prospects or depths. However, many areas have multiple prospects at multiple depths. It is not uncommon to have a shallow imaging prospect that requires a denser, finer sampling pattern combined with a deeper prospect that requires larger apertures and less dense sampling.

A variable density survey design can accomplish the imaging goals of both prospects with the same source effort. In addition to pre-planned survey goals, cableless nodes can be added to survey geometries at any time during acquisition to begin recording additional traces for specific sampling goals.

Source effort

It is common to begin acquiring a seismic survey and discover that areas of the shooting are poorly sampled due to noise or surface features obstructing deployment or shooting. A common practice among seismic operators is to augment those portions of the survey with additional source effort. The cableless nodes change that traditional strategy because they can be added at any stage during the acquisition to increase trace density in designated areas.

This benefit is gained because cableless nodes are continuous recorders. Once deployed, the nodes begin to collect trace data from any shot fired. This allows the seismic contractor to adapt the project as it shoots to achieve better sampling results. This strategy is accomplished with no additional source effort and minimal additional operational effort, so it adds very little cost to the final project.

Cableless nodes also can augment active shooting in less common ways. A great deal of seismic data is currently being recorded to guide completion strategies where accurate geosteering is the primary goal of the seismic. Additionally, cableless nodes are capable of monitoring microseismic events.

The flexibility of nodes is a great benefit on active 3-D projects to record data from a variety of specialized sensors such as downhole arrays. In this case, the downhole data are recorded simultaneous with surface shooting to help calibrate and understand the details in the surface shooting. The same nodes may be used in a passive mode to record microseismic events from either downhole sensors or surface spreads.

The flexibility of the cableless node is only beginning to be fully appreciated by the seismic industry. Nodes can form hybrid systems to create a seismic recording architecture capable of imaging multiple targets or adapt to a diverse set of demands with little additional cost. Because of the continuous recording nature of the node, it can be used in nearly any situation where seismic data are recorded.

Hybrid systems can be scaled from just a few nodes to hundreds of thousands of channels and can be deployed with any geometry type without suffering the inefficiencies arising from pre-configured cables designed to accommodate standard sampling intervals. The benefits of the hybrid system enable seismic contractors to be more flexible and adaptive, while acquisition geophysicists will find that they can design surveys with less restrictions. This creates the opportunity to improve the quality of seismic data in nearly any environment.

References available.

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