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Data Organization
One of the main differences between a cabled system and a nodal system is the organization of the raw recorded data. Cabled systems output “shot records”, or the collection of all the traces recorded by a pre-defined array of receivers for a shot. Nodal systems are the opposite and output “receiver gathers”, or all the traces recorded at a given receiver location for a predefined array of shots.
A cabled recording system activates a set of receivers and then sets off a shot. Traces are recorded at the receivers for a specific amount of time and the seismic data for all the receivers are fed to the recording instrumentation which assembles the shot record. All the receivers start recording and sending data to the recording system at exactly the same time that the shot was activated. Therefore, a shot record is composed of all the traces from all the receivers assigned to that shot for a predefined amount of time after the shot was started. The traces are arranged in recording channel number order. The channel numbers usually follow sequential receiver locations along a receiver line.
Nodal systems record continuously for several hundred or several thousand shots with ”idle” time, where no shooting is occurring, between the live trace segments. The traces are extracted from the continuous stream by selecting the samples starting at the time matching the time of a shot and the samples that follow for the prescribed recording time which is typically 4 to 6 seconds, plus listening time equal to the length of the sweep. Think of it like cutting a long piece of wood into 1ft lengths with a variety of gaps between the 1 ft segments and then lining the 1ft long pieces up next to each other and throwing away the rest. This process creates a receiver gather of traces with all of the samples between live traces being discarded.
The traces are merged with the geometry information, annotating the trace headers with their receiver number, shot number, spatial coordinates, recording channel number and a variety of other attributes.
Seismic data processing systems are commonly “shot based” which means that they expect the data to be organized into shot record “gathers” or “ensembles” coming in from the field. It is also common for there to be multiple sweeps at the same shot location to build up the signal to noise ratio. Cabled systems will usually perform the stacking and correlation of the recorded data prior to assembling the final shot record. The deliverable is typically a single correlated and stacked shot record for each shot location. Nodal systems may or may not have the capability to do the stacking and correlation in the field. It is common for the deliverable to be the raw unstacked and uncorrelated receiver gathers, where those operations would be performed in the data processing center.
Nodal system data volumes are also larger due to the closer spacing of the nodes for single-sensor recording when compared to receiver spacing of cabled systems using geophone arrays. Sometimes multiple nodes are summed to simulate a receiver array, but again, this is generally performed in the processing center. Nodes may be placed much closer together than typical cabled system station spacing, which is controlled by the takeout interval. This results in more traces per shot for nodal systems.
Consider the data explosion. When you combine uncorrelated records, individual traces per sweep and more traces per shot, you can quickly increase the amount of data delivered to a processing center by orders of magnitude when compared to a cabled system.
In the previous article we talked about quality control. To perform the QC operations, the in-field data processing and QC capabilities need to be able to absorb the raw recorded data, correlate and vertically stack it and need to be able to sort the data from receiver to shot domain, at least for a subset of the data.
The ability to handle these processing and data management requirements is one of the advantages of the STRYDE system. STRYDE provides the in-field QC and processing required to ensure the quality of the seismic data meets or exceeds the requirements of the project.
Final Image Quality
This set of articles started by describing a theoretical survey with potentially millions of deployed nodes and talked about some of the logistics related to the collection operations.
Why?.....
Let’s go back to the fundamentals of exploration geophysics. The objective of an exploration project is to generate the most accurate and most interpretable image of the subsurface of the earth possible. The end goal is to enable the explorationists to most effectively locate and most efficiently extract resources from subterranean reservoirs.
In the early days of exploration, the amount of data recorded was very small and as a result the images were very poor. As technology progressed and logistics allowed, the amount of data increased, which yielded images of superior quality. In simple terms, to generate the best images, you need to record as much data as you can.
Cabled systems have physical limitations to the amount of equipment that can be handled and deployed due to the bulk, weight and cost of the equipment. Cabled systems were used for decades and produced large surveys with relatively large data volumes, but the data still suffered from some fundamental deficiencies related to inadequate spatial sampling causing aliasing of the slower noise trains and refracted events which hindered the data processors from being able to remove the noise and enhance the signal.
Early nodal systems were a step forward, but the size and weight of the early generation nodes also physically limited the number that could be deployed, recovered and cycled efficiently.
The STRYDE Node is a significant step forward toward achieving the reality of multi-million receiver surveys with the goal of dramatically increasing the amount of data that can be recorded per unit of area. The STRYDE Nodes are small, light, and cost effective, and are easy to deploy, retrieve and cycle. Their portability, size and weight also allows survey designs with much closer station spacing than any other nodal or cabled technology. It is this dense receiver spacing that dramatically improves the spatial sampling of the low velocity noise that travels along and near the surface so that it is not aliased. Dense receiver spacing also improves the sampling of the subsurface by increasing the number of raypaths that traverse the geology and provides better illumination of the exploration targets.
The processing of the tightly spaced data takes advantage of the noise reduction algorithms and their ability to perform better on data where the noise is not aliased. Other algorithms exploit the large volume of data to help reinforce the signal and reduce the noise via constructive and destructive interference. The data are higher fold in shot, receiver and CDP domains with better offset and azimuth distributions. This improves investigations of fracture-induced azimuthal anisotropy as well as the generation of more accurate AVO information for direct hydrocarbon indications. Every sample that is recorded adds more information to the stacked signal and reduces the random noise resulting in more accurate and more interpretable seismic images.
Summary
There are so many advantages to using nodal systems for land seismic recording that it is inevitable that the majority of future surveys will use this technology. The keys to success lie in the choice of equipment and the ability to utilize that equipment to its fullest extent. Surveys with multi-billions of recorded traces will become the norm and processing systems will continue to evolve to be able to handle the huge data volumes and the various ways that data can be organized. STRYDE is the leader in node technology and has an extensive track record of providing the equipment, training and execution of surveys with some of the highest receiver counts and traces densities ever recorded.
We at STRYDE look forward to helping you achieve your exploration goals and invite you to contact us for more information...
