President/CEO, BJV Design

President/CEO, Absolute Imaging Inc

Software Developer, Absolute Imaging Inc.

We are excited to launch a new column in the RECORDER, named “Ask the Experts”. In this column, we address a technical question submitted by our readers. The Editorial Team chooses a team of experts who are asked to provide a detailed answer to this question. If you wish to have one of your questions answered by one of our panelists, please email us at recordereditor@cseg.ca.

In today’s column, we posed the following question:

Over the past few years, wireless nodes are being deployed widely for onshore seismic programs. What are the relative advantages and/or disadvantages of these nodal systems vs the “conventional” cable systems?

We asked Jason Schweigert, Acquisition Geophysicist at BJV Design Inc., Elvis Floreani, and Dennis Quinn, from Absolute Imaging Inc., for their comments on the above. Below are their responses. Jason’s comments come from the point of view of an Acquisition Geophysicist while Elvis provides what he sees from the processing side.

Jason Schweigert:

Overall, the biggest advantage of Nodal systems over traditional cable systems is their weight and size. Traditional cabled systems are cumbersome due to vast amounts of cables and large batteries, making equipment layout very slow and costly. Nodal systems (especially the latest generation of all-in-one nodes) are light and easy to carry; therefore, a significantly larger number of sensors can be deployed daily (with fewer people) than the traditional cabled system. This equipment evolution has contributed to higher-density surveys being acquired for a reduced cost and has enabled subsurface exploration companies to successfully utilize advanced interpretation methods such as Inversion, AVO, and FWI, resulting in increased economic value of seismic to the company.

Compared to cabled systems, a comparative disadvantage of the Nodal system is the loss of real-time QC in the recorder. In a cabled system, extensive real-time QC can be done in the recorder to give the client confidence in the quality of the recorded data. In Nodal systems, we do not have that luxury; for the most part, you are ‘blind recording.’ That being said, many Nodal systems can QC portions or segments of shot records during recording but nowhere near the flexibility of cabled systems.

One additional comment I would like to make regarding this topic, which is rarely discussed infrequently, is how Nodal systems have made receiver stub lines easy and more cost-effective to use. In contrast, receiver stub lines were extremely difficult and not often used in cabled systems. Receiver stub lines are typically used to improve offset/azimuth statistics within source exclusion areas (e.g. riparian areas). In a cabled system, receiver stub lines had to be ‘snaked’ together, which means that the receiver stub lines had to be connected to adjacent receiver lines via long cables (where the term ‘snaking’ comes from). This process was time-consuming and a headache for field crews, so we rarely did them. Using nodes efficiently and cost-effectively to improve coverage in exclusion zones is one of many areas where nodes have revolutionized the way we model seismic programs today.

Elvis Floreani / Dennis Quinn:

There has been a steady increase in recent years in the amount of seismic data acquired using self-contained nodal detectors as opposed to the traditional cabled geophone arrays. The benefits on the acquisition side are easy to appreciate: (1) the handling of compact and lightweight self-powered units versus handling of groups of geophones connected to miles of cable, and (2) the nodal receivers are typically equipped with internal GPS units simplifying the surveying process. These factors allow for seismic data to be collected faster and at less cost, while at the same time achieving tighter spatial sampling. In addition, each receiver records and stores its own data. The task of retrieving and collating the data from the individual receivers is usually undertaken by the acquisition crew either during the shoot or after, and the data are delivered to the processing shop in the form of traditional shot gathers. From the processor’s point of view, the biggest issues arising from nodal receivers are noise and the question of phase:

1. Noise

Data recorded on nodal detectors tend to be noisier than that recorded on grouped geophones. This is due to the fact that the recorded signal collected from a geophone group is an average of responses of each phone in the group, with some cancellation of random noise coming at the expense of a small amount of spatial smearing. But more important than that is the fact that a geophone group arrayed over the length of a ground-roll wave can significantly reduce the amount of ground-roll contamination in the recorded data, although this effect is less significant in 3D shooting, in which case the ground-roll wave can approach a receiver-array from any direction, not only the in-line direction. With single positioned point-receivers there is no possibility for automatic noise-attenuation resulting simply from detector positioning. Tighter point spacing, however, reduces the likelihood of spatial aliasing of reflections and diffractions allowing for better imaging. Also, steeply dipping linear interferences such as ground roll are easier to estimate and remove if they are not aliased.

2. Phase

Seismic detectors whether they be geophones, MEMS-type (capacitive) nodes, or STRYDE-type (piezoelectric) nodes are all mass-spring systems. Because their resonance frequencies are much higher than typical seismic frequencies, the nodal detectors are classified as accelerometers because the displacement of their proof-mass is proportional to ground acceleration, as opposed to ground velocity in the case of geophones. Though they are both nominally accelerometers, MEMS and STRYDE nodes nevertheless differ significantly in their internal construction, leading to differing character implications in processing, particularly if one is wanting to match the product to collocated geophone data. MEMS-type detectors are termed ‘capacitive’ transducers and consist of a conductive plate attached to the proof-mass and suspended between a pair of charged plates fixed to the detector housing. Output voltage is proportional to the relative displacement between the fixed plates which move with the ground and the suspended plate which stays relatively stationary due to inertia. In STRYDE-type detectors, the proof-mass rests on a crystal of piezoelectric material which is fixed to the detector housing. Output voltage is produced by pressure changes experienced by the crystal resulting from relative acceleration.

Table 1. Impulse Response and Voltage Outputs of Different Detectors

A detector’s output characteristic depends upon both how it experiences the input ground motion and its own internal impulse response. The first is dictated by the difference between the detector’s resonance frequency and the frequencies of interest. In the case of a geophone, the resonance frequency is similar to the typical dominant frequencies of seismic data. For frequencies near the resonant frequency the displacement of the proof-mass is proportional to ground velocity. As mentioned earlier, the resonance frequency of MEMS or STRYDE detectors is much higher (about 1000Hz) than typical seismic frequencies. For frequencies far lower than the resonance frequency the displacement of the proof-mass is proportional to the ground acceleration. The detector’s impulse response is dictated by both the kind of motion it responds to as well as how the voltage is produced in response to that motion. The responses are summarized in the table above.

MEMS-type detectors are approximately phase-consistent with geophones, but STRYDE-type detectors produce an additional time-differentiation by which higher frequencies are further boosted at the expense of the low, with the phase rotated by 90 degrees. This implies that data recorded using STRYDE nodes require a time-integration to compensate for the 90-degree phase shift and frequency-dependent amplitude boost resulting from the extra differentiation in order to render it character-consistent with geophone data.

About the Authors

Jason Schweigert obtained his B.Sc. in Geophysics from the University of Calgary in 1999, followed by a M.Sc. in Geophysics in 2009. Jason has been an Acquisition Geophysicist since 2000, when he joined Veritas DGC. In 2005, Veritas DGC’s seismic survey design department was divested, becoming part of BJV, and Jason became a partner. In 2006, Jason became President and Chief Executive Officer of BJV.

Jason’s practical and personalized approach to seismic survey design has helped make BJV Design a leading Canadian seismic survey design company. During his career, Jason has designed over 4000 seismic programs worldwide for many objectives, including Geothermal, CCS, Helium, Mining, Treasure Hunting, and Oil & Gas projects. He has been an active member of the CSEG, volunteering on the board and chairing the Digital Media Committee. Jason has presented at the GeoConvention, authored and co-authored papers in the CSEG Recorder, and facilitated multiple courses on seismic survey design.

On the personal side, Jason’s greatest joys are his girls (wife and three daughters). Whether participating in school field trips or coaching basketball, he takes great pride in being highly active in his girls’ day-to-day lives.

Elvis Floerani is the President and CEO of Absolute Imaging. Elvis is responsible for creating and implementing the Absolute Imaging’s vision and mission, formulating, and implementing the short and long-term strategic plans, overseeing the over-all operations as well as leading market expansion both domestically and internationally. Elvis’ initial years were spent with Kelman Seismic Processing (Kelman Technologies) where his responsibilities progressed from Processing Geophysicist to Processing Supervisor and eventually to Processing Manager. In 1999 Elvis joined Arcis Corporation as Vice President Processing and was responsible for overseeing the technical and business aspects of the processing division. Elvis is a co-founder of Absolute Imaging, a graduate of the University of Alberta (B.Sc. Geophysics) and Queen’s Executive Program. He is a member of APEGA, CSEG, SEG, GSOC and EAGE.

Dennis Quinn began his career in 1984 at Petty-Ray as a Processing Geophysicist, progressing through more senior roles at DataSpan, Veritas Seismic, Pulsonic and Norex. He began developing software at Arcis in 2003 before joining Absolute Imaging Inc. in 2012 as a Software Developer. In his current role, Dennis is responsible for software development, specializing in noise attenuation methods, spectral enhancement and velocity and anisotropy tools. Dennis has a B.Sc. in Geophysics from the University of Calgary and a Technical Certificate in Computer Science from Mount Royal College, Calgary. He is a member of the APEGA, CSEG and SEG.