13
February 2012 DEW JOURNAL 73 dewjournal.com
February 2012 DEW JOURNAL 73
dewjournal.com
74 DEW JOURNAL February 2012
dewjournal.com
Cableless seismic recording anda new problem for geophysicistsRobert G Heath, Technical Marketing Manager, Seismic Source Co. & iSeis Company, UK
In some countries when medical doctors graduate, they swear the Hypocratic oath, promisingto do no harm to patients and their best to heel all under their care. Scientists and engineerspledge no such oath. However, our profession is as vital to humanity’s well being as any doctor.Hydrocarbons account for about eighty percent of the world’s total energy and almost all of itslow cost fuel. This is important as cheap power is an absolutely vital ingredient to lift peopleout of poverty, to improve their health and life expectancy. And much to the annoyance of thosewho believe in manmade climate change, an interesting fact is that the higher the CO2 outputper person in any country (an indication of how much affordable energy it has at its disposal),generally the lower its child mortality rate, the greater this contribution to mankind’s well-being and (for those who think growing world population is a problem) the lower its birth rate.So the substance we look for is vital to mankind’s future in many ways.
It is our task and duty to keep finding hydrocarbons and it is obvious that we must useeverything at our disposal to do our job as well as any oath would have us pledge. Radicallynew tools and techniques are now available for this task, what I believe come under theumbrella term of “future-seismic”. And just as good medical professionals stay up to datewith the latest surgical procedures and medical hardware, so all those with positions ofresponsibility should make themselves aware of the latest geophysical tools and “experimentaltechniques”. Universal seismic equipment represents the best on offer and I hope our industrycontinues as it has already started, moving ever more quickly to its wider adoption.
L ike many other branches ofscience, geophysics is a practical
field and what we call seismicacquisition is often just another formof data gathering experiment. And, aswith almost all physics, the way wecan conduct this process is stronglyinfluenced by ourunderstanding of the theory,the hardware at our disposaland especially the way we areforced to operate equipmentfor whatever reason.
Unfortunately, it has beenmy experience, in the westanyway, that understandinglimitations forced on ourexperiments by the design ofinstrument and peripheralssometimes seem to be morewidely acknowledged in other
areas of scientific endeavour, such asastrophysics, than our own. Forexample, even those who areuninterested in stars tend to be awarethat equipment limits what can beobserved: the characteristics of arefracting optical telescope are not
those of a reflecting one, and a radiotelescope is different again. Placingany optical telescope near lightsources causes images to be noisy(from light pollution) and distorted(from atmospheric disturbance of lightrays) and this is why the best devices
are located in earth orbit. IfGalileo had the Hubbletelescope at his disposal in1610 and, even moreimportantly known how to useit properly, he would not havethought there were foursatellites of Jupiter but 63.
We recognise noise anddistortion in geophysical dataacquisition but not everyoneappreciates how the use ofparticular types of equipmentsets strict limits on imageFirst use of Sigma universal seismic recorder in India
Geophysics
February 2012 DEW JOURNAL 75
dewjournal.com
quality. This is not just referring topossible data degradation caused byinappropriate or substandarddigitising electronics although, as wewill see, these surely can be issues. Itis the process through which resultsare compromised by the way systemshave to be deployed. This lack ofknowledge is especially relevantwhen it comes to employing hardwarenow becoming available becausesome of it is so capable, in the righthands, of giving us so much more.The fact is that today, more than atany time in our history, equipmentdesign and an understanding of howto use available features are theprimary issues affecting how we canprogress our science to find morehydrocarbons.
I have spent my entireprofessional life, not far short of fourdecades, focussed on land seismicequipment. Recently, I have devotedtime to trying to explain to those whodo not have a strong instrumentationbackground how new options inhardware strongly affect their“experiments” too. I am not sure themessage always gets through. Thisarticle is an attempt to ameliorate thesituation. (A short course is alsounder consideration).
BackgroundAlong with seismic source controllersand sensors, recording instrumentsare the main items in our tool box.But until recently there was littlechoice in the type of recorderavailable. There were just a few cablesystems working in much the sameunsophisticated way they had donefor decades, and with little differencebetween them. It is as though the onlytools were hammers, and when onlyhammers are available every problemmust resemble a nail.
We did not notice this restrictionso much before, possibly because weconcentrated on the easier exploration
tasks where l imited equipmentcapability was not so apparent. Whenthe surveys are basic 2Ds or 3Ds, orwhen we think in terms only of CDPrecording, then cable hardware can bemade to suit in the way that hammerscan also always be made to dosomething. But when we considerwavefield recording and the varietyof survey types that this offers to theimaginative explorationist, we cansee instantly that the geophysicalequivalent of a mallet is not muchuse.
Nowadays, it is not enough justto think of simple symmetricalsampling, or operating only whereobstructions do not exist. It is notacceptable to have a highenvironmental and health/safetyimpact. It is not reasonable to havefield geometry in any sense restrictedby equipment, similar to how cablesforce receivers to be deployed in lineswith limited leeway in trace spacing,offset and azimuth range. It is notcommercially viable to have seriallydependency where a fault with onesmall piece of equipment may bringthe whole survey to come to a halt.And it is strange that seismichardware should still be so expensiveto buy and use when in almost everyother field of electronics, the oppositeis true. It is as though we have notonly have been limiting ourselves tohammers, we have also kept theirprice and cost of use artificially high.For these restrictions, for thelimitations on the “seismicexperiments” we can undertake, andfor poor data quality they output, wecan blame hardware based on cables.
What is wrong with cables?To record routinely on all types ofland operation without telemetrycables has been a desire ofexploration industry for a third of acentury. There are several advantagesto this, many of which relate directly
to cable’s problems: its weight, itscost, its servicing burden, the lownumber of environments it can beused in, its HSE overhead, its relativeunreliability to handle huge data ratesand so on. Cables also force the useof spread connectors which areexpensive and can be the cause ofmuch downtime. Even a mediumsized operation needs thousands ofdigital connector contact points allworking perfectly. Instruments whichsuffer none of these drawbacks offersignificant advantages even to simplesurveys, and would make practicalwhole new types of explorationcurrently impossible with thestraightjacket of cabling.
The total weight of equipmentaffects the number of people andvehicles needed, and thus cost andenvironmental impact. The weight ofthe cable itself can account for up to
Universal seismic recorders offer far wider rangeof affordable “seismic experiments”
Digital cable telemetry, little choice and littlefuture
Geophysics
76 DEW JOURNAL February 2012
dewjournal.com
80% of the total entire of system-specific ground equipment but suchproducts add even more weight inhow they waste energy. They are morepower hungry than is often assumedsince, with the relatively large traceparameters more or less forced on usby cable, the power lost in the DCtransmission may be greater than thatused by the electronics. Doing awaywith cables, therefore, can save powerand weight. (Author’s note: I haveseen some claim that cabled systemsare lighter by referring to specific setsof circumstances which favourparticular types of cable operationand comparing against what are someof the heaviest cableless recordersused in the least efficient way. As thiscaused some confusion, I was askedto write an article for First Break in2010 which enables anyone tocalculate exactly how much lightertheir cableless system will for almostany operation. This is referenced atend of this article).
Cables are also usually the mostcommon cause of lost productionwhen “bringing up the l ine” ifequipment is left out. They areespecially susceptible to harm and,being serially dependent, even a smallamount of damage can bring theentire operation to a halt. Thus, cableslimit the types of experiment we dareto do or can afford to undertake. Noneof this is news to this industry.
However, in my opinion theworst thing about such instruments,and something which is not generallyperceived, is that they represent asimplified and out-of-date view ofacquisition exemplified by the idea ofcommon midpoint recording.Furthermore, suppose we want to useequipment purchased for 3D regionalsurveys in a high resolution 2D mode.That system could definitely not beused as cheaply and effectively as oneunrestricted by cable. Alternatively,imagine we want to record three
components or acquire very largeoffset and/or azimuth surveys. Thisvariety of operations, which is notthat wide, is impractical with a singlecable system. So this inherent lack offlexibility tends to make us employhardware in ways which are far fromoptimal, compromising data quality,safety, productivity and, in somecases, the geophysical model has tobe revised just to allow use of whatequipment contractors happen to haveavailable. What we need is universalhardware which does not limit theparts of the wavefield we can recordeconomically, which does not forcechanges on the experiments we wantto run.
Additionally, wavefieldrecording should not be limited toactive surveys; we may also want toacquire passive data, for examplemicro seismic monitoring, fracmonitoring or some form of (semi)permanent arrays. Should moderninstruments not let us take oneverything?
Cable was already perceived ashaving many restrictions but as westart to appreciate newer explorationmethods, we see it in an even worselight. Fortunately, cableless recordersare available and for those with theright features, the versatility toundertake a much wider variety of“seismic experiments” is now open tous. One system - iSeis’s “Sigma” cantrace its development back ten yearswhen its parent company was the firstto use Wi-fi to control and gather dataremotely from a seismic line. So areall these recorders equally suited tothis new frontier?
Cableless flexibility - risking thewrong choiceSomething to celebrate is that thisindustry’s demands were heard, thereare more than ten cableless recorderson the market so there is great choice.But despite all the time manufacturers
had to undertake market research, itseems that not all these instrumentsoffer the flexibility we wanted.Indeed, it now turns out imperativeto be very careful when choosing acableless recorder because despite notbeing tied by cable, some maynevertheless incline the operator towork under worse restrictions thanwere enforced by older technology. Itseems just because something doesnot need telemetry cables, it does notmean it works better.
This is because there are newthings to go wrong with cableless kitwhich did not affect cabled systems,and users must understand these risksor seismic experiments will becompromised in novel ways. In thewest, which has been using theseinstruments for a while, veryexpensive mistakes have alreadyhappened, with some contractors(who did not realise the risks)finishing with unacceptable data orno data at all in some cases. Choiceis usually good but unless weunderstand what perils each comeswith, then variety can also be verydangerous.
And there are commercial riskstoo. The industry is not large enoughto support many manufacturers andsome have already disappeared.Others might stay in business but notmake enough profit to invest in newsoftware or hardware options. Somebetter systems will probablydisappear simply because ofsomething as mundane as badmarketing whereas conversely, somepoor ones will survive because theyare marketed by richer companies.But how do seismic contractors selectthe right one and how dogeophysicists learn what each systemis capable of? These questions canonly be answered by firstunderstanding more about thedifferences between the technologies.
Beginning with the basics,
Geophysics
February 2012 DEW JOURNAL 77
dewjournal.com
digital telemetry cables are there fora reason - in fact at least three majorreasons. Firstly, out from the centralsystem to the ground units cablescarry timing information. Next theytransport remote control commands.Thirdly, they bring back QC andstatus information along with a greatdeal of seismic data. If we build acableless recorder, then we have toknow how to incorporate thosecapabilities or cope without functionswhich were until now consideredvital . In making suchdecisions, eachmanufacturer has had tocome to terms with whatthey as a company candevelop. However, thatapparently did not alwaysmean that they tried tounderstand all theexperiments the industrywanted to do. Let us startwith system timing.
Many, though not all,cableless instrumentsmanage to do away withcables for distributing timingbecause inside each groundunit is a GPS receiver whichof course primarily is a wayto pick up very accurate timeinformation. Most productsactually use the same GPSreceiver chips, so in thissense there is not much tochoose between them - aslong as there is GPS signalreception, seismic data canbe timed stamped. Buttherein lies the problem.GPS signals are not alwaysavailable. It some locationsit is at best intermittent andin others is non-existent.What happens then?
It is not just junglewhere GPS is notguaranteed; I have been oncrews with wide open skies
where GPS suddenly becameunavailable. After all, the operatorsof GPS satellites do not guaranteeservice everywhere 24 hours per day.There are also times when GPScannot be properly received evenwhen signals are available - someboxes may not receive if they areaccidentally knocked upside down.This is why a few recorders weredesigned not to rely solely on GPS buthave options to use alternatives whenGPS is unavailable. After all, a ground
unit, which for whatever reason,cannot accurately time stamp eachseismic sample is virtually useless.
Further, some systems, in tryingto reduce the problems experiencedwith theft, suggest that ground unitsshould be buried, apparently so as notto attract thieves. GPS receivers aremore sensitive nowadays and workunder some centimetres of dry earthor snow but when that cover becomeswet, then reception may not be soreliable. No wonder that not all
manufacturers rely on GPSexclusively for timing! Allthis makes you realise thatbuilding in importantfunctionality is not easy. Atleast with old technology, ifthe cable broke the observerknew about it - no data lost.Some cableless kit cannoteven alert the observer whenit is not receiving GPS - theworst of all worlds.Fortunately, there ishardware which has givendue consideration to thismost serious of potentialproblems. It has a higherquality on board clock sothat when GPS isintermittent, timing remainsaccurate for some hours. Andin an area of zero reception,it can be enhanced withVHF-based synchronisation.VHF signals, being abouttens times the wavelength ofGPS signals, penetrate heavyfoliage and other difficultareas much better. Further,its design is that it is almostimpossible to tip over.Since seismic instrumentsessentially record voltage(e.g. the output of ageophone) against t ime,ideally both with an accuracyapproaching 1 ppm,equipment which is not able
Cableless line boxes must be tough to protect data and prevent tipping
Choices in time stamping. More accurate clocks and VHF timing optionfor when GPS signals are unavailable
Mesh Radio Networking. Two way communication for cableless systems
Geophysics
78 DEW JOURNAL February 2012
dewjournal.com
to perform reliably cannot reallydescribe itself as viable. So my adviceis to not consider anything whichdoes not always guarantee accuratetiming and also let you know instantlyit is no longer working. This isbecause, if it can only guarantee towork 95% of the time, and you do notknow when the other 5% is, it seemsthere’s a one in twenty chance ofgoing out of business.
Cable’s next function is to carrycommands to remote units. If nowireless alternative is offered, is thisan issue? Providing wireless controlto an entire receiver spread requiressignificant development effort, and toavoid this some manufacturerspretend it is irrelevant and do notoffer it. They claim most surveys aredone without changing acquisitionsettings, that since cableless recordersare generally continuous recordsystems and there is no need torepeatedly start-stop recording, whatwould one need to control? If youhear this, turn away immediately asit ignores one of the major potentialdrawbacks of “going cableless”:cableless equipment uses muchsmaller batteries but higher numbersof them - as many as two per channel(though some need very much less).Large numbers of small cheap
batteries can either be an advantageif you can control them, or a seriousheadache if you cannot.
This does not mean more powernecessarily is being used in cablelessrecording because, as alreadyintimated, cable is wasteful indistributing power (it can doublequoted consumption) andtransmitting data can take as much assome nano joules per bit per transfer.In fact, there are times when havinga small low cost battery run a fewchannels rather than a large onerunning tens of channels is a definiteadvantage - it means one battery canlast weeks instead of just days. But ifyou have hardware which inherentlyneeds very high numbers of batteries,or does not let you monitor remainingbattery power, or requires bespokebatteries and chargers, then you haveproblems you never had with cable.This is why remote control is soimportant - it is nothing to do withchanging sample rates or preampgains. It is to solve the batteryproblem.
Another response from somemanufacturers who do not providecontrol is that their systems can bepreprogrammed to switch on/off orleft on continuously instead. The firstof these suggestions may end up with
thousands of channels allprogrammed to switch off at, say, 6p.m. Perhaps your shooting is runninga bit late, you are left with a handfulof shots but suddenly the wholespread goes dead. Now you are almosta day behind just because you haveno easy control over the equipment.Or you accept the idea you neverswitch off boxes but this makes themhigh energy users as they need power24 hours per day when perhaps theyonly should have been switched onfor eight. In either case, this requiresuse of more expensive or heavierbatteries, or a need to change themmuch more often as well as using upmemory capacity. My advice isalways consider how much energyyour are forced to use as this is whatindicates whether having lots of smallbatteries is an advantage you canprofit from or the exact opposite.
The last purpose of cables is toreturn QC, line equipment status andthe seismic record itself. But in thecableless world it is a mistake to thinkof this all as one set of information.What most cabled systems display tothe observer - or perhaps what mostbother to look at - is not the data itselfbut QC, status and noise levels. Theyfeel that if there is acceptable spreadnoise, if boxes, sensors and batteriesare all working well then probably thedata is fine too.
This is an importantconsideration when discussingcableless technology because thequantity of data per channelcontained in QC, status and noise isless than ten percent of that in real-t ime data fi les. And it is thisdifference which smarter productsmake use of because high bandwidthwireless transmission is expensiveand power hungry but low bandwidthis far simpler to deal with. Thuscableless systems cannot just besegregated into those which sendback nothing (so-called shoot blind
Sigma with optional MRN repeater to increase range
Geophysics
February 2012 DEW JOURNAL 79
dewjournal.com
systems) and those which sendeverything (so-called real-t imesystems). There is a third choice:those that send back just QC/statusand noise. Cabled equipment did nothave this valuable distinction.
Furthermore, instruments withbandwidth capable of returning QC/status also have capability to send outremote control commands, i.e. theessential energy saving featurediscussed earlier. So avoidingrecorders which force you to shoot-blind reduces risk of equipment theftand data loss, and guarantees controlover line power too. This explainswhy, in my view, the only recordersto consider are those which providethis minimal functionality.
So what technologies provide thebandwidth to control boxesindividually or in groups and alsosend back enough information to letthe observers know everything isworking well? There are only a fewchoices but of prime importance ishow simple deployment is in thefield. The ideal solution is that ofmesh radio networking (MRN). Thisis not new but making it work reliablyin the seismic environment has takenconsiderable development effort.
An MRN does not attempt totransmit from each separate groundunit all the way to the central system.It only tries to reach nearby boxeswhich in turn talk to their neighboursand so on until a complete web ofcommunication routes is available,effectively allowing every unit to bein touch with the observer. Networksare self-generating and self-healingmaking them ideal for ourapplication. Also, because they onlycommunicate over short distancesand at low bandwidth (thoughexternal repeaters can increase rangeby a factor of ten) they take littlepower and are less fussy about howthey are deployed than highbandwidth hardware.
So appropriate mesh technologyhas proven the best solution today forcableless applications if it is not vitalto get all data in real-time due to itsadvantages ease of deployment andcost reduction. But note this impliesthat data harvesting must be easy aswell, and this is an issue we mustreturn to. In fact, remote monitoringand flexible harvesting are a pair thatshould always be available in any“universal” system because operatingas an duo they are very powerful toolswhereas one without the other is notso game-changing at all.
Having seen how newinstruments either offer the samefunctionality as cabled systems orforce you to take new risks, we nowlook more closely at other featureswhich cableless kit must providewhich cabled technology did not needto. We continue with systems whichdo not send back all the seismic datawirelessly and consider whatconsequences this has.
Data harvesting issuesWhether the recorder is shoot-blindor offers some form of QC/statusreturn, the fact remains that our datasits in memory inside ground units tillwe harvest it so let us first look atstorage capacity. Even in something
so basic, not all cableless systems arecreated equal. It is simple to calculatenecessary storage size: multiplesample frequency by no. of bytes persample, by number of seconds inoperation (if there is no remotecontrol the number of seconds maybe the those in a whole day), by no.of days between harvests, by numberof channels per ground unit. Add 10-25% for overhead. Now check thecapacity each unit offers as thisdiffers wildly - some are limited toonly 2 or 4 GB of addressablememory space. This is OK for simpletypes of regularly harvested activesurveys but we see that otheroperations can rapidly depleteavailable storage.
So the amount of memory maybe the determining factor when itcomes to how and when you mustharvest - but you do not want to haveto pick up boxes simply because oflimited storage capacity. And here isanother reason not to choose a shoot-blind system: how do you know thememory is filling up? Fortunately,some products not only have farbigger built-in memory capacities butalso larger addressable memory spaceand generally these also do not forceyou to shoot-blind.
Having to collect up ground units
MRN builds it own network of communication routes automatically. This can be monitored by theobserver. (Courtesy Sigma Observer software)
Geophysics
80 DEW JOURNAL February 2012
dewjournal.com
to retrieve data and the problems thiscauses are not new. It was a limitationof the SGR developed by Amoco aboutthirty years ago where data was writtento tape with only some tens ofmegabytes capacity. One would havethought that three decades would notonly have seen advancement in termsof data storage- which most assuredlyin the case given the easy availabilityof large memory chips with tens ofgigabytes capacity. But also that theremight be quicker ways of harvestingvaluable data than actually having topick up boxes, bring them to a stagingarea, connect to a download rack andwait for files to be created.
Yet, surprisingly, this is howsome modern cableless crews still areforced to operate. The disadvantagesshould be clear. If ground units arenot busy acquiring data, then they arecosting money rather than making itas extra channels must be bought tocompensate for when units are not onthe line. And having to use expensiveharvesting racks also adds to the costof most shoot-blind systems.
This unhappy state of affairs ismade still worse if units have internalbatteries, especially those based on
lithium, which can only be chargedwhen they are within a l imitedtemperature range. This may takesome hours to achieve once the boxesare taken off the line. So add this waittime to the charging and harvestingperiods and ground units may beunavailable for recording during animportant fraction of survey time.
Conversely, some systemsoperating even in modes where theyare not transferring the completeseismic file in real-time do not needto be picked up for data to beharvested. They can be left inposition, without interrupting theiressential task of recording while anoperator copies data from each unitinto some mobile device which isthen taken to a central location forSEG file generation. There arevarious ways to do this copying butin my experience no single one isalways best so users should demandmultiple choices as each survey maybenefit from a different approach. Butwe must also consider what mustactually be copied because here issomething else we did not need toworry about with a cabled system.
As cableless recorders also tend
to be continuous record systems, oncethey start digitising, they do not stopuntil told to. Any device with nocommunication capability must bepreprogrammed to stop digitising, oran operator will have to go to everychannel on the spread and make themhalt somehow. So this continuousrecord characteristic comes withimportant issues to consider. A singleseismic channel with a 1 mS sampleinterval will require almost 1 GB ofstorage after two days operations.However, if we consider impulsiveshooting, then perhaps only 1-2% ofthe memory used up contains actualseismic reflection data. So somerecorders - generally only those fewwhich have well integrated sourcecontrol can instruct their harvestingdevices just to download the relevantportions of the memory which holduseful data, accelerating downloadingby a large degree. Data is recoveredfar more quickly and require fewerpeople for the task. This sort of dataretrieval is called “SMARTharvesting”.
Armed with hardware benefitingfrom some download-during-recording SMART feature, whatmethods exist to transfer data frombox to mobile harvesting device? Themost basic is simply connecting alaptop or tablet PC using a shortcable, such as an Ethernet link, to theline unit during acquisition. Such aconnection is very low cost, workswell and enables data download ratesonly limited by memory read speed.This may appear very low-tech but,as we will see, the value of thissimplicity should not beunderestimated. To demonstrate thiswe investigate how some cablelessunits also offer different forms of Wi-fi connectivity allowing variousforms of “pass- by” data retrieval anddiscover this does not always live upto claims made for it.
“Pass-by” is a generic termSMART harvesting using Ethernet cable linkbetween PC and line unit, while not interruptingacquisition
Internal Wi-fi for pass-by harvesting. Specialprojects, Quito, Ecuador
Geophysics
February 2012 DEW JOURNAL 81
dewjournal.com
covering walking, floating, driving oreven flying by ground units, as longas connectivity is achieved longenough between box and Wi-fi-enabled harvesting device for allrelevant data to be collected. Butexperience shows that this periodneeded to allow data transfer is almostcertainly longer than many expect,sometimes hugely so and to the extentthat it greatly slows down operations,especially in memory-limited systems.
The reason is that Wi-ficonnection speeds are not justdetermined by the protocol, but manyvariables in the external seismicenvironment can have surprisinglydeleterious (and very disappointing)effects. Connection stability is amajor issue which can becomesuddenly apparent if there is a lot ofdata to transfer, especially whencommunicating with multiple units.I have seen transfer rates expected inmegabits quickly descend into notmuch better than 100 kpbs. In otherwords, to achieve outdoors in termsof connectivity what is achievedindoors with Wi-fi is no simple matterso do not be seduced by Wi-fi’s hi-tech nature.
Thus, even in something assimple as Wi-fi, there are importantchoices which must be made,especially: where is it best to have theground unit’s Wi-fi? It is possible toinstall internally and, whereas this issometimes convenient, it reduces theconnection range and likelymaximum transfer rate. Similarly, use
of high gain antenna on the harvesteralso affects the efficiency ofoperations (but to a lesser extent).Therefore, Sigma provides choices inexternal or internal Wi-fi, can switchWi-fi off remotely to save powerwhen not needed, and also offersvarious harvesting antenna.Experience shows that to overcomethe already-listed potential drawbacksof Wi-fi harvesting, by far the bestpass-by combination is external Wi-fi plus hi-gain harvester antenna - butalso give yourself the option of beingable to go back to other data transferapproaches when Wi-fi does not liveup to expectations. With more thanten years’ experience at the forefrontof Wi-fi in seismic, the iSeis companywarns of potential disappointment if
this is all you rely on. Such groundunit Wi-fi choices also have knock-on effects in any real-time mode i.e.when seismic data is sent backwirelessly to the observer.
In fact, do we really need Wi-fiat all to copy data from what isessentially one memory to another?Wi-fi relies on 2.4 GHz radiocommunications which also can havenumerous other drawbacks. So Sigmaalso allows the connection ofruggedised types of USB memory towhich data is copied. This has thehighest transfer rates and overcomesall the limitations of Wi-fi. And if thememory is left connected, then datais written to internal memory andsimultaneously to external memory,so harvesting happens as quickly asit is possible to unplug the USBdevice. This option is extremelyuseful where there is no risk of theftand it has many other advantages:PCs and Wi-fi do not need to be takento the line and it lets the crew retrievedata instantly as and when required,perhaps for quality verificationduring start-up. This is all part of thephilosophy of a universal approach.
So it seems multiple methods ofharvesting during recording are vital.Sigma lets operators use all of themon the same survey and for crewswhich have tried all the abovementioned methods it seems that theUSB memory is the preferred. But theimportant issue is to offer choices tomaintain efficiency.
Now let us consider cableless
External Wi-fi for long range pass-by harvesting.Passive acquisition operation
USB memory harvestingSigma data
Real-time cableless data transfer using 2.4GHz semidirectional Wi-fi in daisy chain mode
External USB memory for Sigma system.Potentially the fastest way to harvest data
Geophysics
82 DEW JOURNAL February 2012
dewjournal.com
systems with options to send backdata in real-time. Here the choice israther limited and in all cases rely insome way on the 2.4 GHz licence-freeISM band. Be careful about beingtempted to use anything which offersother connection frequencies, theyare not all legal in many countries.Also be aware that the 2.4 GHz bandhas limitations in terms of effectiveradiated power with 100 mW ERPbeing the maximum accepted in mostjurisdictions. Some new seismichardware may only work well whenit exceeds this.
Because this band is made suchgreat use of in exploration, it is worthextra note of caution when thinkingabout real-time recording. For thosewho are not aware, 2.4 GHz is thefrequency of microwave ovens. Thisfrequency is chosen because it is theone which is most absorbed by the sortof molecules found in the food wewant to warm up, the most commonbeing water. So in our environment,signal is attenuated by continuallyvarying levels of water vapour in theair and in vegetation which maysurround equipment. Therefore, a real-time wireless link based on technologydesigned for use inside and which canprovide high transfer rates only overshort ranges, cannot automatically bethought of as ideal for seismic,whether used for pass-by harvesting orreal-time cableless acquisition.
Therefore, we need to know thevarious failure modes of 2.4 GHz andmake sure equipment suffers fromthem as l i t t le as possible. Forexample, it is possible to set up somecableless systems such that they workperfectly during the day but if left outat night, you may find they do notwork so well the next day. The reasonmay be as simple as that dew hasattached itself to foliage which thenadditionally attenuates transmissionand this can be enough to halt real-time operations and even slow things
down considerably in some types ofWi-Fi-based harvesting.
So it is the way manufacturers“make do with” 2.4 GHz whichdifferentiates real-time recorders. Ifline boxes only offer some form ofomni-directional or internal antennathen you can sure that they probablyalso provide the least flexibility. Ifyou are operating with no vegetation,then use omni-directional antennaonly if you can be sure that receivingantennae density is high enough.Otherwise, use more directionalantenna which will also permit alower density. It is important that the
recorder offers this choice or you mayfind yourself with somethingclaiming to offer real-time but whichdoes so only occasionally. It is alsovery convenient when the ground unitWi-fi subsystem which provides pass-by harvesting can be used for real-time recording as needed, and it muchmore user-friendly when such Wi-fiis external to the ground unit.
On Wi-fi’s positive side, evenlow power 2.4 GHz, with a semi-directional antenna can successfullyconnect over large distances to otherdirectional antenna. The best exampleof this of is the Sigma deployed overan area of 750 sq.km in one of thehilliest states of the USA, withsignificant temperature and humidityswings, sending data back in real-time from around a thousand stations,and all powered by solar arrays. Notonly would this be impossible with acable system, it is impossible withevery other cableless technology. Thearray, used to gather micro-seismicmonitoring data, has been operatingfor over two years, day and night.Cables left deployed for so long insuch a rugged area would quicklyhave been destroyed.
We should also look at whetherelectronic specifications acceptablefor cable are sufficient for cableless.Many think that one set of specs ismuch the same as the next, and in
Permanent array (> 2 years, 24/7) using Sigmafor real-time data transfer over 750 sq.km areaand local storage, USA
Average bit utilisation with 24 bit crew. Bit 10 was highest for largest no. of samples
Geophysics
February 2012 DEW JOURNAL 83
dewjournal.com
active cable-based hardware there istruth in this. There is virtually nodifference at all between suchsystems’ specifications, and wherethere is, it is nothing compared toimprovements which would comefrom such basics such as plantinggeophones well. But are we gettingthe best out of new recorders or, touse our astrophysics analogy, are wetaking the Hubble telescope anddeploying it in the middle a well litcity?
Some geophysicists are under theimpression that 24 bit systems, almostno matter how used, give the bestresults that modern technology canmanage. For example, they believethat their data is coming with thehighest dynamic range data andfrequency content. But this was rarelytrue even with simple surveys usingcabled systems. Let us start bylooking at dynamic range.
Since the introduction of over-sampling convertors in land about 15years ago, we may be losinginstantaneous dynamic rangecompared to older technology, withlarge parts of the active spread onmost 24 bit surveys only triggering 10-12 bits. This is due to the way thatdelta sigma convertors workcompared to floating point amplifiersand successive approximationconvertors. Whereas this may not
always have been noticeable giventhe small range of surveys whichgeophysicists tend to carry out with24 bit cabled systems, the samecannot be said with cablelessrecorders called on to carry out a farwider variety of experiments. Forexample, cableless can be connectedto single sensors in a very quietdownhole environments or to largesurface arrays with potentially highvoltage output from series coupledgeophones. This is a much greaterrange of input signals than we havebeen used to and is why it makessense to choose instruments with 32bit conversion, not just 24. They offerbetter bit utilisation and come withsome more obscure electronicsbenefits important to cableless suchas the way the modulator is designedto prevent overscaling. So 32 bit andcableless can thus be considered
natural bedfellows.The same issues apply to any
survey where we are interested inlow frequencies. Most 24 bit systemshave built-in 3 Hz low cut or DCoffset removal filters. This may nothave been an issue before but inmore and more active recording, andalmost all passive recording, wewant to record below 3 Hz. Forexample, using sensors capable of<1 Hz lets us tap into passive surfaceand body waves to image thesubsurface geology in new ways.Also, the presence of part ia l lysaturated hydrocarbon reservoirs isassociated with spectral anomalies inthe range of 1-6 Hz in microtremors.When measured at the surface, thespectral energy is elevated above ahydrocarbon reservoir comparedwith spectral energy measured atpositions away from a reservoir (1-3.5 or 1-6 Hz) and the spectral ratiobetween vertical and horizontalcomponents can show an anomaly inthe presence of hydrocarbons. Thepolarization of the waves might alsoprovide information about the timevariabi l i ty of the microtremorphenomena related to hydrocarbonreservoirs. So functionality to copewith all this needs to be catered forin anything which wants to describei tself as good for passiveappl icat ions, so check i f your
Long range/high throughput Wi-fi data retrieval(Courtesy iSeis company)
GoogleEarth support for sources and receivers
Geophysics
84 DEW JOURNAL February 2012
dewjournal.com Geophysics
recorder allows recording below 3Hz.
Currently, there is much workunderway in regard to new sensortechnology, both single and multi-component and it seems that analogsensors still offer significant benefits.These include superior performancein terms of noise, the ability to use inarrays and with different recordingsystems, better cost and robustness.Also, as passive generally requiresuse of cableless recorders wherepower is precious, any transducer,such as MEMS accelerometers whichrequire power, should be seen as adrawback. However, there are otheractive technologies now underdevelopment to out-perform MEMSand I believe we shall be seeing otherrecorders follow Sigma’s lead inbeing able to offer internally mountedas well as external sensors of up to 3components before long.
New and better seismicexperimentsActive acquisition is no longer justabout simple surveys as the easy-to-find hydrocarbons have already beenlocated. Exploration today is aboutgoing to new areas with complexgeology and tougher operationalenvironments or returning to existingfields to capture some new aspects ofthe wavefield. However, these are notthe only experiments we want toperform. As well as research, multi-recorder and multi-sourceacquisition, the fastest growingmarket is non-active surveys,covering what may be termed passiveacquisit ion, permanent andmicroseismic recording, everythingfrom frac monitoring to 4D.
Having said all this, this authorat least believes there is still a placefor cabled telemetry. My estimate isthat realistically it offers some benefitsover cableless equipment for as muchas ten percent of the market.
Therefore, contradictory though it mayseem, a universal cableless systemshould also have (flexible) cabledtelemetry option, for example lettingthat part of the spread be a separatecabled local area network which thencommunicates over appropriate Wi-fito the central system, which avoids thedifficulty where older technologyinconveniently forces users to makesure cables always end up connectedto the recording truck.
However, it is not just handy tohave all the above-mentionedfunctionality, it is even better to beable to use all the capabilities at thesame time. This is because most surveysites are not homogenous so it is usefulin one part of the spread to shoot-blindand harvest in a few different wayswhile in another use a MRNmonitoring system, and in another(perhaps where geophysicists want tokeep a close eye on data quality)operate in real-time with cable-LANor Wi-fi. All of this can be loaded intodetailed GoogleEarth imagery, whichthe observer uses as his displaybackdrop, which can track movingsources and other important crewassets in real time. This is trueflexibility and to date, only one systemhas been developed which is capableof all this.
And such universal equipmentalso allows whole new methods ofoperating. There are already“recorderless” crews where thetraditional recording truck no longerexists. With the more modern and
open architecture source controlsystems coupled to cablelesshardware with the rightcommunication facilities, there is noneed.
Cabled and cableless together -harmony or trouble?The desire to use cableless systemsto augment older recorders is growingrapidly but most hardware does nothandle this well . Many newinstruments can only harvest data incommon receiver domain which isOK for SEGY but SEGD does notsupport this. This is of course can bea problem for all the processingcompanies who demand SEGD (eventhough this format sometimes seemsnot to have been designed with theflexibility of cableless in mind).Therefore, to be universal, theessential issue is that an instrumentcan quickly generate files in bothSEGY and SEGD with correctlypopulated headers, as well as anyassociated support files and onlySigma is capable of this. Effectivelyit is clever enough to blend the allinput “raw ingredients” together,handling t iming and phasedifferences along with correlation andstacking etc., producing any/alloutput required by the processingcentre. In other words, universalsystems have to be much moreflexible even in the area of dataoutput than any cabled system neededto be and this seems to be an areawhere some manufacturers of newsystems were not entirely aware of therequirements.
However, even this is not allthere is to it; to make side-by-sideoperations work efficiently alsorequires flexibility in source controlwhich most instruments cannotmanage because of their closedarchitecture. Closed architecture wasdesigned into systems primarily tomake sure that a contractor had to useVery low frequency seismic data acquisition
February 2012 DEW JOURNAL 85
dewjournal.com
as much hardware as possible fromthe same company. This was possiblyacceptable as a philosophy previouslybut now we want multiple systems(sources and reorders) to worktogether, it is clear that it is not theflexible way to go when crews wantto make full use of their inventory.Therefore, it is clear, one contractorwith one cableless system may becalled on to do all of this today andsomething new tomorrow, sospending several hundred thousanddollars or some millions on a productwhich cannot cope is potentialcommercial suicide.
ConclusionIn some countries when medicaldoctors graduate, they swear theHypocratic oath, promising to do noharm to patients and their best to heelall under their care. Scientists andengineers pledge no such oath.However, our profession is as vital tohumanity’s well being as any doctor.Hydrocarbons account for abouteighty percent of the world’s totalenergy and almost all of its low costfuel. This is important as cheap poweris an absolutely vital ingredient to liftpeople out of poverty, to improvetheir health and life expectancy. Andmuch to the annoyance of those whobelieve in manmade climate change,
an interesting fact is that the higherthe CO2 output per person in anycountry (an indication of how muchaffordable energy it has at i tsdisposal), generally the lower its childmortali ty rate, the greater thiscontribution to mankind’s well-beingand (for those who think growingworld population is a problem) thelower its birth rate. So the substancewe look for is vital to mankind’sfuture in many ways.
It is our task and duty to keepfinding hydrocarbons and it is obviousthat we must use everything at ourdisposal to do our job as well as anyoath would have us pledge. Radicallynew tools and techniques are nowavailable for this task, what I believecome under the umbrella term of“future-seismic”. As just as goodmedical professionals stay up to datewith the latest surgical procedures andmedical hardware, so all those withpositions of responsibility shouldmake themselves aware of the latestgeophysical tools and “experimentaltechniques”. Universal seismicequipment represents the best on offerand I hope our industry continues asit has already started, moving evermore quickly to its wider adoption.
For those who would like morespecific advice or explanation inregard to such equipment, I welcome
any feedback and communication.
AcknowledgementsWriting an article of this length anddetail requires input from manyexperts. There are dozens who deserveto be mentioned but I would at leastlike to offer my thanks and recognitionto: John Giles, President of iSeis,Ponca City, USA; Scott Burkholder,Chief Geophysicist, iSeis, Ponca City,USA; Murali Shanmugam, SeniorDesign Engineer, iSeis Co., PoncaCity, USA; Jim O’Donnell, ChiefGeophysicist, BC Geophysics,Nevada, USA; Norm Cooper, MustaghResources, Canada; Ian Jack, (BPretired); Rodney Amstrong, (Europe);Ed Tree, (Amoco retired); DavidBamford, OilVoice, UK.
ReferencesSavazzi, S. And Spagnolini, U. (2011)
Compression and coding forcable-free land acquisit ionsystem. Geophysics 76, 29-39.
Heath, R. (2009) System 2000revisited. The Leading Edge.
Hons, M. Stewart, R. Hauer, G.Lawton, D. Betram, M. (2008)Accelerometer Versus GeophoneResponse, A Field Case History.EAGE Convention Rome.
Heath, R. (2010) Weighing the roleof cableless and cable-basedsystems in the future of landseismic acquisition. First Break,28 (6), 69-77.
Heath, R. (2011) Time to consider thepracticalities of passive seismicacquisition. First Break, 29, 91-98.
Trickett , S. (2011) Reducingacquisition costs with randomsampling and multidimensionalinterpolation. SEG San Antonio.
Heath, R. (2011) Total 3D seismiconshore, a disruptive transition.Finding Petroleum convention,London Nov 2011, openingaddress.
Multiple source and multiple (cable and cableless) recorder testing, only made possible usingSigma-based instruments and source controller
dewjournal.com
Geophysics
