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HD-GNSS FOR LAND SEISMIC SURVEYS

WHITE PAPER

HD-GNSS FOR LAND SEISMIC SURVEYS

TRIMBLE GEOSPATIAL DIVISION

WESTMINSTER, CO, USA

ABSTRACT

Recent advancements in technology have made it possible to conduct GNSS surveys in areas that were once the exclusive realm of optical and inertial instruments. Applied correctly, this technology can provide considerable cost savings and improved quality in seismic stakeout operations. This paper discusses these technological advancements and why they are so important to the seismic industry.

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INTRODUCTION

A recent study of seismic survey positioning specifications shows that most seismic projects require the coordinates of all sources and receivers to be better than one meter in three dimensions at the 95% confidence level. This specification is oblivious to collection technology, whether GNSS, optical, or inertial, as long as the measurement quality is achieved. However, traditional GNSS systems have often been unable to meet the desired precisions under canopy, resulting in the addition of GNSS technology-specific details to the pure seismic specification. Including these additional requirements, such as the minimum number of satellites in the RTK solution is common practice. It is important to remember that a simple survey specification detailing only positioning requirements has therefore always been the truest specification of the needs of the seismic analyst.

The high-end RTK systems of the last decade tend to compute solutions at two distinct levels of precision. The “float” solution offers precisions which are unable to meet the required tolerances for seismic, while the “fixed” solution has precisions which are an order of magnitude better than needed. As such, in addition to stating the required tolerances, many seismic survey specifications either explicitly prohibit “float” solutions or adjust the tolerances to levels which can only be met by “fixed” solutions.

With the release of the Trimble® R10 GNSS receiver, Trimble has introduced a revolutionary concept called HD-GNSS. The implications for the Trimble R10 with HD-GNSS in land seismic surveying are profound: the receiver is capable of obtaining seismic level precisions in tree-covered areas in real time, which previously could be achieved only with optical or inertial instruments. Together, the Trimble R10 and survey specifications appropriate for seismic objectives will deliver dramatic productivity improvements and cost savings compared to other GNSS systems in compromised sky-view environments.

SURVEY TOOLS FOR SEISMIC STAKEOUT

The seismic industry is very competitive and all aspects of the industry are being revolutionized by new technologies. Survey operations are no exception and it is important to minimize the cost to produce every staked location. The cost per point has three major components: the purchase cost of survey equipment; the costs associated with skilled equipment operators; the speed with which the survey equipment and crew can produce results in the field.

The three common positioning technologies used in seismic surveys are optical, inertial, and GNSS. Each of these has advantages and disadvantages in certain operating environments.

Figure 1 - Optical instruments work well under canopy

Optical technology is reliable and has been applied to survey activities for more than a century. There is no question that optical technology allows precise positioning under trees. This comes at the price of at least a two-person crew and a requirement to clear vegetation to ensure line-of-sight between the instrument and surveyed points. The particular optical survey instruments required for seismic stakeout are

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relatively low cost but they require more skilled, more expensive operators. Cutting lines require additional labor that also comes at a cost. In heavily-wooded areas including jungle, conventional instrument use is common for seismic stakeout.

Inertial measurement systems were introduced into land seismic in the mid-1970s. These navigation systems are able to position seismic elements under tree canopy with good precision and without the need to clear vegetation for line-of-sight. However, these capabilities come with very high initial equipment and maintenance costs. Inertial systems also require skilled, relatively expensive operators. In spite of the expense, when operating in the jungle and cutting lines is not practical, inertial technology is used for land seismic stakeout.

Figure 2 - Inertial systems do not require line-of-sight

GNSS survey solutions have a modest initial equipment cost and are relatively easy to use. With GNSS, one highly-qualified surveyor can often support the productive operations of several less-experienced survey technicians. GNSS also offers very rapid point placement without cutting lines. GNSS technology’s normal limitation is that it requires reasonably clear line-of-sight to satellites orbiting the Earth. Tree canopy obstructs this clear path between satellites and the surveyor.

The operational benefits of GNSS technology have made this the dominant technology for seismic survey operations in clear and lightly-wooded prospect areas.

Most seismic stakeout crews rely on RTK GNSS first, and use conventional and/or inertial instruments when required. GNSS positioning technology has continued to evolve since its introduction and there is now reason to rely on GNSS for a significantly greater portion of seismic stakeout work.

Figure 3 - GNSS offers many cost-saving advantages

MAJOR EPOCHS OF RTK INNOVATION

To better understand why it is worth reevaluating the use of GNSS technology for a greater portion of seismic surveys, it is valuable to review the evolution of survey grade GNSS systems. Positioning a GNSS rover with centimeter-level precision using signals broadcast from satellites that are orbiting approximately 20,000 kilometers above the Earth and moving at 14,000 kilometers per hour is quite a formidable task. The basic theory can be easily understood. If we know where the satellites are, and we can measure how far the rover is from each satellite, we can calculate the rover’s location by trilateration. The use of at least four satellites, as shown in Figure 4, eliminates the necessity for the clock within the receiver equipment to be synchronized with the GNSS system time. This time-

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offset, together with the three coordinates of the rover position, become the unknowns solved for by the positioning algorithms.

Figure 4 - Trilateration of satellite ranges to estimate rover position

GNSS satellites broadcast their locations to the rover in the form of ephemerides that describe the orbit and atomic clock offset from GNSS time for each satellite, but only with accuracy at the meter level. Ranges from the rover to each satellite can be measured using broadcast Pseudorandom Noise (PRN) codes – providing the fundamental ranging signal – together with the phase of the received signals. However, because of atmospheric effects on signal propagation and the accuracy of the satellite orbit estimation, the position of a single autonomous rover can be estimated only to approximately 5 to 15 meters. To overcome these fundamental error sources and achieve centimeter-level positioning, a GNSS reference station is required. Some error correction methods even rely on a network of GNSS reference stations.

Figure 5 - Rover positioning with a reference station

In 1988 Trimble introduced the 4000SLD receiver that achieved survey-grade accuracies. This was a kinematic packable receiver that weighed about 44 pounds, excluding the car battery used for power. This receiver was capable of tracking the limited constellation of GPS satellites that were in operation at the time and used differential techniques to obtain accurate positions. To provide a better view of the technology of the day, a quality personal computer at this time used an Intel 80286 processor, had 640KB of RAM, used a 5.25” floppy disk, and supported a 16 color video display.

Figure 6 - 1988 Trimble 4000SD and computer of the same era

A GNSS processing engine like the one in the Trimble 4000 SLD uses the combined data from a rover and

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reference receiver to differentially reduce the effects of orbit and atmospheric errors as these errors are common to both receivers in real time. A software processing engine in an office computer was able to produce postprocessed kinematic solutions using observations logged on the rover receiver combined with data from a base receiver. This processing technique uses the carrier phase of each satellite signal to measure the range from rover to satellite with a precision of millimeters. This is possible using carrier phase because it has a much smaller wavelength than the PRN code signal. The latter has an effective wavelength given by the code bit (or chip) length. For the GPS C/A-code, this is 300 meters, while the carrier wavelength on the L1 frequency is 19 centimeters. Like a measuring tape with a finer graduation, carrier phase can be used more precisely to measure the range to a satellite. Making precise carrier phase measurements requires more processing power and improved signal tracking. Portable receivers in this era were not capable of making these measurements in real time.

In 1994 Trimble introduced the 4000SSE receiver with Real Time Kinematic (RTK) positioning, featuring on-the-fly initialization. This truly portable receiver weighed only 15.4 pounds including batteries, and was able to resolve ambiguities in the field. The breakthroughs in performance were made possible by major increases in processing power and component miniaturization. The very best positional computations were available only through postprocessing, while real-time positioning was possible primarily with a clear sky view. In the PC world, we had moved on to the 3.5” diskette. Microsoft released Windows® 95 the following year. This was the first instance of the mainstream Windows operating system that was not overlaid on the original Disk Operating System (DOS). Computers were becoming quite capable – Windows 95 even had built-in support for dial-up networking! 1995 was also the year that Amazon.com went online.

Figure 7 - 1994 Trimble 4000SSE with Windows-equipped PC of the same era

What was happening in the Trimble 4000SSE that was not possible with previous technologies? Unambiguous carrier phase measurements could be made in real time, and this was no simple task. In the following discussion, the satellite signal can be simply considered as a sine wave, as shown in Figure 8. The carrier phase measurement is actually the difference in the phase of the received signal and the phase of an equivalent signal generated from the receiver’s oscillator (clock) at the nominal transmitted frequency. As the phase of the receiver’s clock (starting at zero when powered up) is arbitrary relative to the satellite’s clock, for the first measurement after acquiring the satellite signal only the fractional part of the phase can be used. The distance between the transmitting and receiving antennas comprises this fraction and an unknown

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integer number of whole cycles (wavelengths). This famous ‘integer ambiguity’ remains constant for subsequent measurements unless there is a cycle-slip or loss of signal tracking, and has to be resolved by the processing engine. Note that the Doppler on the received signal corresponds to the rate of change of the phase measurement as the satellite moves relative to the user, with subsequent phase measurements reflecting this motion.

Figure 8 - Integer ambiguity is the unknown number of whole carrier phase wavelengths between the rover and each satellite

Traditional GNSS processing engines use the combination of the reference station and rover data to attempt to “fix” the number of whole wavelengths between the rover and the satellites. The process would occur in two distinct steps – first the generation of a “float” solution using both PRN code and carrier phase observables in which the ambiguities were not integers, but in fact were “real numbers” containing fractional parts, followed by a search process to resolve the integer portion, after which the solution was “fixed”. The precision of the float solution is primarily driven by the PRN code noise, which is much larger than for the carrier phase due to the respective wavelengths. Typical float precisions are several decimeters and of limited value for survey applications. The convergence from float to fixed is highly polarized, with the float solution maintained for perhaps a considerable time when working in a difficult environment or with a long baseline, followed by an instantaneous switch to a fixed solution with significantly improved precisions.

Over the next few years, the quality of embedded processors continued to improve and electronics shrank rapidly. In 1997 Trimble introduced the 4800 receiver. This was an integrated survey solution offering full RTK capability in a package that weighed only 8.5 pounds. This innovative receiver offered up to 20 channels and could simultaneously track 9 GPS satellites. The integrated design held the GPS receiver and RTK radio modem in a single package with the GPS antenna at the top. This rover worked with the Trimble TSC1 field controller, which offered some of the capabilities of a personal computer in a rugged portable package. Having a more sophisticated microprocessor in the Trimble 4800, coupled with a powerful field controller, created new possibilities for more advanced GPS signal processing. While these advancements provided improved performance, there were not any major changes in the available GPS signals and, therefore, no push to significantly change the signal handling algorithms. The field controller could undertake the survey and workflow provision capabilities while the receiver’s processing remained focused on precise positioning. In the world of personal computers, Windows 98 was about to be introduced and a capable computer included a 233MHz Intel Pentium 2 processor and possibly a graphics card with a separate dedicated processor. Computer games were advancing rapidly as was the use of the Internet.

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Figure 9 - 1997 Trimble 4800 and Pentium 2-equipped PC

The GLONASS satellite navigation system was deemed fully operational in 1995. This system was operated for several years without any major improvements. In 2003, modernized GLONASS satellite design was introduced and system improvement gained renewed focus. Having an additional, reliable, GNSS satellite constellation created new possibilities for GNSS positioning. Using both constellations, positioning availability increased and it became possible to undertake even more complex signal processing methods.

In 2005, Trimble introduced the Trimble® R8 GNSS receiver. This receiver offered RTK solutions using both GPS and GLONASS satellites. The integrated 48-channel receiver also supported the GPS L5 signal and could be equipped with an internal GSM/GPRS cellular modem for data communications. This receiver offered 11 MB of internal memory for data logging. Trimble offered the TSCe™ controller at the same time. This controller offered 512 MB of compact flash memory, a 206 MHz processor, and a touchscreen. There were more satellite signals available and more portable processing power to make use of them. RTK performance increased greatly and it was possible to conduct RTK operations in more locations with compromised sky views than ever before. Surveyors began operating deeper in mine pits and further under tree canopy. At this time in personal computing, many systems began to be equipped with 64-bit processors and laptop sales exceeded those of desktop computers. Sunnyvale, CA became the first city in the US to offer free city-wide Wi-Fi and YouTube.com was launched. We began expecting ubiquitous Internet access and relying on online services everywhere for every purpose.

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Figure 10 - 2005 Trimble R8 with laptop computer from 2005

As the number of available satellite signals increased rapidly along with portable computing power, it became obvious to GNSS scientists that faster and more accurate position determination techniques could be implemented. Work on GNSS reference networks and modeled corrections led to additional significant advancements in error modeling and positioning improvements on server class computers. It was no longer necessary to use so many approximations and simplifications in the determination of signal distortion effects and for ambiguity resolution.

The original less rigorous approaches to ambiguity resolution gained favor because they could be implemented on available embedded processors and effectively cope with the relatively few satellites providing contributing signals at any given time. As surveyors began to rely on GNSS sensors in more difficult observation environments, the disadvantages

of a simplified ambiguity resolution method were encountered more frequently. The most obvious disadvantage to a fixed/float approach is the inability to extract position solutions of good precision while in the float mode. Beyond this, the most troublesome behavior was the possibility of a ‘bad init’ in which an incorrect set of integer ambiguities is selected. Put another way, the correct set was discarded and could not be selected until the search was repeated. This resulted in a position outlier being reported with unrealistically low precisions for many seconds until detected by automated integrity checking processes. For many years, GNSS scientists improved the automated integrity checking process and even implemented multiple parallel GNSS processing engines in order to reduce instances of incorrect fixing. These various conditions are depicted in Figure 11 where the precisions are given by the magnitude of the ellipses. Note that the precision ellipse for an incorrectly fixed solution is deceptively similar to the one for a correctly fixed solution.

Figure 11 - In challenging GNSS environments, a traditional GNSS processor is susceptible to imprecise float solutions and incorrectly fixed integer ambiguity solutions

In 2012 Trimble introduced the Trimble R10 receiver. This 7.9 pound integrated receiver supports 440 GNSS channels and can track all GNSS signals that are currently available and even those that are proposed to be available in the current decade. The Trimble R10 has two application-specific integrated circuits (ASIC) dedicated to GNSS signal tracking and processing. In addition, the receiver integrates non-GNSS sensors to increase productivity. Teamed with a powerful external field controller such as the Trimble® TSC3, it is now possible to undertake previously unimaginable computations in the field.

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The Trimble R10 offers new levels of productivity and includes many of the technologies that are revolutionizing mobile computing. In 2012, Wi-Fi-enabled tablet computers and smart phones changed the public’s expectations for a mobile computing device. The Trimble R10 offers a similar, profound change to the way that seismic surveys are conducted.

Figure 12 - 2012 Trimble R10 with Tablet computer

Many of the world’s leading GNSS scientists have recently implemented fundamental changes to the ambiguity resolution process. This innovative, advanced technology is called HD-GNSS. The specific algorithms developed by Trimble to realize HD-GNSS capability are patented and not freely available. Although many papers have been written on the topic of instantaneous ambiguity resolution, HD-GNSS represents an unprecedented and significant leap forward in this field. The HD-GNSS processing engine performs very detailed calculations accounting for many more of the elements that can affect the determination of precise GNSS positions. With this technique, available computing power is applied to accurately resolving integer ambiguities using advanced statistical methods. The traditional float state of RTK positioning does not occur and the need for continual monitoring to detect incorrect fixes is eliminated. HD-GNSS techniques are an excellent match for the performance and accuracy requirements of a survey sensor for land seismic stakeout. In combination with ever-expanding satellite constellations, HD-GNSS can provide accuracies of better than one meter at the 95% confidence level in challenging GNSS environments where other processing engines would never fix. This means rapid and accurate stakeout operations.

SEISMIC SURVEY REQUIREMENTS

So far, this paper has discussed the need to undertake seismic surveys with speed and appropriate accuracy in order to be competitive. GNSS is the survey sensor of choice when it provides acceptable results. This section discusses the tolerances associated with seismic surveying. This is the final element in analyzing the application of the latest GNSS technology to seismic work in obstructed sky view environments. HD-GNSS technology replaces the float stage of solution determination with other methods and does not report solutions as “float” or “fixed” which makes it essential to examine seismic survey specifications and the traditional use of these labels in them. In January of 2013, a letter was sent to nine major geophysical contractors, most of whom have global

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operations. This letter requested examples of seismic survey specifications for RTK operations that these contractors received from oil companies for recent projects. The letter also requested they check with their staff geophysicists on the appropriate required positional accuracy for sources and receivers. After evaluating more than a dozen replies, it was clear that current RTK survey specifications for seismic include one of three sets of requirements. These are:

• The required positional accuracy for the staking of all sources and receivers is one meter in both horizontal and vertical axes at the 95% confidence level.

• The required positional accuracy for the staking of all sources and receivers is one meter in both horizontal and vertical axes at the 95% confidence level. In addition, integer ambiguities shall be “Fixed”. RTK “Float” solutions are not acceptable.

• The required positional accuracy for the staking of all sources and receivers is 5 centimeters in both horizontal and vertical axes at the 95% confidence level.

Additional criteria specified often include minor variations of a maximum PDOP of 6, a minimum number of satellites of 5, maximum baseline lengths of 10 kilometers, and limits for the layout tolerance (how close the survey evidence must be to the theoretical preplot location). The second two bullets are clearly defensive in nature to prevent the use of RTK float solutions which are unable to meet the tolerance of one meter in three dimensions at the 95% confidence level. Note that all geophysicists who replied confirmed that this level of survey accuracy is all that is needed for modern oil and gas seismic exploration projects.

The one meter accuracy requirement can be easily derived by an analysis of the expected ranges of seismic frequencies and P-wave velocities along with wavelet sampling criteria. Using an extreme case of a near surface weathering layer with an exceptionally low acoustic velocity of 200 m/s, a peak frequency of 100

Hz, and the Nyquist criteria of two samples per wavelength, we compute from v = f*λ where Δz = λ/2:

∆𝑧 (min) =200𝑚/𝑠

2 ∗ 100 𝐻𝑧= 1 𝑚𝑒𝑡𝑒𝑟

We express this 1 meter level at the 95% confidence level (roughly 2 standard deviations) for consistency with SEG and UKOOA standards.

THE IDEAL SURVEY SENSOR FOR SEISMIC

The perfect survey sensor for seismic stakeout would have a combination of the best features of conventional, inertial, and GNSS instruments. It would achieve the required one meter precision requirements at the 95% confidence level. This system would be easy to operate, allowing a single surveyor to stake productively. The solution would not require the maintenance of line-of-sight with the burden of line cutting. The solution would be robust to endure rough handling and use in seismic operations, and not require extensive routine maintenance and calibration. The perfect system would provide all of this at a reasonable initial cost.

Basically, the ideal system would be a GNSS sensor that provides the necessary accuracy even under tree canopy and reports positioning precisions which accurately reflect the true uncertainty in position. This seems to be an unlikely combination of capabilities but the purposeful evolution of RTK technology and the increase in available GNSS signals has been favorable for seismic survey operations.

HD-GNSS APPROACHES THIS IDEAL

To better understand the significance of HD-GNSS to seismic stakeout, let’s look again at the “traditional” RTK solution found in most modern GNSS receivers. Here, the process of ambiguity determination involves a series of sequential and deliberate steps which initially calculate floating point (not integer) estimates of the carrier wavelength ambiguities via a least squares process. This float solution carries precisions on the order of many decimeters which are not suitable for seismic surveying. These initial estimates are then evaluated to determine an optimal set of integer

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ambiguities which minimize the residuals of the position solution. All other combinations of integer ambiguities are ignored until the next search process is initiated. Once “fixed”, the reported precisions then drop to a few centimeters as only carrier phase measurements are considered. Unfortunately, this technique can produce incorrect ambiguities which result in large position errors (meters) yet carry inherently low reported precisions. In addition, the transition from “float” to “fixed” requires favorable tracking conditions with good DOPs and low multipath. When poor tracking environments prevent obtaining the desired precision levels in real time, some users opt to postprocess the data despite the burden it carries.

HD-GNSS technology offers greatly improved performance. HD-GNSS in the Trimble R10 receiver uses a highly optimized and extremely fast microprocessor which supports new statistical techniques for processing GNSS carrier phase data including a generalized method for dealing with biases. Using all available GNSS observables and state-of-the-art proprietary estimation theory, information within the ambiguity search space is used to provide a statistically optimal position. All combinations of integers are constantly evaluated. This rapid ambiguity resolution process is continuous and never enters a traditional float stage. The possibility of false initializations with their misleadingly low reported precisions is eliminated. The realistic precision estimates of the reported positions are a function of satellite geometry (PDOP) and the local signal environment (primarily multipath). The HD-GNSS solution always uses ambiguity-resolved carrier phase-derived ranges where the estimated precisions seamlessly converge from high to low without the polarizing switch from float to fixed. The process is so fast that it may only be obvious on longer baselines or in difficult environments such as under canopy, but even here, acceptable precision values can often be obtained with appropriate field procedures.

RECOMMENDED SEISMIC SURVEY SPECIFICATIONS

There are two general philosophies when writing specifications. The first is to simply define your acceptance criteria and then have the contractor/bidder detail their approach to obtaining these. The second is to define your acceptance criteria and then dictate specific methodologies and/or equipment the contractor will use in their work. Consider the case where you hire a carpenter to build a storage shed. You must provide him with plans, a list of acceptable building materials, and tolerances within which he is allowed to deviate. If you adhere to the second philosophy, you will also dictate what tools he uses and even how he should use them.

For land seismic surveying, there are numerous technologies in use today. Three of these have already been mentioned in this paper; optical, inertial, and RTK. In addition to these are many hybrids where height determination is aided with the use of LiDAR, barometers, and various GNSS postprocessing techniques. For specifications which dictate equipment and procedures, staying current without eliminating new cost-saving technologies is a challenging task.

Just as in the fields of personal computing and seismic acquisition, survey technologies continually evolve. Specifications which reflect systems and methodologies that are no longer current result in a disservice to both client and contractor as new beneficial technologies are either dismissed or forced to work within the parameters of older systems. HD-GNSS represents a significant leap forward which can greatly enhance productivity, minimize line clearing, and reduce cost in land seismic stakeout operations. With HD-GNSS, just as with other survey systems, there are specific procedures to follow and quality indicators to monitor to ensure that the reported positions are within tolerance. For the writer of specifications, one could append this information to an already lengthy set of guidelines covering other technologies, or take a simpler approach and rely on the expertise of their survey contractors to determine the best way to achieve these requirements.

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We suggest the following for oil and gas exploration land seismic survey operations:

• The layout tolerance, defined as how close the survey evidence must be to the theoretical preplot location, shall be two meters in the horizontal axis.

• The accuracy tolerance, defined as the accuracy of the coordinates associated with the survey evidence, shall be one meter in both the horizontal and vertical axes at the 95% confidence level.

There are many alternative systems and associated procedures which can be used to obtain these. It is the responsibility of the contractor/bidder to demonstrate to the client details of their equipment and field procedures they are proposing to ensure these specifications are met.

CONCLUSIONS

To most fully benefit from new technologies in seismic surveying, survey specifications should be written to define the true accuracy and precision requirements of the survey. These specifications should not dictate the sensor type or unrealistic constraints to control operational aspects of the survey that do not affect the outcome of the geophysical analysis. HD-GNSS processing technology offers exceptional positioning performance that supersedes the traditional float/fix solution type. With HD-GNSS it is possible to address the real positioning requirements of land seismic and to meet these goals with speed and efficiency in the field. All of this can be accomplished using the system of choice for land seismic survey - RTK GNSS. Even under canopy, the cost per staked point can be reduced significantly without compromising the survey results.

Figure 13 – Trimble R10 in a challenging GNSS environment