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Understanding fracture geometry and estimating stimulated rock volume (SRV) is the goal of operators in unconventional reservoirs. The industry has long been challenged with getting a good understanding of how wells and completions interact with each other. One of the biggest challenges is how to get a quantitative measure of the extent of fractures. Historically, companies have used many different technologies to better understand their completions. Tilt meters, microseismic geophone arrays, chemical tracers and pressure sensors are just some of the conventional technologies that have been used with a limited coverage to monitor the changes in the reservoir during hydraulic fracturing. The Distributed Acoustic Sensor (DAS) and Distributed Temperature Sensor (DTS) are used for multiple measurements along the entire wellbore. During the hydraulic fracturing treatment, the acoustic energy distribution and temperature profiling are recorded in real time to analyse the fluid allocations per cluster as indicated in Figure 1. The fibre is also used to acquire seismic and microseismic data at different stages of the well completion. Low frequency crosswell strain is also measured in an observation well or in an offset well. DTS data, with a fine resolution of 0.01oC, is used to indicate any hydraulic fluid contact (Hull et al., 2017; Jin and Roy, 2017). The combined microseismic, crosswell strain and temperature data are used to better understand rock properties, well interference and for optimizing the well spacing. The utilization of distributed fibre measurements has been increasing over the past few years. By installing a permanent fibre cable on the outside of a casing string, measurements along the entire wellbore can be achieved (Webster et al., 2013). However, the requirements for the cable orientation and directional perforation adds additional complexity and costs for the installation of the fibre and, therefore, limits the number of wells that can be instrumented with fibre and monitored simultaneously. We present here an advanced Distributed Acoustic Sensor system (referred to here as Carina) that utilizes a new generation of engineered optical fibres (Constellation) with 100x (20 dB) improved sensitivity compared to that of standard fibres.

We use frequency comb-based optical two-way time-frequency transfer (O-TWTFT) to measure the optical frequency ratio of state-of-the-art ytterbium and strontium optical atomic clocks separated by a 1.5 km open-air link. Our free-space measurement is compared to a simultaneous measurement acquired via a noise-cancelled fiber link. Despite non-stationary, ps-level time-of-flight variations in the free-space link, ratio measurements obtained from the two links, averaged over 30.5 hours across six days, agree to $6\times10^{-19}$, showing that O-TWTFT can support free-space atomic clock comparisons below the $10^{-18}$ level.

We demonstrate optical frequency comparison of the 171Yb, 27Al+ and 87Sr atomic clocks with measurement uncertainties below 1 part in 1017, and discuss how phase-coherent and synchronous clock comparisons can be used to improve measurement stability.

This paper describes the recent National Institute of Standards and Technology (NIST) work on incorporating an optical clock into a time scale. We simulate a time scale composed of continuously operating commercial hydrogen masers and an optical frequency standard that does not operate continuously as a clock. The simulations indicate that to achieve the same performance of a continuously operating Cs-fountain time scale, it is necessary to run an optical frequency standard 12 minutes per half a day, or 1 hour per day, or 4 hours per 2.33 day, or 12 hours per week. Following the simulations, a Yb optical clock at NIST was frequently operated during the periods of 2017 March – April and 2017 late October – late December. During this operation, comb-mediated measurements between the Yb clock and a hydrogen maser had durations ranging from a few minutes to a few hours, depending on the experimental arrangements. This paper analyzes these real data preliminarily and discusses the results. More data are needed to make a more complete assessment.

The local position invariance principle of general relativity stipulates that non-gravitational experiments should give outcomes that are independent of the position and orientation of the reference frames in which they have been performed. Here, we study the change in the rates of clocks on Earth with the spatial change of the solar potential, constraining the variation of a non-gravitational interaction—the hyperfine splitting in hydrogen and caesium atoms—to β = (2.2 ± 2.5) × 10−7, a factor of two improvement over previous estimates. Our result is based on the comparison between the long-term fractional frequency variation of four hydrogen masers that are part of an ensemble of clocks comprising the National Institute of Standards and Technology, Boulder, and the fractional frequencies of primary frequency standards operated by leading metrology laboratories in the United States, France, Germany, Italy and the United Kingdom over a period of more than 14 years. Using our results together with the previous best estimates of β, we impose strict limits on the variation of fundamental constants, resulting in a test of general relativity with an unprecedented level of precision.

This paper discusses how to build a time scale with an intermittently-operated optical clock. In particular, it gives suggestions on how long and how often to run an optical clock. It also explores the benefits of having an optical clock in a time scale, by comparing with the current UTC(NIST) performance and the time scale with a continuously-operated Cs fountain., Comment: 11 pages, 10 figures

The pursuit of ever more precise measures of time and frequency motivates redefinition of the second in terms of an optical atomic transition. To ensure continuity with the current definition, based on the microwave hyperfine transition in Cs133, it is necessary to measure the absolute frequency of candidate optical standards relative to primary cesium references. Armed with independent measurements, a stringent test of optical clocks can be made by comparing ratios of absolute frequency measurements against optical frequency ratios measured via direct optical comparison. Here we measure the S01→P03 transition of Yb171 using satellite time and frequency transfer to compare the clock frequency to an international collection of national primary and secondary frequency standards. Our measurements consist of 79 runs spanning eight months, yielding the absolute frequency to be 518 295 836 590 863.71(11) Hz and corresponding to a fractional uncertainty of 2.1×10−16. This absolute frequency measurement, the most accurate reported for any transition, allows us to close the Cs-Yb-Sr-Cs frequency measurement loop at an uncertainty

International audience

A time scale is a procedure for accurately and continuously marking the passage of time. It is exemplified by Coordinated Universal Time (UTC), and provides the backbone for critical navigation tools such as the Global Positioning System (GPS). Present time scales employ microwave atomic clocks, whose attributes can be combined and averaged in a manner such that the composite is more stable, accurate, and reliable than the output of any individual clock. Over the past decade, clocks operating at optical frequencies have been introduced which are orders of magnitude more stable than any microwave clock. However, in spite of their great potential, these optical clocks cannot be operated continuously, which makes their use in a time scale problematic. In this paper, we report the development of a hybrid microwave-optical time scale, which only requires the optical clock to run intermittently while relying upon the ensemble of microwave clocks to serve as the flywheel oscillator. The benefit of using clock ensemble as the flywheel oscillator, instead of a single clock, can be understood by the Dick-effect limit. This time scale demonstrates for the first time sub-nanosecond accuracy for a few months, attaining a fractional frequency uncertainty of 1.45*10-16 at 30 days and reaching the 10-17 decade at 50 days, with respect to UTC. This time scale significantly improves the accuracy in timekeeping and could change the existing time-scale architectures.

We perform absolute measurement of the 27Al+ single-ion and 87Sr neutral lattice clock frequencies at the National Institute of Standards and Technology and JILA at the University of Colorado against a global ensemble of primary frequency standards. Over an eight month period multiple measurements yielded the mean optical atomic transition frequencies ν Al + = 1 121 015 393 207 859.50 ( 0.36 ) Hz and ν Sr = 429 228 004 229 873.19(0.15) Hz, where the stated uncertainties are dominated by statistical noise and gaps in the observation interval (‘dead-time’ uncertainty).

Some investigations have concluded that the diurnal pattern in the time comparison results of present two way satellite time and frequency transfer (TWSTFT) links may come mainly from Doppler dependent errors in the time of arrival (TOA) measurements made by the receivers of the TWSTFT modems. In this paper, several experiments were carried out to test if there is a Doppler dependent error in the 'delay' measurements of the receivers currently used. By simulating quantitative Doppler effects in the time transfer signal both on the carrier and the code, a type of Doppler sensitivity on the code was observed in the receivers, which has about −0.49 ns offset in 'delay' measurement for a 1 × 10−9 fractional Doppler shift. This sensitivity is basically the same for modems with different serial numbers from the same manufacture. We calculated this Doppler caused diurnal pattern in the time comparison results of the transatlantic TWSTFT link between NIST and PTB and found that it is very small and negligible, because the Doppler dependent error is almost identical in the NIST and PTB measurements and therefore it is nearly canceled in the TWSTFT difference.

This paper describes the recent NIST work on incorporating an optical clock into a time scale. We simulate a time scale composed of continuously-operating commercial hydrogen masers and an optical frequency standard that does not operate continuously as a clock. The simulations indicate that to achieve the same performance of a continuously-operating Cs-fountain time scale, it is necessary to run an optical frequency standard 12 min per half a day, or 1 hour per day, or 4 hours per 2.33 day, or 12 hours per week. Following the simulations, a Yb optical clock at NIST was frequently operated during the periods of 2017 March – April and 2017 late October – late December. During this operation, comb-mediated measurements between the Yb clock and a hydrogen maser had durations ranging from a few minutes to a few hours, depending on the experimental arrangements. This paper analyzes these real data preliminarily, and discusses the results. More data are needed to make a more complete assessment.

International audience; The International Bureau of Weights and Measures and the Consultative Committee for Time and Frequency Working Group on Two-Way Satellite Time and Frequency Transfer (TWSTFT) have stimulated a pilot study on using software defined radio (SDR) receivers for TWSTFT in the framework of the realization of Coordinated Universal Time. The SDR receivers based on a software developed by TL have been set up at PTB, OP and NIST during the summer and fall of 2016. Continuous SDR TWSTFT measurements have been established for the OP/PTB, NIST/OP and NIST/PTB links. From the SDR TWSTFT clock comparison results (TWSTFT difference), we observed that the commonly present but undesired diurnal pattern in the OP/PTB SDR TWSTFT difference is greatly reduced compared to the diurnal pattern in the difference obtained from the regular (nonSDR) TWSTFT equipment. However, the diurnal reduction in the NIST/OP and NIST/PTB SDR TWSTFT differences is not as significant as in the OP/PTB link. This is a strong indication that the application of SDR receivers has the ability to significantly improve the stability of some TWSTFT links, but is not a solution for all links. In this paper, we will analyze the diurnal reduction using the SDR receivers for the three links and study the dominant causes for the diurnal effect in the EuropeEurope and transatlantic TWSTFT links.

The conventional wisdom suggests that a direct Two-Way Satellite Time and Frequency Transfer (TWSTFT or TW) time link should result in a smaller uncertainty than that of an indirect TW link over the same baseline [12]. This is why all Coordinated Universal Time (UTC) TW links are direct and therefore assumed to be the most precise. This paper shows an exception to this logic. Following the use of the Global Positioning System (GPS) all-in-view [1], the UTC links, including Global Navigation Satellite Systems (GNSS) and TWSTFT, became the direct links between Lab(k) and the PTB (Physikalisch-Technische Bundesanstalt, Germany), which is the single pivot laboratory of the international UTC network. Currently, the daily variation (diurnal) in TW differences is the dominant source of instability in TWSTFT. This, at least in the case of the inner-European network, may be significantly reduced by an indirect link through a third intermediate laboratory in the USA, acting as a relaying laboratory [2, 3]. In this study, we examined the direct and indirect Europe-Europe TW links with data collected by SATRE modems and using the NIST (National Institute of Standards and Technology) or the USNO (US Naval Observatory) laboratory as an intermediate laboratory. We applied the Time deviation (Tdev), triangle closures, and the gains obtained by comparing with the results of the GPS Precise Point Positioning (GPSPPP) [4] and the TW software-defined radio (SDR) receiver [5, 6, 7] as indicators to verify the improvements in the indirect links versus the direct links. Considerable improvements were obtained by a factor of two or three. On the same TW baselines, we further investigated the TW SDR indirect links and gains continued to be observed. In addition, the rapidly developing new time and frequency transfer techniques, such as optical fibres, are often not linked directly with the present pivot laboratory, the PTB. Therefore, indirect links or the multi-pivot UTC time transfer network configuration for future UTC generation is meaningful and has to be reconsidered. Finally, one of the most important characteristics of the indirect link is that it does not require new equipment or additional measurements. Only minor modifications were required to the software and this has already been carried out in the BIPM UTC/TAI computation software package, Tsoft.

We report on a preliminary design of a new Kalman-filter Hydrogen-maser time scale at NIST. The time scale is composed of a few Hydrogen masers and a Cs clock. The Cs clock is used as a reference clock, to ease operations with existing data. All the data used in this paper are real measurement data, except mentioned specifically. Unlike most other time scales, this time scale uses three basic time-scale equations, instead of only one equation. Also, this time scale can detect a clock error (i.e., time error, frequency error, or frequency drift error) automatically. A frequency step of 6.8×10-15 s/s (which typically corresponds to ~ 20C temperature change of a H-maser chamber) in a clock only leads to ~ 2.5 ns change in the time scale in 100 days, thanks to the advanced error-detection technique. A frequency-drift step of 5.4×10-21 s/s2 in a clock only leads to ~ 11 ns change in the time scale in 100 days. These features make the new time scale stiff and less likely to be affected by a bad clock. Tests show that the time scale deviates from the UTC by less than ± 5 ns for ~100 days, when the time scale is initially aligned to the UTC and then is completely free running. Once the time scale is steered to an external frequency standard (such as a Cs fountain), it can maintain the time with little error even if the external frequency standard stops working for tens of days. At NIST, we have the Cs fountain running in 2015 September (23 days), 2015 December (14 days) and 2016 February (18 days). Although the Cs fountain runs for only 55 days in total, the time scale steered to the Cs fountain has a deviation of less than 4 ns (peak-to-peak) from the UTC, during MJD 57265 – 57500 (235 days). This can be helpful when we do not have a continuously-running fountain, or when the continuously-running fountain accidentally stops, or when optical clocks run occasionally.

We report the first accuracy evaluation of NIST-F2, a second-generation laser-cooled caesium fountain primary standard developed at the National Institute of Standards and Technology (NIST) with a cryogenic (liquid nitrogen) microwave cavity and flight region. The 80?K atom interrogation environment reduces the uncertainty due to the blackbody radiation shift by more than a factor of 50. Also, the Ramsey microwave cavity exhibits a high quality factor (>50?000) at this low temperature, resulting in a reduced distributed cavity phase shift. NIST-F2 has undergone many tests and improvements since we first began operation in 2008. In the last few years NIST-F2 has been compared against a NIST maser time scale and NIST-F1 (the US primary frequency standard) as part of in-house accuracy evaluations. We report the results of nine in-house comparisons since 2010 with a focus on the most recent accuracy evaluation. This paper discusses the design of the physics package, the laser and optics systems and the accuracy evaluation methods. The type B fractional uncertainty of NIST-F2 is shown to be 0.11???10?15 and is dominated by microwave amplitude dependent effects. The most recent evaluation (August 2013) had a statistical (type A) fractional uncertainty of 0.44???10?15.

The development of atomic frequency standards at NIST is discussed and three of the key frequency-standard technologies of the current era are described. For each of these technologies, the most recent NIST implementation of the particular type of standard is described in greater detail. The best relative standard uncertainty achieved to date for a NIST frequency standard is 1.5×10(-15). The uncertainties of the most recent NIST standards are displayed relative to the uncertainties of atomic frequency standards of several other countries.

Most of the Two-Way Satellite Time and Frequency Transfer (TWSTFT or TW) links exhibit a daily variation (diurnal) on the order of 1 ns in the differences. The stability of TW is degraded by the diurnal. Many studies on the sources of diurnal have been carried out, but no dominating cause of the diurnal has been found. In this study, we examine the diurnal in several Europe-to-Europe TW links and report that the diurnal and the short-term transfer noise can be reduced by using the triangle difference of the transatlantic TW. We will also analyze where the improvement comes from.

Results are presented from a theoretical and experimental investigation of the frequency transfer uncertainty (FTU) in long-distance comparisons of frequency standards. The FTU can be an important component of the total uncertainty in such comparisons. The use of the Allan deviation in characterizing the FTU is analysed theoretically and it is shown that for certain noise types the Allan deviation is biased high. A potentially more accurate first difference statistic that can be used in certain situations is also discussed. In addition, an experimental determination of the noise types and levels in common transfer techniques is presented. It is shown that FTUs approaching 1 part in 1016 at 30 days are possible with current transfer methods. Finally, a method is presented for estimating the FTU in calibrating International Atomic Time (TAI) with a primary frequency standard.

There are currently nine caesium fountain primary frequency standards regularly reporting calibrations of International Atomic Time to the Bureau International des Poids et Measures (BIPM). An investigation has been carried out using data from the BIPM publication Circular T to evaluate the frequency differences among these standards and to determine whether these offsets are consistent with the stated uncertainties. The fractional frequency uncertainties of some Cs fountains are now in the range of 4 × 10−16 to 5 × 10−16. The results of this investigation show that the standards agree well with each other. An overall estimate of the caesium frequency is made using the weighted mean of all the fountains.

The record of atomic clock frequency comparisons at NIST over the past half-decade provides one of the tightest constraints of any present-day temporal variations of the fundamental constants. Notably, the 6-year record of increasingly precise measurements of the absolute frequency of the Hg+ single-ion optical clock (using the cesium primary frequency standard NIST-F1) constrains the temporal variation of the fine structure constant α to less than 2 · 10−6yr−1 and offers a Local Position Invariance test in the framework of General Relativity. The most recent measurement of the frequency ratio of the Al+ and Hg+ optical clocks is reported with a fractional frequency uncertainty of ±5.2 · 10−17. The record of such measurements over the last year sensitively tests for a temporal variation of α and constrains $\dot{\alpha}/\alpha = (-1.6 \pm 2.3)\ .\ 10^{-17} {\rm yr}^{-1}$ , consistent with zero.

The absolute frequency of the 1S0-3P0 clock transition of 87Sr has been measured to be 429 228 004 229 873.65 (37) Hz using lattice-confined atoms, where the fractional uncertainty of 8.6x10-16 represents one of the most accurate measurements of an atomic transition frequency to date. After a detailed study of systematic effects, which reduced the total systematic uncertainty of the Sr lattice clock to 1.5x10-16, the clock frequency is measured against a hydrogen maser which is simultaneously calibrated to the US primary frequency standard, the NIST Cs fountain clock, NIST-F1. The comparison is made possible using a femtosecond laser based optical frequency comb to phase coherently connect the optical and microwave spectral regions and by a 3.5 km fiber transfer scheme to compare the remotely located clock signals.

We report the technical aspects of the optical-to-microwave comparison for our recent measurements of the optical frequency of the mercury single-ion frequency standard in terms of the SI second as realized by the NIST-F1 cesium fountain clock. Over the course of six years, these measurements have resulted in a determination of the mercury single-ion frequency with a fractional uncertainty of less than 7×10-16, making it the most accurately measured optical frequency to date. In this paper, we focus on the details of the comparison techniques used in the experiment and discuss the uncertainties associated with the optical-to-microwave synthesis based on a femtosecond laser frequency comb. We also present our most recent results in the context of the previous measurements of the mercury single-ion frequency and arrive at a final determination of the mercury single-ion optical frequency: f(Hg+)=1 064 721 609 899 145.30(69) Hz.

A theory is presented for estimating the uncertainty of a frequency comparison in the presence of distributed dead time or measurement interval offset using an extension of the method of Douglas and Boulanger (1997 Proc. 11th European Frequency and Time Forum pp 345–9). The uncertainties due to the distributed dead time and lumped dead time with mixed power law noise type are calculated and compared. It is shown that the use of distributed measurements of frequencies can greatly reduce the uncertainty as compared with that of lumped measurements. When a measurement interval offset is present, two different methods are possible for the frequency estimation and uncertainty evaluation. We compare and discuss the different results for the different methods.

We report a two-detector measurement of the propagation speed of neutrinos over a baseline of 734 km. The measurement was made with the NuMI beam at Fermilab between the near and far MINOS detectors. The fractional difference between the neutrino speed and the speed of light is determined to be $(v/c-1) = (1.0 \pm 1.1) \times 10^{-6}$, consistent with relativistic neutrinos., 10 pages, six figures, after refereeing re-submitted to PRD

Advances in opto-electronics and associated signal processing have enabled the development of Distributed Acoustic and Temperature Sensors. Unlike systems relying on discrete optical sensors a distributed system does not rely upon manufactured sensors but utilises passive custom optical fibre cables resistant to harsh environments, including high temperature applications (600°C). The principle of distributed sensing is well known from the distributed temperature sensor (DTS) which uses the interaction of the source light with thermal vibrations (Raman scattering) to determine the temperature at all points along the fibre. Distributed Acoustic Sensing (DAS) uses a novel digital optical detection technique to precisely capture the true full acoustic field (amplitude, frequency and phase) over a wide dynamic range at every point simultaneously. A number of signal processing techniques have been developed to process a large array of acoustic signals to quantify the coherent temporal and spatial characteristics of the acoustic waves. Predominantly these systems have been developed for the oil and gas industry to assist reservoir engineers in optimising the well lifetime. Nowadays these systems find a wide variety of applications as integrity monitoring tools in process vessels, storage tanks and piping systems offering the operator tools to schedule maintenance programs and maximize service life.

The techniques of GPS time and frequency transfer (code based and carrier phase) and TWSTFT are widely used in remote clock comparison and in the computation of TAI and UTC. Many timing laboratories in the world utilize both techniques (GPS and TWSTFT transfer links) to compare each other's clocks. A time link must be calibrated to assure the time transfer accuracy. In many cases, calibration campaigns have been very infrequent due to the expense and lack of suitable equipment. In lieu of repeated calibrations, some information regarding the long-term stability of these links can be obtained through comparisons between the two links (a so called double difference). Without frequent calibrations it is impossible to tell where the instabilities originate, but information regarding the magnitude of the instabilities can be obtained from double difference data. We have been investigating the combined variations of GPS and TWSTFT links for a number of laboratory pairs, including both long and short baselines. Our results show that the relative change between GPS and TWSTFT transfer links can be as large as 6 to 7 ns over a few years and that all of the laboratory pairs that have been investigated show similar magnitudes in the double difference data. Currently the longest set of good double difference data is about 7 years. The study results point out the need for frequent calibration campaigns if accuracies at the nanosecond level are required.

Primary frequency standards serve the function of calibrating the rate of International Atomic Time, TAI, and therefore play a critical role in the accuracy of the world's time. The Working Group on Primary and Secondary Frequency Standards, WGPSFS, is an advisory body to the Time Department of the Bureau International des Poids et Mesures and to the Consultative Committee for Time and Frequency on matters related to primary and secondary frequency standards that are used to determine the rate of TAI. A current issue being considered by the WGPSFS is establishing guidelines for deciding when and how to make corrections for newly discovered frequency biases in primary frequency standards. This paper is intended to generate discussions on this topic in an audience wider than just the WGPSFS.

Although Global Positioning System (GPS) carrier-phase time transfer (GPSCPTT) offers frequency stability approaching I0-15 at averaging times of 1 d, a discontinuity occurs in the time-transfer estimates between the end of one processing batch (1-3 d in length) and the beginning of the next. The average frequency over a multiday analysis period often has been computed by first estimating and removing these discontinuities, i.e., through concatenation. We present a new frequency-estimation technique in which frequencies are computed from the individual batches then averaged to obtain the mean frequency for a multiday period. This allows the frequency to be computed without the uncertainty associated with the removal of the discontinuities and requires fewer computational resources. The new technique was tested by comparing the fractional frequency-difference values it yields to those obtained using a GPSCPTT concatenation method and those obtained using two-way satellite time-and-frequency transfer (TWSTFT). The clocks studied were located in Braunschweig, Germany, and in Boulder, CO. The frequencies obtained from the GPSCPTT measurements using either method agreed with those obtained from TWSTFT at several parts in 1016. The frequency values obtained from the GPSCPTT data by use of the new method agreed with those obtained using the concatenation technique at 1 - 4 middot 10-16.

International audience; Istituto Elettrotecnico Nazionale Galileo Ferraris (IEN), National Institute of Standards and Technology (NIST), National Physical Laboratory (NPL), Laboratoire National de Métrologie et d'Essais---Observatoire de Paris/Systèmes de Référence Temps Espace (OP) and Physikalisch-Technische Bundesanstalt (PTB) operate cold-atom based primary frequency standards which are capable of realizing the SI second with a relative uncertainty of 1 × 10-15 or even below. These institutes performed an intense comparison campaign of selected frequency references maintained in their laboratories during about 25 days in October/November 2004. Active hydrogen maser reference standards served as frequency references for the institutes' fountain frequency standards. Three techniques of frequency (and time) comparisons were employed. Two-way satellite time and frequency transfer (TWSTFT) was performed in an intensified measurement schedule of 12 equally spaced measurements per day. The data of dual-frequency geodetic Global Positioning System (GPS) receivers were processed to yield an ionosphere-free linear combination of the code observations from both GPS frequencies, typically referred to as GPS TAI P3 analysis. Last but not least, the same GPS raw data were separately processed, allowing GPS carrier-phase (GPS CP) based frequency comparisons to be made. These showed the lowest relative frequency instability at short averaging times of all the methods. The instability was at the level of 1 part in 1015 at one-day averaging time using TWSTFT and GPS CP. The GPS TAI P3 analysis is capable of giving a similar quality of data after averaging over two days or longer. All techniques provided the same mean frequency difference between the standards involved within the 1sigma measurement uncertainty of a few parts in 1016. The frequency differences between the three fountains of IEN (IEN-CsF1), NPL (NPL-CsF1) and OP (OP-FO2) were evaluated. Differences lower than the 1sigma measurement uncertainty were observed between NPL and OP, whereas the IEN fountain deviated by about 2sigma from the other two fountains.

The operation of a caesium fountain primary frequency standard is greatly influenced by the characteristics of two other important facilities. The first is a stable frequency reference and the second is the frequency-transfer system. A stable frequency reference such as a hydrogen maser is a virtual necessity since essentially no fountain dead time can be tolerated without it. The frequency stability of this reference has a significant impact on the procedures for evaluating certain systematic biases in the fountain. State-of-the-art frequency transfer technology is also necessary if the fountain is intended to contribute to TAI or to be compared with other remotely located frequency standards without excessive degradation of stated uncertainties. We discuss the facilities available at the National Institute of Standards and Technology (NIST) and how they impact the operation of NIST-F1, the primary frequency standard at NIST.

In the last several years we have made many improvements to NIST-F1 (a laser-cooled Cs fountain primary frequency standard at the National Institute of Standards and Technology (NIST) in Boulder, Colorado) resulting in a reduction by over a factor of 2 in the uncertainty of the realization of the SI second. The two most recent accuracy evaluations of NIST-F1 had combined standard fractional uncertainties of 0.61 × 10−15 (June 2004) and 0.53 × 10−15 (January 2005), which were submitted to the Bureau International des Poids et Mesures with total fractional uncertainties (including time-transfer contributions) of, respectively, 0.88 × 10−15 and 0.97 × 10−15. Here we discuss the improvements and evaluation methods and present an updated uncertainty budget.

Over the last several years, we have made many improvements to NIST-F1 (a laser-cooled cesium fountain primary frequency standard) resulting in nearly a factor of 2 reduction in the uncertainty in the realization of the SI second at the National Institute of Standards and Technology. We recently submitted an evaluation with a combined standard fractional uncertainty of 0.61/spl times/10/sup -15/ to the Bureau International des Poids et Mesures (BIPM). The total fractional uncertainty of the evaluation (including dead time and time transfer contributions) was 0.88/spl times/10/sup -15/. This is the smallest uncertainty in a frequency standard yet submitted to the BIPM.

The evaluation procedure of a new laser-cooled caesium fountain primary frequency standard developed at the National Institute of Standards and Technology (NIST) is described. The new standard, NIST-F1, is described in some detail and typical operational parameters are discussed. Systematic frequency biases for which corrections are made - second-order Zeeman shift, black-body radiation shift, gravitational red shift and spin-exchange shift - are discussed in detail. Numerous other frequency shifts are evaluated, but are so small in this type of standard that corrections are not made for their effects. We also discuss comparisons of this standard both with local frequency standards and with standards at other national laboratories.

We report on a new Kalman-filter hydrogen-maser time scale (i.e. JY1 time scale) designed at the National Institute of Standards and Technology (NIST). The JY1 time scale is composed of a few hydrogen masers and a commercial Cs clock. The Cs clock is used as a reference clock to ease operations with existing data. Unlike other time scales, the JY1 time scale uses three basic time-scale equations, instead of only one equation. Also, this time scale can detect a clock error (i.e. time error, frequency error, or frequency drift error) automatically. These features make the JY1 time scale stiff and less likely to be affected by an abnormal clock. Tests show that the JY1 time scale deviates from the UTC by less than ±5 ns for ~100 d, when the time scale is initially aligned to the UTC and then is completely free running. Once the time scale is steered to a Cs fountain, it can maintain the time with little error even if the Cs fountain stops working for tens of days. This can be helpful when we do not have a continuously operated fountain or when the continuously operated fountain accidentally stops, or when optical clocks run occasionally.

Historically in-well flow measurement has been challenged by downhole power requirements and post-installation inflexibility. Distributed acoustic sensing makes it possible to monitor the acoustic field along the entire length of a standard fiber optic cable, and therefore facilitate flow monitoring without any downhole electric power requirements. Further, the distributed nature of such systems enables post-installation adaptability as the production profile evolves. Partly as a result of this flexibility, distributed acoustic sensors have now started to gain momentum as a recognized solution for in-well flow surveillance. In this paper, Silixa will present recent development work showing how distributed technology can enable high accuracy flow monitoring. Emerging results will be used to show how array processing of flow noise can be used to develop production data. Examples from the laboratory will be used to demonstrate these advanced techniques. It will be seen that advanced signal processing can be combined with knowledge concerning the physics of fluid-acoustic interactions to make determinations concerning the flow within a well. Finally, it will be shown that there exists strong promise for this upstream technology in a broad variety of well-types.

We report a high-accuracy direct measurement of the blackbody radiation shift of the 133Cs ground-state hyperfine transition. This frequency shift is one of the largest systematic frequency biases encountered in realizing the current definition of the International System of Units (SI) second. Uncertainty in the blackbody radiation frequency shift correction has led to its being the focus of intense theoretical effort by a variety of research groups. Our experimental measurement of the shift used three primary frequency standards operating at different temperatures. We achieved an uncertainty a factor of five smaller than the previous best direct measurement. These results tend to validate the claimed accuracy of the recently calculated values.

Many Statoil operated wells are completed with fibre optic cables, typically installed for transmitting data from pressure/temperature gauges in the wells. The use of fibre optic cables for Distributed Acoustic Sensing (DAS) to acquire borehole seismic data in producing wells has been tested in a cooperation project between Silixa, Weatherford and Statoil, supported by Technology Strategy Board UK. Offshore field trials have been successfully carried out by retrofitting Silixa iDAS units to fibre optic cables previously installed in producing wells. Seismic signals were generated by a towed source and recorded by iDAS using the fibres in the wells as distributed sensors. The iDAS measurements were carried out without disturbing the normal operation of the wells, which were all producing during the acquisition of down hole seismic data. The fibre optic cables used for the tests were installed strapped to tubing .i.e. without direct coupling to the formation. Even so, and in the presence of noise from production, it was possible to acquire seismic data with clear first arrivals from which travel times and interval velocities could reliably be obtained. Also reflected and refracted events are clearly visible. We will present results from DAS measurements in four producing offshore wells and discuss the potential of this exciting technology.

We have conducted global positioning system (GPS) carrier-phase time-transfer experiments between the master clock (MC) at the U.S. Naval Observatory (USNO) in Washington, DC and the alternate master clock (AMC) at Schriever Air Force Base near Colorado Springs, Colorado. These clocks are also monitored on an hourly basis with two-way satellite time-transfer (TWSTT) measurements. We compared the performance of the GPS carrier phase and TWSTT systems over a 236-d period. Because of power problems and data outages during the carrier-phase experiment, the longest continuous time span is 96 d. The data from this period show agreement with TWSTT within /spl plusmn/1 ns, apart from an overall constant time offset (caused by unknown delays in the GPS hardware at both ends). For averaging times of a day, the carrier-phase and TWSTT systems have a frequency uncertainty of 2.5 and 5.5 parts in 10/sup 15/, respectively.

It is necessary to have a complete understanding of the environmental sensitivities of cavity-tuned hydrogen masers to obtain optimum frequency stability and to avoid common-mode frequency fluctuations. Measurements of environmental sensitivities (temperature, relative humidity, atmospheric pressure, line voltage and magnetic field) made at the National Institute of Standards and Technology have demonstrated that the frequency stability of a cavity-tuned, active hydrogen maser is not significantly degraded if the maser is operated in a moderately controlled environment. Under these conditions, common-mode frequency fluctuations caused by the observed environmental factors also are negligible.

The recent addition of new commercial high stability frequency standards to the National Institute of Standards and Technology (NIST) real time AT1 time scale has resulted in significant improvements in the performance of the scale. The frequency stability of the scale at one day has increased by a factor of 2 to 4/spl times/10/sup -15/ and the stability at 100 days has improved to approximately 1/spl times/10/sup -15/. As a result UTC (NIST) has been kept within 50 ns of UTC (Coordinated Universal Time) for the last year. Further improvements are anticipated as more high performance clocks are added to the AT1 ensemble.

We have investigated the earth station errors in two-way time transfer. A 3.7-m earth station and a very small aperture terminal earth station were used in these experiments along with a transponder located on a mountain about 10 km away. The temperature coefficient of the receive delay for the 3.7 m NIST earth station was (-150/spl plusmn/30) ps/K, and the temperature coefficient for the transmit delay was (-50/spl plusmn/10) ps/K. The overall temperature coefficient for the earth station was (100/spl plusmn/30) ps/K. Aside from environmental errors there are apparent changes in delay due to characteristics of the modulation/demodulation scheme. Apparent delay changes due to nonlinearity effects in the earth station were as high as 10 ns. The errors due to code dependence and third code interference were sometimes as high as 3 ns.

Acquiring vertical seismic profiling (VSP) data in a well using conventional wireline technology requires an array of seismic sensors to be lowered down the well to record the seismic signals generated by a shooting vessel on the surface. Due to the expenses involved when interrupting production this is rarely done in producing wells and VSP has not been economically feasible for monitoring purposes. The intelligent Distributed Acoustic Sensing (iDAS) technology enables a fibre-optic cable to be used as a massive acoustic sensor array. Presently, fibre optic cables suitable for iDAS measurements are deployed along many wells for other purposes. The iDAS can be retrofitted to existing optical fibres to acquire densely sampled acoustic measurements at low expense because the normal operation of the well is not disturbed.