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Novel cable coupling technique for improved shallow distributed acoustic sensor VSPs
Journal of Applied Geophysics 138 (2017) 72–79
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Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo
Novel cable coupling technique for improved shallow distributed acoustic sensor VSPs Jonathan D. Munn a,⁎, Thomas I. Coleman a,b, Beth L. Parker a, Michael J. Mondanos b, Athena Chalari b a b
G360 – Centre for Applied Groundwater Research, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada Silixa Ltd., 230 Centennial Park, Centennial Avenue, Elstree, Hertfordshire WD6 3SN, UK
a r t i c l e
i n f o
Article history: Received 16 February 2016 Received in revised form 12 December 2016 Accepted 6 January 2017 Available online 07 January 2017 Keywords: Distributed acoustic sensing Cable coupling VSP Seismic Fiber optic Flexible borehole liner
a b s t r a c t Vertical seismic profiles (VSPs) collected using fiber optic distributed acoustic sensors (DAS) are becoming increasingly common; yet, ensuring good cable coupling with the borehole wall remains a persistent challenge. Traditional cable deployment techniques used in the petroleum industry are either not possible or do not provide data of sufficient quality for shallow applications. Additionally, no direct field comparison of coupling techniques in the same borehole exists to determine the impacts of poor coupling on DAS VSP data quality. This paper addresses these issues by: (1) presenting a novel cable coupling solution using a removable and relatively inexpensive FLUTe™ flexible borehole liner; and (2) presenting field examples of DAS VSPs under different coupling conditions. The proposed coupling technique is analogous to a fully cemented deployment in that the cable is continuously coupled directly to the formation. Field experiments conducted to assess and validate the technique demonstrate a marked improvement in VSP data quality when the cable is coupled with a flexible borehole liner. Without the liner, seismic profiles are dominated by a high-amplitude cable wave and the p-wave arrival is not observed; however, with cable coupling provided by a borehole liner inflated using hydrostatic pressure, the cable wave is suppressed and clear p-wave arrivals are visible. Additional tests examining the influence of fiber optic cable structure on seismic responses demonstrate that tight buffered fibers are more sensitive to dynamic strain than loose tube fibers making them potentially better suited for certain DAS applications. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Fiber optic distributed acoustic sensors (DAS) measure the acoustic energy along the full length of an optical fiber, making them versatile tools for a wide range of applications. Borehole deployments using DAS are becoming increasingly common; yet, inherent challenges with coupling the cable to the borehole wall persist. DAS is based on optical time-domain reflectometry where incident pulses of light are sent down a standard single-mode optical fiber and a small amount of light is continuously backscattered towards the source due to the fiber impurities (Rayleigh scattering). Acoustic waves impart localized strain on the optical fiber and alter the optical path, which creates interference effects in the backscattered light. The DAS can continuously analyze these interference effects and relate them to the local dynamic strain along the fiber. Since the speed of light is well known, the location of backscatter can be calculated from the 2-way travel time through the fiber. Sampling at acoustic frequencies for each location of backscatter allows seismic waveforms to be resolved. In practice, these principles ⁎ Corresponding author: G360 – Centre for Applied Groundwater Research, University of Guelph, 50 Stone Road E, Guelph, ON N1G 2W1, Canada. E-mail address: [email protected] (J.D. Munn).
http://dx.doi.org/10.1016/j.jappgeo.2017.01.007 0926-9851/© 2017 Elsevier B.V. All rights reserved.
enable DAS to be utilized as a continuous array of geophones or hydrophones for recording seismic data such as a vertical seismic profile (VSP). VSPs allow the one-way travel time of seismic waves through geological media to be constrained which, when integrated with borehole geophysics, lithological logs, and surface seismic data provide insights into the extent of lithological contacts and structures such as fracture zones and faults away from boreholes. The first use of DAS for a VSP was presented by Mestayer et al. (2011), and subsequent field studies have demonstrated successful applications of the approach (e.g. Barberan et al., 2012; Cox et al., 2012; Madsen et al., 2012; Miller et al., 2012; Daley et al., 2013; Hartog et al., 2014; Mateeva et al., 2014; Harris et al., 2016). DAS has several advantages over geophones in borehole seismic surveys which have been outlined in recent literature (Mateeva et al., 2014; Li et al., 2015). Of these, one primary advantage is the ability to sample all intervals along the optical fiber simultaneously, allowing data to be collected along the full length of the borehole at once rather than incrementally moving a finite number of geophones on a string up and down the borehole to obtain full coverage. Additionally, fiber optic cables can be deployed in horizontal or slim boreholes, and can include additional fibers for other distributed sensors such as a distributed
J.D. Munn et al. / Journal of Applied Geophysics 138 (2017) 72–79
temperature sensor (DTS). The primary limitation of DAS however, is the lower signal to noise ratio relative to geophones (Cox et al., 2012; Daley et al., 2013; Mateeva et al., 2014; Li et al., 2015; Harris et al., 2016). Efficient transfer of energy from the seismic source to the receiver (optical fiber) is therefore extremely important and requires the cable to be firmly coupled to the borehole wall. With geophones, coupling is achieved through physical clamping to the borehole wall. With DAS surveys, it is more difficult and one of three cable coupling techniques are typically implemented: (1) permanently cementing the cable behind well casing; (2) clamping the cable to production tubing inside the casing; and (3) a wireline or slickline deployment where the cable is installed loose inside the borehole. Of the three options, cementing provides the highest data quality (signal to noise ratio), followed by clamping the cable to the production tubing, and lastly the unclamped wireline deployment typically produces the lowest data quality (Mateeva et al., 2014; Li et al., 2015). All of the traditional deployment techniques have inherent trade-offs between data quality, expense or complexity of installation, and removability (Table 1). Recent studies have worked at improving the performance of certain DAS deployment techniques such as adding extra slack to cables in wireline deployments to improve coupling (Schilke et al., 2016; Constantinou et al., 2016), and using helically wound fiber optic cables to improve broadside sensitivity (Kuvshinov, 2016). While often suitable for deep boreholes, these traditional deployment techniques are typically not possible for shallower applications such as environmental, geotechnical, or hydrogeological investigations where the well casing may not extend beyond the top of bedrock or production tubing may not be present. There is a need for an alternative deployment technique for shallow applications, and also for a direct field comparison between a well coupled cable (analogous to a cemented cable) and a poorly coupled cable (analogous to a wireline deployment) to clearly demonstrate the necessity of good cable coupling. This paper addresses these needs by: (1) providing a novel cable coupling solution using flexible borehole liners to continuously couple the cable against the borehole wall; and (2) demonstrating the strong impact cable coupling can have on DAS VSP data quality using field data collected in the same borehole under different coupling conditions. Additionally, the influence of the fiber optic cable itself is examined by collecting data using two different cable structures, loose tube and tight buffered, to determine if the physical structure of the cable can influence the sensitivity of DAS for measuring dynamic strain. The coupling technique presented here makes use of commercially available, flexible, impermeable, and removable fabric borehole liners which, when inflated with water, can press the cable against the borehole wall providing continuous coupling against the formation. The liners are manufactured by Flexible Liner Underground Technologies (FLUTe, Alcalde, New Mexico, USA, www.flut.com) and have a multitude of established commercial applications including: temporarily sealing open boreholes to prevent vertical hydraulic cross-connection (Pehme et al., 2010, 2013, 2014a; Coleman et al., 2015), transmissivity profiling of boreholes (Keller et al., 2014), multilevel monitoring wells (Cherry et al., 2007), and temporary deployments of temperature and
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pressure sensor strings (Pehme et al., 2014b) among others. The first combined use of flexible borehole liners and distributed fiber optic sensing was conducted by Coleman (2013) and Coleman et al. (2015) where the liner was used in conjunction with a fiber optic cable to conduct active distributed temperature sensor (DTS) experiments to identify natural groundwater flow. The coupling technique introduced in this paper builds upon these studies and presents the first use of flexible liners with DAS for seismic surveying. Due to physical constraints of the liners, the coupling technique presented here is best suited for shallow boreholes with depths of 425 m or less, though the manufacturer notes that deeper applications are possible with appropriate borehole conditions (www.flut.com). Nonetheless, the comparison of DAS VSPs under different coupling conditions is relevant to surveys of any depth and provides a good analog for deep cemented vs. wireline deployments. The field trial to demonstrate the proof of concept was conducted at a research station at the University of Guelph, in Guelph, Ontario, Canada. 2. Materials and methods 2.1. Site description The field trial was conducted at the G360 Bedrock Aquifer Field Facility (BAFF) on the University of Guelph campus in Guelph, Ontario, Canada. The facility was designed to study the Silurian dolostone aquifer used by the City of Guelph (population 125,000) for its municipal water supply and includes a cluster of 9 coreholes (6 vertical and 3 inclined), closely spaced from 7.5 to 70 m apart in a roughly 75 m × 75 m area. The coreholes have a vertical depth of 73 m below ground surface (m bgs) and have continuous polyvinyl chloride (PVC) casing cemented from the ground surface to the top of bedrock at approximately 13 m bgs. Below this, the coreholes are open through the bedrock sequence to the bottom of the holes. The VSPs were conducted in one of the vertical, PQ-diameter (12.3 cm) bedrock coreholes, which has an open hole water level of approximately 17 m bgs. In addition to continuous rock cores from each of the coreholes, a robust suite of high resolution hydraulic and geophysical datasets has been collected to constrain the hydrogeologic units and physical properties of the aquifer for subsequent studies. All depths presented below are relative to ground surface. 2.2. Field equipment An iDAS™ manufactured by Silixa (Elstree, Hertfordshire, UK, www. silixa.com) was used to collect the VSPs. This DAS has a maximum sampling resolution of 0.25 m and can be configured to sample cables of 40 km in length or longer with amplifiers (Parker et al., 2014), though this would require decreasing the sampling resolution. The seismic data presented here were recorded at 20 kHz at 0.5 m intervals and were later down-sampled in time for processing. This provided 146 discrete intervals along the 73 m of cable in the well, which is analogous to a continuous string of 146 geophones spaced 0.5 m apart. A 4.5 kg (10 lb) sledge hammer was used in conjunction with a steel I-beam as
Table 1 Summary of traditional DAS fiber optic cable deployment techniques. Cable deployment technique
Relative data quality
Relative cost/complexity
Removable
Examples
Cemented behind casing
Higha,b,c
Higha,b
Nob
Clamped to production tubing
Moderatea,b,c
Moderatea,b
Semib
Inside production tubing or loose in borehole (wireline or slickline)
Lowa,b
Lowa,b
Yesb
Mestayer et al. (2011), Cox et al. (2012), Mateeva et al. (2014), Harris et al. (2016) Barberan et al. (2012), Daley et al. (2013, 2015), Mateeva et al. (2013, 2014) Miller et al. (2012), Parker et al. (2014), Hartog et al. (2014), Constantinou et al. (2016)
a b c
Mateeva et al. (2014). Li et al. (2015). Parker et al. (2014).
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a compressional (p-wave) seismic source. A triggering mechanism consisting of a magnetic accelerometer and an input module was used to initiate recording on the DAS when the steel plate was struck with the hammer. Two different cable structures were interrogated by the DAS (Fig. 1). The first was a composite cable with a diameter of 9.1 mm and contained two tight buffered single mode fibers, two tight buffered multimode fibers, and two 18-gauge copper conductors. The second had a diameter of 7.7 mm and contained two loose tube single mode fibers, two loose tube multimode fibers, and two 18-gauge copper conductors. The primary difference between the cables is the way the fibers are incorporated into the cable structure. In tight buffered cables, the fibers are individually encapsulated in solid buffering materials, whereas in loose tube cables, the fibers are bundled together in a tube filled with hydrogen scavenging gel. Only the single mode fibers were used in the study, though the multimode fibers and copper conductors increase the versatility of the cables and allow active DTS data to be collected using the same cable and deployment techniques (Coleman et al., 2015). The flexible borehole liner is made from 400-denier, urethane coated nylon. The liners are essentially a waterproof sleeve that is open at the top end and sealed at the bottom. A tether is attached to the sealed end of the sleeve allowing control of installation depth and removal of the liner. The liner used in this study was 71 m long and had a diameter of 12.7 cm (5 in.); though, liners can be manufactured in many different diameters and thicknesses to suite the properties of a given borehole. They can also be manufactured for stepped boreholes where the diameter changes along the length of the wellbore or reinforced at certain locations for added strength. Typically, the liners are slightly wider than the borehole diameter to allow the material to fully expand and move into fractures and small voids to provide a good seal. 2.3. Field deployment The deployment of the cable and flexible liner was relatively simple (Fig. 2). The first step was to install the fiber optic cable in the borehole. The bottom of each cable had a relatively heavy stainless steel termination housing that allowed the cables to be lowered by hand to the bottom of the borehole without difficulty (Fig. 2A). Once the cable reached the bottom, it was lifted approximately 10 cm to ensure it was straight and taut. The borehole liner was then installed using the eversion process described by Cherry et al. (2007). To summarize, the open end of the liner was clamped to the top of the casing and a small amount of material was everted into the borehole to form a pocket. Water was added to the pocket until it had enough weight to start everting the liner down into the borehole (Fig. 2B). Once the water table was reached, water was continuously added inside the liner to maintain a positive head differential and to drive the eversion until it was fully installed. The liner seals the borehole as it descends and acts as a piston that forces the water in the wellbore into the formation.
Consequently, the transmissivity of the borehole below the liner will dictate the rate at which it will descend. At this site, the liners took approximately 1.5 h to reach the bottom. A borehole with lower transmissivity would take longer, and vice versa. A fully cased borehole, or borehole with extremely low transmissivity may require tubing to be installed in the borehole prior to the liner to allow water in the borehole to be removed during liner descent. Once installed to the desired depth, the tether was tied to a steel cross-bar across the top of the casing to secure the base of the liner in place (Fig. 2C). At this point, the fiber optic cable was continuously coupled to the formation between the depths of 13 m (the water level in the liner) and 70 m (the base of the liner) and the liner provided a mean outward pressure of approximately 49 kPa (7.1 psi) below 17 m (Fig. 2D). This outward pressure is the result of the pressure differential between the water level inside the liner and the water level within the rock formation (13 and 17 m respectively). It is applied evenly along the length of the cable below 17 m as the formation hydrostatic pressure increases with depth at the same rate as the hydrostatic pressure inside the liner. Some deviation may occur at a given depth due to natural variations in hydraulic head throughout the aquifer sequence. 2.4. Data acquisition Initial offset VSPs were collected using the tight buffered fiber under two different coupling conditions: (1) the fiber optic cable was installed in the open borehole without a liner (i.e. simply hanging in the borehole comparable to a wireline installation); and (2) the fiber optic cable was installed in the borehole and coupled to the borehole wall with the flexible liner. The first scenario relied simply on friction between the cable and borehole wall to couple the cable whereas the second utilized the outward pressure of the inflated borehole liner. For these tests, 15 shots were recorded at offset distances of 2, 5, 10, and 20 m. The VSPs were repeated using the second cable design (loose tube) to assess the effect of cable structure on the measured dynamic strain. Source offsets of 5 and 10 m were used for this second round of VSPs. Minimal data processing was performed on the datasets including a bandpass filter (10, 20–200, 250 Hz) and stacking of the individual shots (15 shots in total). The measured strain rates presented below are in arbitrary units but have the same reference so a direct comparison can be made. 3. Results and discussion 3.1. Unlined deployment The VSP collected without the flexible liner coupling the fiber (Fig. 3A), had a very high amplitude reverberating event beginning at the top of the borehole and propagating down at a near constant velocity of approximately 2200 m/s. This wave repeated roughly every 15 ms. Two example waveforms from traces at 26 and 50 m show the high strain rates (analogous to amplitude) measured at these locations (Fig. 3B). The velocity of the wave was much lower than the expected
Fig. 1. Schematic showing cable cross sections for the tight buffered composite cable (left) and the loose tube composite cable (right), with the primary difference being the positioning of optical fibers. The tight buffered cable has each fiber encapsulated separately in solid buffering materials whereas the loose tube cable has all fibers together in a gel filled tube. The yellow material between the cable components is aramid fiber. Only the single mode fibers were used in this study. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. Schematic showing the installation procedure for the fiber optic cable and borehole liner: (A) Cable lowered into the borehole; (B) borehole liner clamped to top of casing and water is added to evert liner down the borehole; (C) liner is fully installed and tethered off. The fiber optic cable is continuously coupled against the borehole wall due to hydrostatic pressure inside the liner; (D) (inset) cross section looking down the lined borehole demonstrating how the liner forces the cable against the borehole wall.
Fig. 3. Comparison between the unlined and lined DAS VSP (2 m offset, tight buffered cable, stacked). (A) Profile from unlined scenario showing reverberating high-amplitude cable wave (trace normalized). Note the p-wave arrival is not present in this profile; (B) waveform from traces 53 (26 m) and 101 (50 m) showing high strain rate of the cable wave; (C) profile where the cable is coupled with a borehole liner (trace normalized). Note the clear p-wave arrival and later arrival interpreted to be a tube wave generated at the surface from ground roll; (D) waveform from trace 53 (26 m) and 101 (50 m) showing the much lower strain rate of the seismic source signal relative to the cable wave measured in the unlined scenario.
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causes reflection interference effects that appear as slight variations in velocity or amplitude and can be seen in the far right portion of (Fig. 3A). Aside from the cable wave, no additional arrivals were observed in this VSP. Consequently, lowering the cable into the borehole and relying on friction between the cable and borehole similar to a wireline or slickline deployment, did not produce data of any appreciable quality and does not seem to be a viable deployment option for shallow investigations. 3.2. Lined deployment
Fig. 4. Upgoing energy for the unlined DAS VSP (2 m offset, tight buffered cable). Data is stacked (15 shots) and includes a bandpass filter (10, 20–200, 250 Hz) and downgoing energy subtracted. Reverberating upgoing cable waves can be seen reflecting up from the base of the cable.
p-wave velocity through the competent dolostone, which based on fullwaveform sonic logs of the borehole should average approximately 5300 m/s across the full bedrock sequence. Seismic refraction surveys of the site also indicated similar shallow bedrock p-wave velocity of 5200 m/s (based on a delay time analysis) and overburden velocities between 600 and 800 m/s. A clear travel-time inflection should have occurred across this interface; however, the velocity of this wave remained essentially constant throughout the entire profile. There was no evidence of a p-wave first arrival before the slower, high amplitude wave even after stacking and examination of individual traces. This high-amplitude reverberating event is interpreted to represent a cable wave generated at the surface by high-amplitude ground roll. The cable was suspended from a spool sitting on the ground directly above the borehole and it is likely that this spool would have experienced some movement due to ground roll resulting in the propagation of a wave down the cable. The 2200 m/s velocity of this wave is largely consistent with the longitudinal ultrasonic velocity of polyamide (2200 m/s) and polyurethane (1900 m/s) from which the cable is made. A comparable signal, generated at the wellhead due to high-amplitude ground roll is observed in Miller et al. (2012) at the top of a DAS VSP. They note that the signal is similar to that observed in other borehole tests and interpret it as “extensional vibration of the optical cable where static friction between the cable and borehole is negligible or has been overcome by strong motion”. Daley et al. (2013) also notes reverberations in the upper 300 m of the borehole during a DAS VSP. With the downgoing energy subtracted from the profile (Fig. 4), clear reverberating upgoing waves are observed resulting from reflection of the downgoing energy when it reaches the bottom of the cable. This
In the VSP where cable coupling was provided by a flexible borehole liner (Fig. 3C), the cable wave is also present, but only propagates to a depth of around 22 m. The cable wave appears to arrive earlier in the lined profile but examination of individual traces confirms a first arrival at 6.5 ms in both the lined and unlined scenarios. This cable wave first arrival has much lower amplitude than the rest of the wave and is not visible in the unlined VSP due to the plot scale and trace normalization. The presence of this wave was to be expected to a depth of 13 m (water level in the flexible liner) since the liner had no outward hydrostatic pressure to couple the fiber above this level; however, the cable wave continued 9 m below this point suggesting that the coupling force of the liner was not sufficient to attenuate the cable wave between 13 and 22 m. Raising the water level inside the liner would increase the coupling pressure and could potentially help reduce the propagation of this wave though this was not tested. The dynamic strain rate from the cable wave is slightly lower, and dissipates faster in the lined scenario when compared to the unlined scenario (Fig. 5). In this example trace at 10 m, most of the energy dissipates by 0.3 s in the lined profile whereas it persists beyond 0.45 s before reaching background levels in the unlined scenario. This is likely the result of the more restricted cable movement and contact with the liner in the upper portion of the corehole. Though the liner is not inflated here, the material is still in contact with the cable and corehole wall, which would have a damping effect on the wave. The frequency spectra of the cable wave in the lined and unlined scenarios also varies with a strong peak at 65 Hz in the unlined spectrum relating to the strong reverberation at that frequency (Fig. 5). These strong peaks are absent in the lined scenario and the overall magnitude is slightly lower; however, there are some similarities such as the increased frequency content between 50 and 100 Hz. The apparent variations in velocity and frequency distribution in the lined scenario can likely be attributed to the reflection interference which is more prominent in the early time data due to the shorter length of the reverberating cable and thus, more frequent reflections. Below 22 m, much lower amplitude seismic signals were observed in the trace normalized and stacked profile including a clear p-wave first arrival and later arrival interpreted to be a tube wave. The p-wave had an average velocity of 5235 m/s which is mostly consistent with the
Fig. 5. Measured dynamic strain (upper) and frequency spectra (lower) of the cable wave at 10 m below ground surface.
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units) whereas in the lined scenario, the same trace had a maximum measured strain rate of 61 (arbitrary units, absolute value), nearly two orders of magnitude lower. Also notable was the lower amplitude strain rate measured at the deeper trace (50 m) relative to the shallower trace (26 m) in the lined scenario. Thus, the seismic energy traveling through the formation is significantly lower in amplitude than that traveling along the cable, and the measured amplitude of the signal traveling through the formation decreases with depth. The attenuation of the cable wave and presence of clear first arrivals in the VSP demonstrates that the borehole liner coupling technique significantly improved data quality over the unlined scenario. When compared to traditional cable deployment techniques (Fig. 7), the flexible borehole liner offers several advantages: (1) once the cable wave is attenuated, it continuously couples the cable directly against the formation allowing high quality VSPs to be collected; (2) it is completely removable; and (3) it is relatively inexpensive and easy to install. The main limitation of the technique is the depth restriction of approximately 425 m. Nevertheless, it provides a unique and effective solution for shallow borehole deployments where otherwise no practical alternatives exist. 3.3. Cable comparison Fig. 6. Schematic showing (from left to right): borehole and deployment summary, DAS VSP showing average p-wave pick and corresponding velocity, and comparison of mean VSP velocity with detailed and mean velocities from the full waveform sonic (FWS) probe. Mean p-wave velocities are highly congruent between the VSP and FWS probe.
mean p-wave velocity measured with the full-waveform sonic (FWS) probe through the sequence (5304 m/s). A more powerful and consistent source for the DAS VSP would be required for a detailed velocity comparison between the DAS and FWS; nonetheless, the similar mean velocities between the two techniques confirms this to be an accurate measure of the p-wave arrival (Fig. 6). The later arrival interpreted to be a tube wave had a velocity of approximately 1300 m/s. Tube waves of a similar velocity along the full length of the cable were also observed by Daley et al. (2013) and are interpreted to be generated along the casing due to ground roll. Two example traces (Fig. 3D) taken from the same depths as the unlined scenario (26 m) demonstrate the much lower signal amplitude (strain rate) of the p-wave relative to the cable wave near the top. At the 26 m trace, the cable wave in the unlined scenario had a maximum measured strain rate of 4899 (arbitrary
Comparison of the two different cable structures (tight buffered and loose tube), both coupled with the flexible liner, revealed notable differences. Example traces from a similar depth for the tight buffered and loose tube cable structures (26 and 26.2 m, respectively) demonstrate that the tight buffered cable registers more dynamic strain than the loose tube cable for the same type of seismic signal (15 shots stacked) (Fig. 8). While both cable structures were able to record enough dynamic strain to produce interpretable VSPs, the maximum amplitude of measured strain rate for the 26 m trace using tight buffered cable was 44 (arbitrary units, absolute value) whereas the loose tube was 13 (arbitrary units), less than a third of the strain recorded using the tight buffered cable. While this example is from a single trace, a similar relationship was observed across numerous traces where overlapping data existed for comparison. The two cable types also have different frequency distributions with the tight buffered cable registering more power in the 0 to 175 Hz range relative to the loose tube cable. The higher signal amplitude and frequency distribution measured in the tight buffered cable can be explained through the more efficient transfer of seismic energy from the formation to the
Fig. 7. Schematic displaying the various traditional deployment options for fiber optic cables in boreholes next to the proposed deployment technique with flexible borehole liners. Relative data quality, expense, removability and effective depth are compared at the bottom of the schematic. The flexible liner technique offers a low cost, removable deployment technique that provides high quality data. The main limitation of the technique is the depth limitation of approximately 425 m. (After Li et al., 2015 and expanded to include flexible borehole liner technique).
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Fig. 8. Measured dynamic strain (upper) and frequency spectra (lower) of the two cable structures tested: tight buffered and loose tube. The tight buffered cable registers more dynamic strain from the seismic source below 175 Hz relative to the loose tube cable structure.
optical fiber through the entirely solid cable materials versus the loose tube cable where the fibers are suspended in a gel. This higher sensitivity increases the signal to noise ratio and consequently, could allow the identification of a seismic signal farther along an optical fiber than what would be possible with loose tube fibers. It should be noted however, that tight buffered fibers are subject to optical signal attenuation with depth due to hydrostatic pressure, which would limit the maximum depth to which they could be deployed in boreholes. The field-scale observations presented here are consistent with the independently developed bench-scale tests of Papp et al. (2016) who demonstrated that different cable coatings influence the ability of individual tight buffered fibers to measure strain. It is clear that cable design and structure can have a strong impact on DAS seismic performance and should therefore be carefully considered for each deployment.
Acknowledgements
4. Conclusions
References
Traditional cable deployment techniques for DAS borehole seismic surveys such as cementing behind casing or clamping to production tubing are typically not suitable for shallow boreholes often utilized in environmental, geotechnical, or hydrogeological investigations. The flexible borehole coupling technique presented here provides a novel and effective solution by continuously pressing the cable against the borehole wall using the outward hydrostatic pressure of the liner. It is relatively inexpensive, easy to deploy, and completely removable. Field validation of the technique demonstrated a marked improvement in the data quality of the VSP when compared to the same test collected without the liner. Without the liner, the VSP was dominated by a high amplitude cable wave and no p-wave arrival was observed; however, with the flexible liner coupling the cable, the high amplitude cable wave was attenuated below 22 m and a clear p-wave arrival could be observed. These findings not only provide a solution for cable coupling in shallow boreholes, but also demonstrate the necessity of good cable coupling for DAS VSPs at any depth. Comparison of two fiber optic cable structures, tight buffered and loose tube, suggests that the tight buffered cable is more sensitive to dynamic strain than the loose tube cable. This is interpreted to be the result of inefficient strain transfer through the cable materials in the loose tube cable as the seismic energy must propagate through the gel that surrounds the fibers versus the fully solid materials in tight buffered cables. Consequently, the use of tight buffered cables in shallow DAS borehole or surface seismic deployments could help maximize the sensitivity of the receiver to the seismic source.
Funding for this research was provided through the Natural Sciences and Engineering Research Council of Canada (NSERC) (Grant number IRCPJ363783-11 with Dr. Beth Parker as the Principal Investigator) and in-kind support from Silixa Ltd. and Flexible Liner Underground Technologies, LLC. The authors would like to thank Peeter Pehme, Colby Steelman, and Carlos Maldaner (G360 – Centre for Applied Groundwater Research, University of Guelph) for their assistance in planning and reviews of the paper. Special thanks to Andrey Fomenko (G360) who collected several of the background geophysical datasets at the research site. The authors would also like to thank Dr. Herb Wang (University of Wisconsin) for his thoughtful comments. Enthusiastic field operation support was provided by Rumen Karaulanov (Silixa Ltd.). Software and support was provided by VSProwess Ltd.
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J.D. Munn et al. / Journal of Applied Geophysics 138 (2017) 72–79 Keller, C.E., Cherry, J.A., Parker, B.L., 2014. New method for continuous transmissivity profiling in fractured rock. Groundwater 52 (3):352–367. http://dx.doi.org/10.1111/ gwat.12064. Kuvshinov, B.N., 2016. Interaction of helically wound fibre-optic cables with plane seismic waves. Geophys. Prospect. 64 (3):671–688. http://dx.doi.org/10.1111/1365-2478. 12303. Li, M., Wang, H., Tao, G., 2015. Current and future applications of distributed acoustic sensing as a new reservoir geophysics tool. Open Pet. Eng. J. 8 (1):272–281. http:// dx.doi.org/10.2174/1874834120150625E008. Madsen, K.N., Parker, T., Gatson, G., 2012. A VSP field trial using distributed acoustic sensing in a producing well in the North Sea. 74th EAGE Conference and Exhibition, Expanded Abstracts. Mateeva, A., Lopez, J., Mestayer, J., Wills, P., Cox, B., Kiyashchenko, D., et al., 2013. Distributed acoustic sensing for reservoir monitoring with VSP. Lead. Edge 32 (10), 1278–1283. Mateeva, A., Lopez, J., Potters, H., Mestayer, J., Cox, B., Kiyashchenko, D., Wills, P., Grandi, S., Hornman, K., Kuvshinov, B., Berlang, W., Yang, Z., Detomo, R., 2014. Distributed acoustic sensing for reservoir monitoring with vertical seismic profiling. Geophys. Prospect. 62:679–692. http://dx.doi.org/10.1111/1365-2478.12116. Mestayer, J., Cox, B., Wills, P., Kiyashchenko, D., Lopez, J., Costello, M., Bourne, S., Ugueto, G., Lupton, R., Solano, G., Hill, D., Lewis, A., 2011. Field trials of distributed acoustic sensing for geophysical monitoring. SEG Annual Meeting, San Antonio, TX, USA: pp. 4253–4257. http://dx.doi.org/10.1190/1.3628095. Miller, D., Parker, T., Kashikar, S., Todorov, M., Bostick, T., 2012. Vertical seismic profiling using a fiber-optic cable as a distributed acoustic sensor. 74th EAGE Conference and Exhibition, Expanded Abstracts.
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Papp, B., Donno, D., Martin, J.E., Hartog, A.H., 2016. A study of the geophysical response of distributed fibre optic sensors through laboratory-scale experiments. Geophys. Prospect. http://dx.doi.org/10.1111/1365-2478.12471. Parker, T., Shatalin, S., Farhadiroushan, M., 2014. Distributed acoustic sensing–a new tool for seismic applications. First Break 32 (2):61–69. http://dx.doi.org/10.3997/13652397.2013034. Pehme, P., Parker, B., Cherry, J., Greenhouse, J., 2010. Improved resolution of ambient flow through fractured rock with temperature logs. Ground Water 48 (2):191–205. http:// dx.doi.org/10.1111/j.1745-6584.2009.00639.x. Pehme, P., Parker, B., Cherry, J., Molson, J., Greenhouse, J., 2013. Enhanced detection of hydraulically active fractures by temperature profiling in lined heated bedrock boreholes. J. Hydrol. 484:1–15. http://dx.doi.org/10.1016/j.jhydrol.2012.12.048. Pehme, P., Parker, B.L., Cherry, J.A., Blohm, D., 2014a. Detailed measurement of the magnitude and orientation of thermal gradients in lined boreholes for characterizing groundwater flow in fractured rock. J. Hydrol. 513:101–114. http://dx.doi.org/10. 1016/j.jhydrol.2014.03.015. Pehme, P., Chapman, S.W., Parker, B.L., Cherry, J.A., 2014b. Temporary Sensor Deployments: a Method for Improved Insight into Hydraulic Variations and Design of Permanent Multilevel Installations. DFNE Vancouver, Vancouver, Canada-British Columbia, pp. 19–22 (October). Schilke, S., Donno, D., Chauris, H., Hartog, A.H., Farahani, A., Pico, Y., 2016. Numerical evaluation of sensor coupling of distributed acoustic sensing systems in vertical seismic profiling. 86th SEG Annual Meeting, Dallas, USA, Expanded Abstracts, pp. 677–681.
Contents lists available at ScienceDirect
Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo
Novel cable coupling technique for improved shallow distributed acoustic sensor VSPs Jonathan D. Munn a,⁎, Thomas I. Coleman a,b, Beth L. Parker a, Michael J. Mondanos b, Athena Chalari b a b
G360 – Centre for Applied Groundwater Research, University of Guelph, 50 Stone Road East, Guelph, Ontario N1G 2W1, Canada Silixa Ltd., 230 Centennial Park, Centennial Avenue, Elstree, Hertfordshire WD6 3SN, UK
a r t i c l e
i n f o
Article history: Received 16 February 2016 Received in revised form 12 December 2016 Accepted 6 January 2017 Available online 07 January 2017 Keywords: Distributed acoustic sensing Cable coupling VSP Seismic Fiber optic Flexible borehole liner
a b s t r a c t Vertical seismic profiles (VSPs) collected using fiber optic distributed acoustic sensors (DAS) are becoming increasingly common; yet, ensuring good cable coupling with the borehole wall remains a persistent challenge. Traditional cable deployment techniques used in the petroleum industry are either not possible or do not provide data of sufficient quality for shallow applications. Additionally, no direct field comparison of coupling techniques in the same borehole exists to determine the impacts of poor coupling on DAS VSP data quality. This paper addresses these issues by: (1) presenting a novel cable coupling solution using a removable and relatively inexpensive FLUTe™ flexible borehole liner; and (2) presenting field examples of DAS VSPs under different coupling conditions. The proposed coupling technique is analogous to a fully cemented deployment in that the cable is continuously coupled directly to the formation. Field experiments conducted to assess and validate the technique demonstrate a marked improvement in VSP data quality when the cable is coupled with a flexible borehole liner. Without the liner, seismic profiles are dominated by a high-amplitude cable wave and the p-wave arrival is not observed; however, with cable coupling provided by a borehole liner inflated using hydrostatic pressure, the cable wave is suppressed and clear p-wave arrivals are visible. Additional tests examining the influence of fiber optic cable structure on seismic responses demonstrate that tight buffered fibers are more sensitive to dynamic strain than loose tube fibers making them potentially better suited for certain DAS applications. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Fiber optic distributed acoustic sensors (DAS) measure the acoustic energy along the full length of an optical fiber, making them versatile tools for a wide range of applications. Borehole deployments using DAS are becoming increasingly common; yet, inherent challenges with coupling the cable to the borehole wall persist. DAS is based on optical time-domain reflectometry where incident pulses of light are sent down a standard single-mode optical fiber and a small amount of light is continuously backscattered towards the source due to the fiber impurities (Rayleigh scattering). Acoustic waves impart localized strain on the optical fiber and alter the optical path, which creates interference effects in the backscattered light. The DAS can continuously analyze these interference effects and relate them to the local dynamic strain along the fiber. Since the speed of light is well known, the location of backscatter can be calculated from the 2-way travel time through the fiber. Sampling at acoustic frequencies for each location of backscatter allows seismic waveforms to be resolved. In practice, these principles ⁎ Corresponding author: G360 – Centre for Applied Groundwater Research, University of Guelph, 50 Stone Road E, Guelph, ON N1G 2W1, Canada. E-mail address: [email protected] (J.D. Munn).
http://dx.doi.org/10.1016/j.jappgeo.2017.01.007 0926-9851/© 2017 Elsevier B.V. All rights reserved.
enable DAS to be utilized as a continuous array of geophones or hydrophones for recording seismic data such as a vertical seismic profile (VSP). VSPs allow the one-way travel time of seismic waves through geological media to be constrained which, when integrated with borehole geophysics, lithological logs, and surface seismic data provide insights into the extent of lithological contacts and structures such as fracture zones and faults away from boreholes. The first use of DAS for a VSP was presented by Mestayer et al. (2011), and subsequent field studies have demonstrated successful applications of the approach (e.g. Barberan et al., 2012; Cox et al., 2012; Madsen et al., 2012; Miller et al., 2012; Daley et al., 2013; Hartog et al., 2014; Mateeva et al., 2014; Harris et al., 2016). DAS has several advantages over geophones in borehole seismic surveys which have been outlined in recent literature (Mateeva et al., 2014; Li et al., 2015). Of these, one primary advantage is the ability to sample all intervals along the optical fiber simultaneously, allowing data to be collected along the full length of the borehole at once rather than incrementally moving a finite number of geophones on a string up and down the borehole to obtain full coverage. Additionally, fiber optic cables can be deployed in horizontal or slim boreholes, and can include additional fibers for other distributed sensors such as a distributed
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temperature sensor (DTS). The primary limitation of DAS however, is the lower signal to noise ratio relative to geophones (Cox et al., 2012; Daley et al., 2013; Mateeva et al., 2014; Li et al., 2015; Harris et al., 2016). Efficient transfer of energy from the seismic source to the receiver (optical fiber) is therefore extremely important and requires the cable to be firmly coupled to the borehole wall. With geophones, coupling is achieved through physical clamping to the borehole wall. With DAS surveys, it is more difficult and one of three cable coupling techniques are typically implemented: (1) permanently cementing the cable behind well casing; (2) clamping the cable to production tubing inside the casing; and (3) a wireline or slickline deployment where the cable is installed loose inside the borehole. Of the three options, cementing provides the highest data quality (signal to noise ratio), followed by clamping the cable to the production tubing, and lastly the unclamped wireline deployment typically produces the lowest data quality (Mateeva et al., 2014; Li et al., 2015). All of the traditional deployment techniques have inherent trade-offs between data quality, expense or complexity of installation, and removability (Table 1). Recent studies have worked at improving the performance of certain DAS deployment techniques such as adding extra slack to cables in wireline deployments to improve coupling (Schilke et al., 2016; Constantinou et al., 2016), and using helically wound fiber optic cables to improve broadside sensitivity (Kuvshinov, 2016). While often suitable for deep boreholes, these traditional deployment techniques are typically not possible for shallower applications such as environmental, geotechnical, or hydrogeological investigations where the well casing may not extend beyond the top of bedrock or production tubing may not be present. There is a need for an alternative deployment technique for shallow applications, and also for a direct field comparison between a well coupled cable (analogous to a cemented cable) and a poorly coupled cable (analogous to a wireline deployment) to clearly demonstrate the necessity of good cable coupling. This paper addresses these needs by: (1) providing a novel cable coupling solution using flexible borehole liners to continuously couple the cable against the borehole wall; and (2) demonstrating the strong impact cable coupling can have on DAS VSP data quality using field data collected in the same borehole under different coupling conditions. Additionally, the influence of the fiber optic cable itself is examined by collecting data using two different cable structures, loose tube and tight buffered, to determine if the physical structure of the cable can influence the sensitivity of DAS for measuring dynamic strain. The coupling technique presented here makes use of commercially available, flexible, impermeable, and removable fabric borehole liners which, when inflated with water, can press the cable against the borehole wall providing continuous coupling against the formation. The liners are manufactured by Flexible Liner Underground Technologies (FLUTe, Alcalde, New Mexico, USA, www.flut.com) and have a multitude of established commercial applications including: temporarily sealing open boreholes to prevent vertical hydraulic cross-connection (Pehme et al., 2010, 2013, 2014a; Coleman et al., 2015), transmissivity profiling of boreholes (Keller et al., 2014), multilevel monitoring wells (Cherry et al., 2007), and temporary deployments of temperature and
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pressure sensor strings (Pehme et al., 2014b) among others. The first combined use of flexible borehole liners and distributed fiber optic sensing was conducted by Coleman (2013) and Coleman et al. (2015) where the liner was used in conjunction with a fiber optic cable to conduct active distributed temperature sensor (DTS) experiments to identify natural groundwater flow. The coupling technique introduced in this paper builds upon these studies and presents the first use of flexible liners with DAS for seismic surveying. Due to physical constraints of the liners, the coupling technique presented here is best suited for shallow boreholes with depths of 425 m or less, though the manufacturer notes that deeper applications are possible with appropriate borehole conditions (www.flut.com). Nonetheless, the comparison of DAS VSPs under different coupling conditions is relevant to surveys of any depth and provides a good analog for deep cemented vs. wireline deployments. The field trial to demonstrate the proof of concept was conducted at a research station at the University of Guelph, in Guelph, Ontario, Canada. 2. Materials and methods 2.1. Site description The field trial was conducted at the G360 Bedrock Aquifer Field Facility (BAFF) on the University of Guelph campus in Guelph, Ontario, Canada. The facility was designed to study the Silurian dolostone aquifer used by the City of Guelph (population 125,000) for its municipal water supply and includes a cluster of 9 coreholes (6 vertical and 3 inclined), closely spaced from 7.5 to 70 m apart in a roughly 75 m × 75 m area. The coreholes have a vertical depth of 73 m below ground surface (m bgs) and have continuous polyvinyl chloride (PVC) casing cemented from the ground surface to the top of bedrock at approximately 13 m bgs. Below this, the coreholes are open through the bedrock sequence to the bottom of the holes. The VSPs were conducted in one of the vertical, PQ-diameter (12.3 cm) bedrock coreholes, which has an open hole water level of approximately 17 m bgs. In addition to continuous rock cores from each of the coreholes, a robust suite of high resolution hydraulic and geophysical datasets has been collected to constrain the hydrogeologic units and physical properties of the aquifer for subsequent studies. All depths presented below are relative to ground surface. 2.2. Field equipment An iDAS™ manufactured by Silixa (Elstree, Hertfordshire, UK, www. silixa.com) was used to collect the VSPs. This DAS has a maximum sampling resolution of 0.25 m and can be configured to sample cables of 40 km in length or longer with amplifiers (Parker et al., 2014), though this would require decreasing the sampling resolution. The seismic data presented here were recorded at 20 kHz at 0.5 m intervals and were later down-sampled in time for processing. This provided 146 discrete intervals along the 73 m of cable in the well, which is analogous to a continuous string of 146 geophones spaced 0.5 m apart. A 4.5 kg (10 lb) sledge hammer was used in conjunction with a steel I-beam as
Table 1 Summary of traditional DAS fiber optic cable deployment techniques. Cable deployment technique
Relative data quality
Relative cost/complexity
Removable
Examples
Cemented behind casing
Higha,b,c
Higha,b
Nob
Clamped to production tubing
Moderatea,b,c
Moderatea,b
Semib
Inside production tubing or loose in borehole (wireline or slickline)
Lowa,b
Lowa,b
Yesb
Mestayer et al. (2011), Cox et al. (2012), Mateeva et al. (2014), Harris et al. (2016) Barberan et al. (2012), Daley et al. (2013, 2015), Mateeva et al. (2013, 2014) Miller et al. (2012), Parker et al. (2014), Hartog et al. (2014), Constantinou et al. (2016)
a b c
Mateeva et al. (2014). Li et al. (2015). Parker et al. (2014).
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a compressional (p-wave) seismic source. A triggering mechanism consisting of a magnetic accelerometer and an input module was used to initiate recording on the DAS when the steel plate was struck with the hammer. Two different cable structures were interrogated by the DAS (Fig. 1). The first was a composite cable with a diameter of 9.1 mm and contained two tight buffered single mode fibers, two tight buffered multimode fibers, and two 18-gauge copper conductors. The second had a diameter of 7.7 mm and contained two loose tube single mode fibers, two loose tube multimode fibers, and two 18-gauge copper conductors. The primary difference between the cables is the way the fibers are incorporated into the cable structure. In tight buffered cables, the fibers are individually encapsulated in solid buffering materials, whereas in loose tube cables, the fibers are bundled together in a tube filled with hydrogen scavenging gel. Only the single mode fibers were used in the study, though the multimode fibers and copper conductors increase the versatility of the cables and allow active DTS data to be collected using the same cable and deployment techniques (Coleman et al., 2015). The flexible borehole liner is made from 400-denier, urethane coated nylon. The liners are essentially a waterproof sleeve that is open at the top end and sealed at the bottom. A tether is attached to the sealed end of the sleeve allowing control of installation depth and removal of the liner. The liner used in this study was 71 m long and had a diameter of 12.7 cm (5 in.); though, liners can be manufactured in many different diameters and thicknesses to suite the properties of a given borehole. They can also be manufactured for stepped boreholes where the diameter changes along the length of the wellbore or reinforced at certain locations for added strength. Typically, the liners are slightly wider than the borehole diameter to allow the material to fully expand and move into fractures and small voids to provide a good seal. 2.3. Field deployment The deployment of the cable and flexible liner was relatively simple (Fig. 2). The first step was to install the fiber optic cable in the borehole. The bottom of each cable had a relatively heavy stainless steel termination housing that allowed the cables to be lowered by hand to the bottom of the borehole without difficulty (Fig. 2A). Once the cable reached the bottom, it was lifted approximately 10 cm to ensure it was straight and taut. The borehole liner was then installed using the eversion process described by Cherry et al. (2007). To summarize, the open end of the liner was clamped to the top of the casing and a small amount of material was everted into the borehole to form a pocket. Water was added to the pocket until it had enough weight to start everting the liner down into the borehole (Fig. 2B). Once the water table was reached, water was continuously added inside the liner to maintain a positive head differential and to drive the eversion until it was fully installed. The liner seals the borehole as it descends and acts as a piston that forces the water in the wellbore into the formation.
Consequently, the transmissivity of the borehole below the liner will dictate the rate at which it will descend. At this site, the liners took approximately 1.5 h to reach the bottom. A borehole with lower transmissivity would take longer, and vice versa. A fully cased borehole, or borehole with extremely low transmissivity may require tubing to be installed in the borehole prior to the liner to allow water in the borehole to be removed during liner descent. Once installed to the desired depth, the tether was tied to a steel cross-bar across the top of the casing to secure the base of the liner in place (Fig. 2C). At this point, the fiber optic cable was continuously coupled to the formation between the depths of 13 m (the water level in the liner) and 70 m (the base of the liner) and the liner provided a mean outward pressure of approximately 49 kPa (7.1 psi) below 17 m (Fig. 2D). This outward pressure is the result of the pressure differential between the water level inside the liner and the water level within the rock formation (13 and 17 m respectively). It is applied evenly along the length of the cable below 17 m as the formation hydrostatic pressure increases with depth at the same rate as the hydrostatic pressure inside the liner. Some deviation may occur at a given depth due to natural variations in hydraulic head throughout the aquifer sequence. 2.4. Data acquisition Initial offset VSPs were collected using the tight buffered fiber under two different coupling conditions: (1) the fiber optic cable was installed in the open borehole without a liner (i.e. simply hanging in the borehole comparable to a wireline installation); and (2) the fiber optic cable was installed in the borehole and coupled to the borehole wall with the flexible liner. The first scenario relied simply on friction between the cable and borehole wall to couple the cable whereas the second utilized the outward pressure of the inflated borehole liner. For these tests, 15 shots were recorded at offset distances of 2, 5, 10, and 20 m. The VSPs were repeated using the second cable design (loose tube) to assess the effect of cable structure on the measured dynamic strain. Source offsets of 5 and 10 m were used for this second round of VSPs. Minimal data processing was performed on the datasets including a bandpass filter (10, 20–200, 250 Hz) and stacking of the individual shots (15 shots in total). The measured strain rates presented below are in arbitrary units but have the same reference so a direct comparison can be made. 3. Results and discussion 3.1. Unlined deployment The VSP collected without the flexible liner coupling the fiber (Fig. 3A), had a very high amplitude reverberating event beginning at the top of the borehole and propagating down at a near constant velocity of approximately 2200 m/s. This wave repeated roughly every 15 ms. Two example waveforms from traces at 26 and 50 m show the high strain rates (analogous to amplitude) measured at these locations (Fig. 3B). The velocity of the wave was much lower than the expected
Fig. 1. Schematic showing cable cross sections for the tight buffered composite cable (left) and the loose tube composite cable (right), with the primary difference being the positioning of optical fibers. The tight buffered cable has each fiber encapsulated separately in solid buffering materials whereas the loose tube cable has all fibers together in a gel filled tube. The yellow material between the cable components is aramid fiber. Only the single mode fibers were used in this study. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 2. Schematic showing the installation procedure for the fiber optic cable and borehole liner: (A) Cable lowered into the borehole; (B) borehole liner clamped to top of casing and water is added to evert liner down the borehole; (C) liner is fully installed and tethered off. The fiber optic cable is continuously coupled against the borehole wall due to hydrostatic pressure inside the liner; (D) (inset) cross section looking down the lined borehole demonstrating how the liner forces the cable against the borehole wall.
Fig. 3. Comparison between the unlined and lined DAS VSP (2 m offset, tight buffered cable, stacked). (A) Profile from unlined scenario showing reverberating high-amplitude cable wave (trace normalized). Note the p-wave arrival is not present in this profile; (B) waveform from traces 53 (26 m) and 101 (50 m) showing high strain rate of the cable wave; (C) profile where the cable is coupled with a borehole liner (trace normalized). Note the clear p-wave arrival and later arrival interpreted to be a tube wave generated at the surface from ground roll; (D) waveform from trace 53 (26 m) and 101 (50 m) showing the much lower strain rate of the seismic source signal relative to the cable wave measured in the unlined scenario.
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causes reflection interference effects that appear as slight variations in velocity or amplitude and can be seen in the far right portion of (Fig. 3A). Aside from the cable wave, no additional arrivals were observed in this VSP. Consequently, lowering the cable into the borehole and relying on friction between the cable and borehole similar to a wireline or slickline deployment, did not produce data of any appreciable quality and does not seem to be a viable deployment option for shallow investigations. 3.2. Lined deployment
Fig. 4. Upgoing energy for the unlined DAS VSP (2 m offset, tight buffered cable). Data is stacked (15 shots) and includes a bandpass filter (10, 20–200, 250 Hz) and downgoing energy subtracted. Reverberating upgoing cable waves can be seen reflecting up from the base of the cable.
p-wave velocity through the competent dolostone, which based on fullwaveform sonic logs of the borehole should average approximately 5300 m/s across the full bedrock sequence. Seismic refraction surveys of the site also indicated similar shallow bedrock p-wave velocity of 5200 m/s (based on a delay time analysis) and overburden velocities between 600 and 800 m/s. A clear travel-time inflection should have occurred across this interface; however, the velocity of this wave remained essentially constant throughout the entire profile. There was no evidence of a p-wave first arrival before the slower, high amplitude wave even after stacking and examination of individual traces. This high-amplitude reverberating event is interpreted to represent a cable wave generated at the surface by high-amplitude ground roll. The cable was suspended from a spool sitting on the ground directly above the borehole and it is likely that this spool would have experienced some movement due to ground roll resulting in the propagation of a wave down the cable. The 2200 m/s velocity of this wave is largely consistent with the longitudinal ultrasonic velocity of polyamide (2200 m/s) and polyurethane (1900 m/s) from which the cable is made. A comparable signal, generated at the wellhead due to high-amplitude ground roll is observed in Miller et al. (2012) at the top of a DAS VSP. They note that the signal is similar to that observed in other borehole tests and interpret it as “extensional vibration of the optical cable where static friction between the cable and borehole is negligible or has been overcome by strong motion”. Daley et al. (2013) also notes reverberations in the upper 300 m of the borehole during a DAS VSP. With the downgoing energy subtracted from the profile (Fig. 4), clear reverberating upgoing waves are observed resulting from reflection of the downgoing energy when it reaches the bottom of the cable. This
In the VSP where cable coupling was provided by a flexible borehole liner (Fig. 3C), the cable wave is also present, but only propagates to a depth of around 22 m. The cable wave appears to arrive earlier in the lined profile but examination of individual traces confirms a first arrival at 6.5 ms in both the lined and unlined scenarios. This cable wave first arrival has much lower amplitude than the rest of the wave and is not visible in the unlined VSP due to the plot scale and trace normalization. The presence of this wave was to be expected to a depth of 13 m (water level in the flexible liner) since the liner had no outward hydrostatic pressure to couple the fiber above this level; however, the cable wave continued 9 m below this point suggesting that the coupling force of the liner was not sufficient to attenuate the cable wave between 13 and 22 m. Raising the water level inside the liner would increase the coupling pressure and could potentially help reduce the propagation of this wave though this was not tested. The dynamic strain rate from the cable wave is slightly lower, and dissipates faster in the lined scenario when compared to the unlined scenario (Fig. 5). In this example trace at 10 m, most of the energy dissipates by 0.3 s in the lined profile whereas it persists beyond 0.45 s before reaching background levels in the unlined scenario. This is likely the result of the more restricted cable movement and contact with the liner in the upper portion of the corehole. Though the liner is not inflated here, the material is still in contact with the cable and corehole wall, which would have a damping effect on the wave. The frequency spectra of the cable wave in the lined and unlined scenarios also varies with a strong peak at 65 Hz in the unlined spectrum relating to the strong reverberation at that frequency (Fig. 5). These strong peaks are absent in the lined scenario and the overall magnitude is slightly lower; however, there are some similarities such as the increased frequency content between 50 and 100 Hz. The apparent variations in velocity and frequency distribution in the lined scenario can likely be attributed to the reflection interference which is more prominent in the early time data due to the shorter length of the reverberating cable and thus, more frequent reflections. Below 22 m, much lower amplitude seismic signals were observed in the trace normalized and stacked profile including a clear p-wave first arrival and later arrival interpreted to be a tube wave. The p-wave had an average velocity of 5235 m/s which is mostly consistent with the
Fig. 5. Measured dynamic strain (upper) and frequency spectra (lower) of the cable wave at 10 m below ground surface.
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units) whereas in the lined scenario, the same trace had a maximum measured strain rate of 61 (arbitrary units, absolute value), nearly two orders of magnitude lower. Also notable was the lower amplitude strain rate measured at the deeper trace (50 m) relative to the shallower trace (26 m) in the lined scenario. Thus, the seismic energy traveling through the formation is significantly lower in amplitude than that traveling along the cable, and the measured amplitude of the signal traveling through the formation decreases with depth. The attenuation of the cable wave and presence of clear first arrivals in the VSP demonstrates that the borehole liner coupling technique significantly improved data quality over the unlined scenario. When compared to traditional cable deployment techniques (Fig. 7), the flexible borehole liner offers several advantages: (1) once the cable wave is attenuated, it continuously couples the cable directly against the formation allowing high quality VSPs to be collected; (2) it is completely removable; and (3) it is relatively inexpensive and easy to install. The main limitation of the technique is the depth restriction of approximately 425 m. Nevertheless, it provides a unique and effective solution for shallow borehole deployments where otherwise no practical alternatives exist. 3.3. Cable comparison Fig. 6. Schematic showing (from left to right): borehole and deployment summary, DAS VSP showing average p-wave pick and corresponding velocity, and comparison of mean VSP velocity with detailed and mean velocities from the full waveform sonic (FWS) probe. Mean p-wave velocities are highly congruent between the VSP and FWS probe.
mean p-wave velocity measured with the full-waveform sonic (FWS) probe through the sequence (5304 m/s). A more powerful and consistent source for the DAS VSP would be required for a detailed velocity comparison between the DAS and FWS; nonetheless, the similar mean velocities between the two techniques confirms this to be an accurate measure of the p-wave arrival (Fig. 6). The later arrival interpreted to be a tube wave had a velocity of approximately 1300 m/s. Tube waves of a similar velocity along the full length of the cable were also observed by Daley et al. (2013) and are interpreted to be generated along the casing due to ground roll. Two example traces (Fig. 3D) taken from the same depths as the unlined scenario (26 m) demonstrate the much lower signal amplitude (strain rate) of the p-wave relative to the cable wave near the top. At the 26 m trace, the cable wave in the unlined scenario had a maximum measured strain rate of 4899 (arbitrary
Comparison of the two different cable structures (tight buffered and loose tube), both coupled with the flexible liner, revealed notable differences. Example traces from a similar depth for the tight buffered and loose tube cable structures (26 and 26.2 m, respectively) demonstrate that the tight buffered cable registers more dynamic strain than the loose tube cable for the same type of seismic signal (15 shots stacked) (Fig. 8). While both cable structures were able to record enough dynamic strain to produce interpretable VSPs, the maximum amplitude of measured strain rate for the 26 m trace using tight buffered cable was 44 (arbitrary units, absolute value) whereas the loose tube was 13 (arbitrary units), less than a third of the strain recorded using the tight buffered cable. While this example is from a single trace, a similar relationship was observed across numerous traces where overlapping data existed for comparison. The two cable types also have different frequency distributions with the tight buffered cable registering more power in the 0 to 175 Hz range relative to the loose tube cable. The higher signal amplitude and frequency distribution measured in the tight buffered cable can be explained through the more efficient transfer of seismic energy from the formation to the
Fig. 7. Schematic displaying the various traditional deployment options for fiber optic cables in boreholes next to the proposed deployment technique with flexible borehole liners. Relative data quality, expense, removability and effective depth are compared at the bottom of the schematic. The flexible liner technique offers a low cost, removable deployment technique that provides high quality data. The main limitation of the technique is the depth limitation of approximately 425 m. (After Li et al., 2015 and expanded to include flexible borehole liner technique).
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Fig. 8. Measured dynamic strain (upper) and frequency spectra (lower) of the two cable structures tested: tight buffered and loose tube. The tight buffered cable registers more dynamic strain from the seismic source below 175 Hz relative to the loose tube cable structure.
optical fiber through the entirely solid cable materials versus the loose tube cable where the fibers are suspended in a gel. This higher sensitivity increases the signal to noise ratio and consequently, could allow the identification of a seismic signal farther along an optical fiber than what would be possible with loose tube fibers. It should be noted however, that tight buffered fibers are subject to optical signal attenuation with depth due to hydrostatic pressure, which would limit the maximum depth to which they could be deployed in boreholes. The field-scale observations presented here are consistent with the independently developed bench-scale tests of Papp et al. (2016) who demonstrated that different cable coatings influence the ability of individual tight buffered fibers to measure strain. It is clear that cable design and structure can have a strong impact on DAS seismic performance and should therefore be carefully considered for each deployment.
Acknowledgements
4. Conclusions
References
Traditional cable deployment techniques for DAS borehole seismic surveys such as cementing behind casing or clamping to production tubing are typically not suitable for shallow boreholes often utilized in environmental, geotechnical, or hydrogeological investigations. The flexible borehole coupling technique presented here provides a novel and effective solution by continuously pressing the cable against the borehole wall using the outward hydrostatic pressure of the liner. It is relatively inexpensive, easy to deploy, and completely removable. Field validation of the technique demonstrated a marked improvement in the data quality of the VSP when compared to the same test collected without the liner. Without the liner, the VSP was dominated by a high amplitude cable wave and no p-wave arrival was observed; however, with the flexible liner coupling the cable, the high amplitude cable wave was attenuated below 22 m and a clear p-wave arrival could be observed. These findings not only provide a solution for cable coupling in shallow boreholes, but also demonstrate the necessity of good cable coupling for DAS VSPs at any depth. Comparison of two fiber optic cable structures, tight buffered and loose tube, suggests that the tight buffered cable is more sensitive to dynamic strain than the loose tube cable. This is interpreted to be the result of inefficient strain transfer through the cable materials in the loose tube cable as the seismic energy must propagate through the gel that surrounds the fibers versus the fully solid materials in tight buffered cables. Consequently, the use of tight buffered cables in shallow DAS borehole or surface seismic deployments could help maximize the sensitivity of the receiver to the seismic source.
Funding for this research was provided through the Natural Sciences and Engineering Research Council of Canada (NSERC) (Grant number IRCPJ363783-11 with Dr. Beth Parker as the Principal Investigator) and in-kind support from Silixa Ltd. and Flexible Liner Underground Technologies, LLC. The authors would like to thank Peeter Pehme, Colby Steelman, and Carlos Maldaner (G360 – Centre for Applied Groundwater Research, University of Guelph) for their assistance in planning and reviews of the paper. Special thanks to Andrey Fomenko (G360) who collected several of the background geophysical datasets at the research site. The authors would also like to thank Dr. Herb Wang (University of Wisconsin) for his thoughtful comments. Enthusiastic field operation support was provided by Rumen Karaulanov (Silixa Ltd.). Software and support was provided by VSProwess Ltd.
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