Imaging the wave velocity field surrounding a borehole while drilling is a promising and urgently needed approach for extending the exploration range of the borehole point.This paper develops a drilling process detect...Imaging the wave velocity field surrounding a borehole while drilling is a promising and urgently needed approach for extending the exploration range of the borehole point.This paper develops a drilling process detection(DPD)system consisting of a multifunctional sensor and a pilot geophone installed at the top of the drilling rod,geophones at the tunnel face,a laser rangefinder,and an onsite computer.A weighted adjoint-state first arrival travel time tomography method is used to invert the P-wave velocity field of rock mass while borehole drilling.A field experiment in the ongoing construction of a deep buried tunnel in southwestern China demonstrated the DPD system and the tomography method.Time-frequency analysis of typical borehole drilling detection data shows that the impact drilling source is a pulse-like seismic exploration wavelet.A velocity field of the rock mass in a triangular area defined by the borehole trajectory and geophone receiving line can be obtained.Both the borehole core and optical image validate the inverted P-wave velocity field.A numerical simulation of a checkerboard benchmark model is used to test the tomography method.The rapid convergence of the misfits and consistent agreement between the inverted and observed travel times validate the P-wave velocity imaging.展开更多
The microbial induced calcium carbonate precipitation(MICP)technology provides a new approach to solve borehole destabilization in broken formations;however,the high-temperature and alkaline environments inhibit the g...The microbial induced calcium carbonate precipitation(MICP)technology provides a new approach to solve borehole destabilization in broken formations;however,the high-temperature and alkaline environments inhibit the growth of microorganisms,which in turn affects the performance of their wall enhancement performance.In this study,a pH and temperature-coupled induced domestication method was applied to Bacillus pasteurii,and its wall enhancement performance was evaluated.Post domestication,Bacillus pasteurii exhibited high growth activity at pH 10.3 and temperature 45℃.In a sodium carboxymethyl cellulose(CMC)drilling fluid environment,bacterial concentration reached 1.373 with urease activity at 1.98 after 24 h,and in a xanthan gum(XG)environment,the figures were 0.931 and 1.76,respectively—significantly higher than those before domestication.The Bacillus pasteurii-CMC system exhibited enhanced performance with the unconfined compressive strength of the specimen up to 1.232 MPa,permeability coefficient as low as 0.024,and calcium carbonate production up to 24.685 g.The crushed specimen portions remained lumpy with even calcium carbonate distribution.In contrast,the Bacillus pasteurii-XG system exhibited the highest unconfined compressive strength of 0.561 MPa,lowest permeability coefficient of 0.081,and the greatest calcium carbonate production of 16.03 g,with an externally cemented shell but internally loose structure and uneven calcium carbonate distribution,resulting in weaker mechanical properties.The Bacillus pasteurii induced predominantly vaterite calcium carbonate crystals in the CMC drilling fluid.In the XG drilling fluid,the crystals were mainly calcite.Both types effectively cemented the broken particles,improving formation strength and reducing permeability.However,under the same conditions,the Bacillus pasteurii-CMC system demonstrated a more pronounced enhancement effect.展开更多
The purpose of this paper is to establish the existence of the critical condition of borehole stability during air drilling. Rock Failure Process Analysis Code 20 was used to set up a damage model of the borehole exca...The purpose of this paper is to establish the existence of the critical condition of borehole stability during air drilling. Rock Failure Process Analysis Code 20 was used to set up a damage model of the borehole excavated in strain-softening rock. Damage evolution around the borehole was studied by tracking acoustic emission. The study indicates that excavation damaged zone （EDZ） is formed around borehole because of stress concentration after the borehole is excavated. There is a critical condition for borehole stability; the borehole will collapse when the critical damage condition is reached. The critical condition of underground excavation exists not only in elastic and ideal plastic material but in strainsoftening material as well. The research is helpful to developing an evaluation method of borehole stability during air drilling.展开更多
Hydrocarbons are very often associated with salt structures. The oil and gas industry is often required to drill along and through long salt sections to reach and recover hydrocarbons. The unique physical properties o...Hydrocarbons are very often associated with salt structures. The oil and gas industry is often required to drill along and through long salt sections to reach and recover hydrocarbons. The unique physical properties of salt require special techniques to ensure borehole stability and adequate casing design. This paper assumed that the mechanical behavior of salt is regulated by the magnitude of mean stress and octahedral shear stress and under the influence of different stress conditions the deformation of rock salt can be represented by three domains, i.e. compression domain, volume unchanged domain, and dilatancy domain, which are separated by a stress dependent boundary. In the compression domain, the volume of salt decreases until all microcracks are closed, with only elastic deformation and pure creep; in the volume unchanged domain the deformation is considered steady incompressible flow controlled by pure creep; and in the dilatancy domain the volume of salt increases during deformation due to micro-cracking, causing damage and accelerating ＂creep＂ until failure. This paper presents a hypothesis that the borehole is stable only when the magnitude of octahedral shear stress is below the dilatancy boundary. It gives the design method for determining drilling fluids density, and calculates the closure rate ofborehole with the recommended drilling fluids density. If the closure rate of the borehole is less than 0.1%, the drilling fluids density window can be used during drilling through extremely thick salt formations.展开更多
Rock damage appears in brittle shale even prior to peak stress(i.e.,before failure)due to the occurrence of microcracks in these rocks.In this work,a coupled hydromechanical model was built by incorporating the mechan...Rock damage appears in brittle shale even prior to peak stress(i.e.,before failure)due to the occurrence of microcracks in these rocks.In this work,a coupled hydromechanical model was built by incorporating the mechanical and fluid seepage induced stresses around a wellbore during drilling.The borehole instability mechanism of hard-brittle shale was studied.The results show that even if a well is simply drilled into a hard-brittle shale formation,the formation around the borehole can be subjected to rock damage.The maximum failure ratio of the formation around the borehole increases with drilling time.A lower drilling fluid density corresponds to a faster increase in the failure ratio of the borehole with time and a shorter period of borehole collapse.When the initial drilling fluid density is too low,serious rock damage occurs in the formation around the borehole.Even though a high-density drilling fluid is used after drilling,long-term borehole stability is difficult to maintain.While drilling in hard-brittle shale,drilling fluid with a proper density should be used rather than increasing the density of the drilling fluid only after borehole collapse occurs,which is more favorable for maintaining long-term borehole stability.展开更多
There are many problems associated with coiled tubing drilling operations, such as great circulation pressure loss inside pipe, difficulties in weight on bit(WOB) transferring, and high probability of differential sti...There are many problems associated with coiled tubing drilling operations, such as great circulation pressure loss inside pipe, difficulties in weight on bit(WOB) transferring, and high probability of differential sticking. Aiming at these problems, solids-free brine drilling fluid system was developed on the basis of formulation optimization with brine base fluid experiment, which was evaluated and applied to field drilling. Based on the optimization of flow pattern regulator, salt-resisting filtrate reducer, high performance lubricant and bit cleaner, the basic formula of the solids-free brine drilling fluid system was formed: brine +(0.1%-0.2%) Na OH +(0.2%-0.4%) HT-XC +(2.0%-3.0%) YLJ-1 +(0.5%-2.0%) SDNR +(1.0%-2.5%) FT-1 A +(1.0%-5.0%) SD-505 + compound salt density regulator. Lab evaluation showed that the fluid had satisfactory temperature resistance(up to 150 ℃), excellent cuttings tolerance(up to 25%), and strong inhibition(92.7% cuttings recovery); Moreover, its lubrication performance was similar to that of all oil-based drilling fluid. The wellbore could be fairly cleaned at annular up-flow velocity of more than 0.8 m/s if the ratio of yield point to plastic viscosity was kept above 0.5. This fluid system has been applied in the drilling of three coiled tubing sidetracking wells in the Liaohe Oilfield, during which the system was stable and easy to adjust, resulting in excellent cuttings transportation, high ROP, regular hole size, and no down hole accidents. In summary, the solids-free brine drilling fluid system can meet the technical requirements of coiled tubing drilling.展开更多
To solve the problem of borehole trajectory uncertainty, some methods such as error ellipsoid posture characterization, sectional error ellipse solution and error elliptic cylinder construction were proposed and an ap...To solve the problem of borehole trajectory uncertainty, some methods such as error ellipsoid posture characterization, sectional error ellipse solution and error elliptic cylinder construction were proposed and an application example was given. According to the definition of inclination, azimuth and tool-face angle, a characterization method of error ellipsoid posture of borehole trajectory was presented. By intercepting the error ellipsoid with an arbitrary plane in space, the general concept and algorithm of sectional error ellipse were established to analyze the borehole trajectory errors in horizontal plane, plumb plane, normal plane, etc. Based on the theory of surface tangency and curve projection, a construction method of error elliptic cylinder of borehole trajectory was put forward to evaluate the axial enveloping error of borehole trajectory and its variation along well depth. The research shows that the deeper the well, the greater the borehole trajectory error will be. In deep and ultra-deep wells measured using conventional measurement while drilling(MWD),the borehole trajectory position error reaches tens of meters. The research results provide a complete set of analysis methods for borehole trajectory error, which can evaluate the accuracy and reliability of borehole trajectory monitoring.展开更多
Downhole acoustic telemetry(DAT),using a long drill string with periodical structures as the channel,is a prospective technology for improving the transmission rate of logging while drilling(LWD)data.Previous studies ...Downhole acoustic telemetry(DAT),using a long drill string with periodical structures as the channel,is a prospective technology for improving the transmission rate of logging while drilling(LWD)data.Previous studies only focused on the acoustic property of a free drill string and neglected the coupling between pipes and fluid-filled boreholes.In addition to the drill-string waves,a series of fluid waves are recorded in the DAT channel,which has not been investigated yet.Unpredictable channel characteristics result in lower transmission rates and stability than expected.Therefore,a more realistic channel model is needed considering the fluid-filled borehole.In this paper,we propose a hybrid modeling method to investigate the response characteristics of the DAT channel.By combining the axial wavenumbers and excitation functions of mode waves in radially layered LWD structures,the channel model is approximated to the 1-D propagation,which considers transmission,reflection,and interconversion of the drillstring and fluid waves.The proposed 1-D approximation has been well validated by comparing the 2-D finite-difference modeling.It is revealed that the transmitted and converted fluid waves interfere with the drill-string wave,which characterizes the DAT channel as a particular coherent multi-path channel.When a fluid-filled borehole surrounds the drill string,the channel responses exhibit considerable delay as well as strong frequency selectivity in amplitude and phase.These new findings suggest that the complexity of the channel response has been underestimated in the past,and therefore channel measurements on the ground are unreliable.To address these channel characteristics,we apply a noncoherent demodulation strategy.The transmission rate for synthetic data reaches 15 bps in a 94.5 m long channel,indicating that the acoustic telemetry is promising to break the low-speed limitation of mud-pulse telemetry.展开更多
In order to accurately calculate drilled trajectories,the method of quantitatively recognizing borehole trajectory models was provided,and a case analysis was conducted.Because the measurement-while-drilling data prov...In order to accurately calculate drilled trajectories,the method of quantitatively recognizing borehole trajectory models was provided,and a case analysis was conducted.Because the measurement-while-drilling data provide with measured values of tool-face angle besides inclination angle and azimuth angle,this paper presents the technological approach of recognizing borehole trajectory models based on tool-face angle.A universal tool-face angle equation was established based on the directional deflection mechanism of steerable drilling tools,and it can calculate the tool-face angles with characteristic parameters of various borehole trajectory models.Then,by evaluating the error between the theoretical values and the measured values of tool-face angle,the trajectory model most consistent with the actual well trajectory can be selected.The model recognition of borehole trajectory provides with the quantitative evaluation index and selection basis of survey calculation methods,which can avoid subjectively and randomly selecting the survey calculation method,and consequently improve the monitoring accuracy and reliability of borehole trajectory.展开更多
Tool waves, also named collar waves, propagating along the drill collars in acoustic logging while drilling （ALWD）, strongly interfere with the needed P- and S-waves of a penetrated formation, which is a key issue i...Tool waves, also named collar waves, propagating along the drill collars in acoustic logging while drilling （ALWD）, strongly interfere with the needed P- and S-waves of a penetrated formation, which is a key issue in picking up formation P- and S-wave velocities. Previous studies on physical insulation for the collar waves designed on the collar between the source and the receiver sections did not bring to a satisfactory solution. In this paper, we investigate the propagation features of collar waves in different models. It is confirmed that there exists an indirect collar wave in the synthetic full waves due to the coupling between the drill collar and the borehole, even there is a perfect isolator between the source and the receiver. The direct collar waves propagating all along the tool and the indirect ones produced by echoes from the borehole wall are summarized as the generalized collar waves. Further analyses show that the indirect collar waves could be relatively strong in the full wave data. This is why the collar waves cannot be eliminated with satisfactory effect in many cases by designing the physical isolators carved on the tool.展开更多
基金the support of the National Natural Science Foundation of China(Nos.42207211,42202320 and 42172296)Key Laboratory of Geotechnical and Underground Engineering of Ministry of Education,Tongji University(No.KLE-TJGE-G2304).
文摘Imaging the wave velocity field surrounding a borehole while drilling is a promising and urgently needed approach for extending the exploration range of the borehole point.This paper develops a drilling process detection(DPD)system consisting of a multifunctional sensor and a pilot geophone installed at the top of the drilling rod,geophones at the tunnel face,a laser rangefinder,and an onsite computer.A weighted adjoint-state first arrival travel time tomography method is used to invert the P-wave velocity field of rock mass while borehole drilling.A field experiment in the ongoing construction of a deep buried tunnel in southwestern China demonstrated the DPD system and the tomography method.Time-frequency analysis of typical borehole drilling detection data shows that the impact drilling source is a pulse-like seismic exploration wavelet.A velocity field of the rock mass in a triangular area defined by the borehole trajectory and geophone receiving line can be obtained.Both the borehole core and optical image validate the inverted P-wave velocity field.A numerical simulation of a checkerboard benchmark model is used to test the tomography method.The rapid convergence of the misfits and consistent agreement between the inverted and observed travel times validate the P-wave velocity imaging.
基金supported by the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection Independent Research Project(SKLGP2023Z012)the National Natural Science Foundation of China(Grant No.41702388)the Everest Technology Research Proposal of Chengdu University of Technology(Grant No.80000-2023ZF11411)。
文摘The microbial induced calcium carbonate precipitation(MICP)technology provides a new approach to solve borehole destabilization in broken formations;however,the high-temperature and alkaline environments inhibit the growth of microorganisms,which in turn affects the performance of their wall enhancement performance.In this study,a pH and temperature-coupled induced domestication method was applied to Bacillus pasteurii,and its wall enhancement performance was evaluated.Post domestication,Bacillus pasteurii exhibited high growth activity at pH 10.3 and temperature 45℃.In a sodium carboxymethyl cellulose(CMC)drilling fluid environment,bacterial concentration reached 1.373 with urease activity at 1.98 after 24 h,and in a xanthan gum(XG)environment,the figures were 0.931 and 1.76,respectively—significantly higher than those before domestication.The Bacillus pasteurii-CMC system exhibited enhanced performance with the unconfined compressive strength of the specimen up to 1.232 MPa,permeability coefficient as low as 0.024,and calcium carbonate production up to 24.685 g.The crushed specimen portions remained lumpy with even calcium carbonate distribution.In contrast,the Bacillus pasteurii-XG system exhibited the highest unconfined compressive strength of 0.561 MPa,lowest permeability coefficient of 0.081,and the greatest calcium carbonate production of 16.03 g,with an externally cemented shell but internally loose structure and uneven calcium carbonate distribution,resulting in weaker mechanical properties.The Bacillus pasteurii induced predominantly vaterite calcium carbonate crystals in the CMC drilling fluid.In the XG drilling fluid,the crystals were mainly calcite.Both types effectively cemented the broken particles,improving formation strength and reducing permeability.However,under the same conditions,the Bacillus pasteurii-CMC system demonstrated a more pronounced enhancement effect.
文摘The purpose of this paper is to establish the existence of the critical condition of borehole stability during air drilling. Rock Failure Process Analysis Code 20 was used to set up a damage model of the borehole excavated in strain-softening rock. Damage evolution around the borehole was studied by tracking acoustic emission. The study indicates that excavation damaged zone （EDZ） is formed around borehole because of stress concentration after the borehole is excavated. There is a critical condition for borehole stability; the borehole will collapse when the critical damage condition is reached. The critical condition of underground excavation exists not only in elastic and ideal plastic material but in strainsoftening material as well. The research is helpful to developing an evaluation method of borehole stability during air drilling.
文摘Hydrocarbons are very often associated with salt structures. The oil and gas industry is often required to drill along and through long salt sections to reach and recover hydrocarbons. The unique physical properties of salt require special techniques to ensure borehole stability and adequate casing design. This paper assumed that the mechanical behavior of salt is regulated by the magnitude of mean stress and octahedral shear stress and under the influence of different stress conditions the deformation of rock salt can be represented by three domains, i.e. compression domain, volume unchanged domain, and dilatancy domain, which are separated by a stress dependent boundary. In the compression domain, the volume of salt decreases until all microcracks are closed, with only elastic deformation and pure creep; in the volume unchanged domain the deformation is considered steady incompressible flow controlled by pure creep; and in the dilatancy domain the volume of salt increases during deformation due to micro-cracking, causing damage and accelerating ＂creep＂ until failure. This paper presents a hypothesis that the borehole is stable only when the magnitude of octahedral shear stress is below the dilatancy boundary. It gives the design method for determining drilling fluids density, and calculates the closure rate ofborehole with the recommended drilling fluids density. If the closure rate of the borehole is less than 0.1%, the drilling fluids density window can be used during drilling through extremely thick salt formations.
基金financially supported by the National Natural Scienceof China(52074224,U1762216)the Key Research and Development Program of Shandong Province(2019GGX103025)
文摘Rock damage appears in brittle shale even prior to peak stress(i.e.,before failure)due to the occurrence of microcracks in these rocks.In this work,a coupled hydromechanical model was built by incorporating the mechanical and fluid seepage induced stresses around a wellbore during drilling.The borehole instability mechanism of hard-brittle shale was studied.The results show that even if a well is simply drilled into a hard-brittle shale formation,the formation around the borehole can be subjected to rock damage.The maximum failure ratio of the formation around the borehole increases with drilling time.A lower drilling fluid density corresponds to a faster increase in the failure ratio of the borehole with time and a shorter period of borehole collapse.When the initial drilling fluid density is too low,serious rock damage occurs in the formation around the borehole.Even though a high-density drilling fluid is used after drilling,long-term borehole stability is difficult to maintain.While drilling in hard-brittle shale,drilling fluid with a proper density should be used rather than increasing the density of the drilling fluid only after borehole collapse occurs,which is more favorable for maintaining long-term borehole stability.
基金Supported by the China National Science and Technology Major Project(2016ZX05020-004)
文摘There are many problems associated with coiled tubing drilling operations, such as great circulation pressure loss inside pipe, difficulties in weight on bit(WOB) transferring, and high probability of differential sticking. Aiming at these problems, solids-free brine drilling fluid system was developed on the basis of formulation optimization with brine base fluid experiment, which was evaluated and applied to field drilling. Based on the optimization of flow pattern regulator, salt-resisting filtrate reducer, high performance lubricant and bit cleaner, the basic formula of the solids-free brine drilling fluid system was formed: brine +(0.1%-0.2%) Na OH +(0.2%-0.4%) HT-XC +(2.0%-3.0%) YLJ-1 +(0.5%-2.0%) SDNR +(1.0%-2.5%) FT-1 A +(1.0%-5.0%) SD-505 + compound salt density regulator. Lab evaluation showed that the fluid had satisfactory temperature resistance(up to 150 ℃), excellent cuttings tolerance(up to 25%), and strong inhibition(92.7% cuttings recovery); Moreover, its lubrication performance was similar to that of all oil-based drilling fluid. The wellbore could be fairly cleaned at annular up-flow velocity of more than 0.8 m/s if the ratio of yield point to plastic viscosity was kept above 0.5. This fluid system has been applied in the drilling of three coiled tubing sidetracking wells in the Liaohe Oilfield, during which the system was stable and easy to adjust, resulting in excellent cuttings transportation, high ROP, regular hole size, and no down hole accidents. In summary, the solids-free brine drilling fluid system can meet the technical requirements of coiled tubing drilling.
基金Supported by the China National Science and Technology Major Project(2017ZX05005-005)
文摘To solve the problem of borehole trajectory uncertainty, some methods such as error ellipsoid posture characterization, sectional error ellipse solution and error elliptic cylinder construction were proposed and an application example was given. According to the definition of inclination, azimuth and tool-face angle, a characterization method of error ellipsoid posture of borehole trajectory was presented. By intercepting the error ellipsoid with an arbitrary plane in space, the general concept and algorithm of sectional error ellipse were established to analyze the borehole trajectory errors in horizontal plane, plumb plane, normal plane, etc. Based on the theory of surface tangency and curve projection, a construction method of error elliptic cylinder of borehole trajectory was put forward to evaluate the axial enveloping error of borehole trajectory and its variation along well depth. The research shows that the deeper the well, the greater the borehole trajectory error will be. In deep and ultra-deep wells measured using conventional measurement while drilling(MWD),the borehole trajectory position error reaches tens of meters. The research results provide a complete set of analysis methods for borehole trajectory error, which can evaluate the accuracy and reliability of borehole trajectory monitoring.
基金supported by the National Natural Science Foundation of China(Grant Nos.12174421 and 11734017)the Scientific Instrument Developing Project of the Chinese Academy of Sciences,China(Grant Nos.YJKYYQ20200072 and GJJSTD20210008).
文摘Downhole acoustic telemetry(DAT),using a long drill string with periodical structures as the channel,is a prospective technology for improving the transmission rate of logging while drilling(LWD)data.Previous studies only focused on the acoustic property of a free drill string and neglected the coupling between pipes and fluid-filled boreholes.In addition to the drill-string waves,a series of fluid waves are recorded in the DAT channel,which has not been investigated yet.Unpredictable channel characteristics result in lower transmission rates and stability than expected.Therefore,a more realistic channel model is needed considering the fluid-filled borehole.In this paper,we propose a hybrid modeling method to investigate the response characteristics of the DAT channel.By combining the axial wavenumbers and excitation functions of mode waves in radially layered LWD structures,the channel model is approximated to the 1-D propagation,which considers transmission,reflection,and interconversion of the drillstring and fluid waves.The proposed 1-D approximation has been well validated by comparing the 2-D finite-difference modeling.It is revealed that the transmitted and converted fluid waves interfere with the drill-string wave,which characterizes the DAT channel as a particular coherent multi-path channel.When a fluid-filled borehole surrounds the drill string,the channel responses exhibit considerable delay as well as strong frequency selectivity in amplitude and phase.These new findings suggest that the complexity of the channel response has been underestimated in the past,and therefore channel measurements on the ground are unreliable.To address these channel characteristics,we apply a noncoherent demodulation strategy.The transmission rate for synthetic data reaches 15 bps in a 94.5 m long channel,indicating that the acoustic telemetry is promising to break the low-speed limitation of mud-pulse telemetry.
基金Supported by the China National Science and Technology Major Project(2017ZX05005-005)
文摘In order to accurately calculate drilled trajectories,the method of quantitatively recognizing borehole trajectory models was provided,and a case analysis was conducted.Because the measurement-while-drilling data provide with measured values of tool-face angle besides inclination angle and azimuth angle,this paper presents the technological approach of recognizing borehole trajectory models based on tool-face angle.A universal tool-face angle equation was established based on the directional deflection mechanism of steerable drilling tools,and it can calculate the tool-face angles with characteristic parameters of various borehole trajectory models.Then,by evaluating the error between the theoretical values and the measured values of tool-face angle,the trajectory model most consistent with the actual well trajectory can be selected.The model recognition of borehole trajectory provides with the quantitative evaluation index and selection basis of survey calculation methods,which can avoid subjectively and randomly selecting the survey calculation method,and consequently improve the monitoring accuracy and reliability of borehole trajectory.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11134011 and 11374322)the Foresight Research Project,Institute of Acoustics,Chinese Academy of Sciences
文摘Tool waves, also named collar waves, propagating along the drill collars in acoustic logging while drilling （ALWD）, strongly interfere with the needed P- and S-waves of a penetrated formation, which is a key issue in picking up formation P- and S-wave velocities. Previous studies on physical insulation for the collar waves designed on the collar between the source and the receiver sections did not bring to a satisfactory solution. In this paper, we investigate the propagation features of collar waves in different models. It is confirmed that there exists an indirect collar wave in the synthetic full waves due to the coupling between the drill collar and the borehole, even there is a perfect isolator between the source and the receiver. The direct collar waves propagating all along the tool and the indirect ones produced by echoes from the borehole wall are summarized as the generalized collar waves. Further analyses show that the indirect collar waves could be relatively strong in the full wave data. This is why the collar waves cannot be eliminated with satisfactory effect in many cases by designing the physical isolators carved on the tool.
