Research On Imaging Methods And Experiments Of Borehole Radar

Borehole Radar(BHR),an efficient electromagnetic(EM)detection and imaging tool,has been widely used in many geophysical explorations.Compared with the conventional surface-penetrating radar,the BHR is able to work in a much deeper formation,but at the expense of a more complicated woring enviroment.So,there are much higher requirements for the detection and imaging methods.In this dissertation,combined with the BHR surveys,the detection and imaging techniques under the condtions of both non-highly-conductive(NHC)borehole,in which the NHC medium is filled in the borehole,and highly-conductive(HC)borehole,in which the HC medium is filled in the borehole,are studied.The former belongs to the conventional BHR application,in which we focused on the suppression methods of reflected guided waves caused by the surface-downhole conductive communication cable directly connected to the radar,and the improved reverse-time migration(RTM)algorithm(Energy flow domain RTM);While for the later,an unconventional BHR application,the BHR guided waves caused by the high conductivity of the borehoe are utilized to detect and image the geophysical structures near the wellbore.This technique is a valuable supplement and development for the conventional BHR detections.The specific researches of this dissertation are mainly carried out from the following aspects.1.In the normal BHR surveys,the NHC medium,such as air and clean water,are generally filled in the borehole for a purpose of remote detection.In this dissertation,combined with the GeoMole BHR experiments in an NHC borehole,the BHR detection principles and echo signals components are introduced and analyzed.In addition,the armored cables(i.e.,they have a conductive metal sheath)are normally used to cope with the stress induced by the BHR or real-time data-transmission,which together with the borehole consititute the transmission system of guided-waves.When this armored cable is directly connected to the BHR(closer to the transmitting antenna),the radiation energy from the BHR antenna can be coupled on the condutive surface of cable and excited the current,which will further arouse and maintain the BHR guided waves propagating along the borehole.Once these guided waves encounter the discontinuous impedance along the borehole(mainly caused by discontinuous structures near the wellbore,such as cross-hole fractures,stratigraphic interfaces,etc.),the reflected guided waves might be formed and detected by the BHR antenna.In the BHR's section,the reflected guided waves are presented as the oblique striped aptterns,which seriousely contaminate the normal side-reflections of interest,and make imaging difficult.In order to solve the problem of reflected guided-waves interference,we proposed a migration-based filtering strategry.This method can also be used for other geophysical processings,such as suppressing ground-rolls in surface-ground seismic surveying,extracting Stoneley-waves from the full sonic logging data,and separating up-and down-going waves in VSP seismic measurements.2.After the preprocessings of the original BHR data(such as removing direct-waves,and suppressing reflected guided-waves propagating along the borehole),the high-quality imaging processing becomes the essential procedure in the BHR detection and imaging technology under the NHC-borehole condition.Based on the traditional RTM algorithm,this dissertation utilized a newly-developed wave-field separation technique to form a modified RTM,i.e.,energy-flow domain RTM(EF-RTM).EF-RTM first performs the wave-field separation in the source wave field and receiveing wave field,and then the imaging condition should be applied to the both groups of energy-flow components.Finally,re-combining the above results according the principles and four imaging sections with different characteristics can be obtained.Except the high-precision characteristic as the conventional RTM,EF-RTM algorithm can effectively suppress migration noise,improve the focus degree of the target's shape and migrated cross-range resolution.In addition,EF-RTM can also directly decompose the structures with different dips.3.When the HC medium(such as saline-based mud for logging)is filled in the borehole,most of the EM energy radiated from the BHR antenna is confined in the vicinity of the wellbore,which deteriorates the BHR's remote detection.This also seriousely wastes BHR energy and reduces BHR efficiency.In order to make effective use of this part of energy,this dissertation conducted the logging experiments of the radar logging and imaging prototype(RLIP)made by University of Electronic Science and Technology(UESTC),and the analyzed the BHR detection principles and echo signals components in the HC-borehole condition.In addition,based on the short-time Fourier transform(STFT)and singular value decomposition(SVD),this dissertation realizes the fast extraction of the normalized apparent-resistivity curve from the radar echo data collected in the HC borehole,which represents the conductivity variety of near-well stratum.The curve is in good correlation with the conventional resistivity logging.This BHR application extends the functions of the RLIP,and provides the valuable information for the logging evaluation.Moreover,the normalized apparent-resistivity curve can also support the imaging analysis under the HC-borehole condition.In addition,based on the propagation characteristics of guided-waves formed in the HC borehole,the relationship between the apparent velocity of the reflected guided-waves and the EM wave velocity in the near-well stratum is also studied.This will lay a foundation for obtaining near-well stratum permittivity from BHR data.4.Based on the studies of the BHR guided-wave characteristics under the HC-borehole condition,this dissertation further developed the imaging technique for near-wellbore geological structures,using BHR guided-waves propagating along the vicinity of the borehole wall.This technique is called BHR guided-waves detection and imaging.The feasibility of guided-wave detection and imaging is verified by synthetic data and field data.Actually,this method can be used to detect and image the horizonal(relative to the borehole)near-wellbore geological structures,and is an effective supplement and development for the conventional BHR applications(NHC-borehole conditions).This research also extends and popularizes the BHR's application fields,especially for some BHR logging projects under the HC-borehole condition.