Phase Velocity Processing For Acoustic Logging-While-Drilling Full Waveform Data
This paper presents the principle and demonstrates the advantages of using a Phase Velocity Processing technique (PVP) applied to the full wave acoustic data recorded by a logging-while-drilling tool. Phase Velocity Processing is particularly well suited for handling the characteristics of LWD data that may degrade the results of Semblance method. Among them are: large tool-mode content, high signal attenuation across the receiver array, and strong road noise. Another advantage of applying PVP is its high vertical resolution: due to the common receiver mode of operation vertical resolution of the calculated slowness is equal to the offset between adjacent receivers. By comparison, the Semblance method delivers results averaged across the entire receiver array span. The PVP algorithm employs series of Fast Fourier Transform procedures to (a) filter the data using Heisenberg’s method, (b) define time domain distribution of the phase arrivals using a Hilbert transform, and finally (c) perform the Phase Velocity analysis. Large number of phase arrival points provides efficient mechanism to calculate standard deviation of the results, quality indicators are added to show how well the algorithm was able to calculate both compressional and shear slownesses. Processing package includes filters operating in the frequency, time, slowness or space domains to minimize competing acoustic modes, especially the tool arrivals that are frequently present in LWD waveform data. Examples demonstrate improvement of the slowness calculation performed with the PVP over the results delivered by the Semblance method. The results obtained in attenuating formations, across the bed boundaries, and in thin beds are presented. The ability of the new filtering technique to identify and suppress the tool mode arrivals is also demonstrated. INTRODUCTION The acoustic logging-while-drilling tools that were introduced six years ago (Minear et. al., 1995, Heysse et. al., 1996, Minear et. al., 1996) are by necessity full waveform recording devices. The drilling noise and the requirement for rigidity in an LWD environment prevented the use of a simple threshold technique as employed in the older wireline acoustic tools. The semblance method was the most commonly utilized processing technique at the time. It was implemented in the form of embedded firmware for downhole processing and as the surface system application for memory waveform data analysis. Semblance processing has a number of drawbacks, such as its requirement for constant amplitude and velocity across the receiver array, the smoothing of the waveforms across the receiver span, the ability to mask defective data collected by an under performing tool, and not in the least its numerical complexity and therefore long execution time. We introduce here the Phase Velocity Processing algorithm implemented to process full waveform data recorded with a loggingwhile-drilling acoustic tool. ACOUSTIC LWD WAVEFORM AND LWD TOOL ARCHITECTURE The very nature of acoustic measurements performed under drilling conditions has necessitated the recording of full waveform data. This has complicated not only the construction of the acoustic LWD tools but also the downhole processing, as realtime sonic slowness calculations are required. A constant presence in LWD waveforms is tool mode, defined as the signal traveling from the transmitters to the receivers, either directly in the tool body or deflected by the borehole wall. The isolator sections in acoustic LWD tools are not perfect due to the fact that the LWD devices, contrary to wireline units, need to be absolutely rigid: the collar strength cannot be compromised by the introduction of the electronics and isolators. Tool mode signal is
16. DATA REPORT: REPROCESSING OF WIRELINE SONIC LOGS IN TURBIDITES AND HEMIPELAGIC SEDIMENTS AT ODP SITE 1173
High-quality sonic log waveforms were recorded during Leg 190 from 65to 360 m below seafloor (mbsf) in Hole 1173A. Schlumberger’s wireline Dipole Shear Imager (DSI) tool was used, and the data were reprocessed using a phase-picking method to improve the shipboard Pand S-wave logs. This postcruise work focused on evaluating the high variability observed in the P-wave log in the upper interval of the hole and an inversion (low VP and VS) below 350 mbsf. In these intervals, sonic signals were weak and difficult to identify using standard shipboard processing methods. The outcome of the reprocessing was successful and extracted improved P-wave logs and S-wave values as low as 280 m/ s in the upper interval of the hole and below 350 mbsf. The reprocessed data are currently available online at www.ldeo.columbia.edu/BRG/ ODP/DATABASE/index.html. INTRODUCTION AND BACKGROUND Leg 190 drilling on the Nankai Trough accretionary prism was the first of a two-leg program concentrating on coring and sampling a transect of sites across the prism within a three-dimensional (3-D) seismic survey. Previous drilling in this area, however, provides relatively little constraint on seismic velocities (also see Moore et al., 2001). During Ocean Drilling Program (ODP) Leg 190, six sites along two transects across the Nankai Trough accretionary prism were successfully drilled. 1Goldberg, D., 2003. Data report: Reprocessing of wireline sonic logs in turbidites and hemipelagic sediments at ODP Site 1173. In Mikada, H., Moore, G.F., Taira, A., Becker, K., Moore, J.C., and Klaus, A. (Eds.), Proc. ODP, Sci. Results, 190/196, 1–15 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 2Lamont-Doherty Earth Observatory, Borehole Research Group, PO Box 1000, 61 Route 9W, Palisades NY 10964, USA. goldberg@ldeo.columbia.edu Initial receipt: 17 November 2002 Acceptance: 15 May 2003 Web publication: 25 July 2003 Ms 190SR-209 D. GOLDBERG SONIC LOG REPROCESSING 2 Only one reference site (Site 1173) at the seaward end of the Muroto Transect was logged using the ODP wireline tool strings (Shipboard Scientific Party, 2000). Leg 196 used logging-while-drilling (LWD) technology to collect additional in situ physical properties data from Site 1173 as well as from Site 808 along the Muroto Transect (Shipboard Scientific Party, 2001c). The wireline and LWD data sets are complementary; when combined, they can provide accurate constraints on the in situ physical properties. Sonic logs provide one of the best means to investigate the acoustic properties of a formation as a continuous function of depth and to tie logging data with seismic and core measurements (e.g., Goldberg, 1997). During Leg 190, the Schlumberger Dipole Shear Sonic Imager (DSI) tool was used to record sonic logs (see Shipboard Scientific Party, 2001a). This tool utilizes a combination of monopole and dipole transducers in an array (Fig. F1) and is deployed typically in combination with other tools such as the Formation MicroScanner and natural gamma ray tools (www.ldeo.columbia.edu/BRG/ODP/LOGGING/ TOOLS/tool.shtml). Dipole sonic tools such as the DSI are designed to excite both compressional and flexural energy in the borehole (Winbow, 1985) and are thus able to directly measure both Pand S-wave speeds in all type of formations so long as good borehole conditions persist. In unconsolidated formations, such as the turbidites and hemipelagic mud drilled at Site 1173, the flexural mode is excited by a lowfrequency asymmetric source in the tool and travels at the S-wave velocity at its low-frequency cutoff (Winbow, 1985). This provides a reliable measure of the S-wave velocity where it would otherwise not be measurable. LOGGING OPERATIONS AND HOLE CONDITIONS Logging operations in Hole 1173A are described in Shipboard Scientific Party (2001b). For this work, it is important to note that two passes of the DSI were made over the interval from 65 to 360 m below the seafloor (mbsf) through lithostratigraphic units composed of sandy to muddy turbidites, some volcanic ash, and silty and siliceous claystones (Shipboard Scientific Party, 2001d). The hole was drilled with a 9.875-in bit that was raised to a depth of 65 mbsf prior to logging. The logging tool was not able to reach the total drilled depth in Hole 1173A due to bridging of the hole at 380 mbsf, and efforts to clear this obstruction deteriorated the shallow hole conditions to some extent. Before logging, 50 bbl of sepiolite mud was pumped into the hole, and then, to further stabilize the hole, ~225 bbl of 10.5-ppg weighted barite mud was pumped in the hole to 637 mbsf (well below the first log reading). As a result, the borehole fluid properties and any change in fluid invasion into the formation are likely to be similar between the two passes. Both of the DSI passes were run at the same logging speed using the wireline heave compensator (Goldberg, 1990). Pass 1 recorded the highfrequency upper dipole plus Pand S-wave modes, and pass 2 recorded the low-frequency lower dipole plus Pand S-wave modes. The lowfrequency (<1 kHz) dipole source was used for the second pass to enhance the flexural wave energy. In Figure F2, the caliper log shows the hole diameter in orthogonal directions from both logging passes. Hole diameter is variable and enlarged from 100 to 220 mbsf along one axis and otherwise relatively constant to a depth of 360 mbsf. The measured shape of the borehole 13 ft
Standoff and caliper measurements while drilling using a new formation-evaluation tool with three ultrasonic transducers
A new formation-evaluation LWD tool that employs three ultrasonic transducers to determine tool standoff and borehole diameter is presented. The ultrasonic data are used to improve density and neutron-porosity measurements obtained from the same tool. The paper describes the ultrasonic transducer hardware and operating characteristics, as well as a unique technique for setting the ultrasonic threshold. Data processing procedures and caliper equations are also presented. Downhole phenomena that adversely affect the measurements are discussed, along with solutions used to handle them. Log examples illustrate the effectiveness of the ultrasonic measurements and include evidence of sticking, lateral bouncing, and whirling of the toolstring while drilling. Although monitoring of these undesirable bit motions is not currently available, future tools will be able to communicate this information to the surface to aid drillers.