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
Identification of Mixed Acoustic Modes in the Dipole Full Waveform Data using Instantaneous Frequency-Slowness Method
Dipole full waveform acoustic tools are used to estimate shear wave velocities, especially in soft and poorly consolidated formations. Under ideal conditions dipole source employed by those tools excites only borehole flexural wave that is propagating along fluidsolid interface This frequency dispersed flexural wave is used to estimate the velocity of the formation shear wave. In very soft formations, the dipole source may also excite a phase reversed compressional mode, sometimes referred to as a slow compressional wave (primarily due to its dispersed character).
FULL-WAVE ACOUSTIC LOGGING.
Conventional acoustic logs are based on a small fraction of the potential information available in the acoustic wave train propagating down the borehole. The objective of full-wave acoustic logging is to record, process, and interpret he entire acoustic wave train. New acoustic logging tools, sophisticated signal processing techniques, and improved computer processing capabilities at the well site are providing the ability to extract more of the information. The acoustic wave train associated with the borehole is composed of compressional, shear, normal mode and Stoneley waves. Characteristics of each of these phases are discussed briefly and illustrated with waveforms recorded in different lithologies. These characteristics have guided the design of full-wave logging systems: tools, data acquisition, and processing. New generation full-wave acoustic logging tools have more receivers, greater transmitter-to-receiver spacings, and lower frequency response than their short-spaced predecessors. Waveforms are generally recorded and processed igitally. Nonsymmetrical sources and receivers are used to generate shear waves preferentially. The reasons for these differences from conventional acoustic tools are discussed. Seismic-type data processing is used to extract velocity and amplitude information from full-wave acoustic signals. Phase velocities of compressional, shear and Stoneley waves are determined almost entirely by some type of correlation technique. These techniques include crosscorrelation, semblance and linear and nonlinear delay and sum. Other processing being employed includes depth stacking and time-domain filtering. Several examples of processing full-wave data with different echniques are discussed. Full-wave acoustic logging has some problems as well as benefits. The use of relatively sophisticated digital processing and the fact that full-wave tools generate about ten times more data than do other logging tools places considerable demand on computing facilities, particularly at the well site. The greater transmitter-to-receiver spacings result in greater signal attenuation and require greater dynamic range than do short spaced tools. Array-type processing requires new borehole compensation techniques, Some of the most important applications of full-wave acoustic information to formulation evaluation are presented. These include lithology, gas zone identification, and estimates of porosity, elastic modulii, and formation strength. The newest uses include estimation of structural dip, fracture height, permeability, and generation of shear wave synthetic seismograms. Geologic Implications of LargeScale Trends in Well-Log Response, Northern Green River Basin, Wyoming S. E. Prensky, U.S. Geological Survey SPWLA-7