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
Instantaneous frequency-slowness analysis applied to borehole acoustic data
The methods most frequently used to process borehole acoustic data are based on semblance analysis. Two most commonly utilized semblance implementations are: slowness-time coherence and slowness-frequency coherence. Both of them are relatively robust under noisy well conditions. They deliver slowness value across the receiver array, and, as the quality control measures, coherence peak value and frequency dispersion curve. Semblance processing might be substituted by instantaneous frequency-slowness method based on complex wave form analysis. Instantaneous frequency -slowness delivers rich set of quality control measures. Among them are the velocities, the goodness and standard deviation across the receiver array, and instantaneous frequency and slowness wave forms computed between adjacent receiver pairs. Furthermore, since computations are performed across adjacent receivers, the vertical resolution is limited to the offset between receivers. Thus the effect of multiple semblance peaks observed while the receiver array is passing through the high acoustic impedance contrast is eliminated. Also, the method is capable to detect underperforming receivers. Finally it can help to control mixed acoustic mode conditions. Instantaneous frequency-slowness method delivers robust results under good to moderately noisy well data. The set of quality measures it delivers is much broader than the one generated by the semblance method.
Archaeological Survey of Dangoyama Kofun by 3DGPR
We presented a three-dimensional ground penetrating radar (3DGPR) system for the archeological investigation in a harsh site, where we have to deal with many trees and/or great topographic variations. This system consists of a commercial GPR system with a laser positioning system. With this system, we can move the GPR antenna freely, on which a positioning detector can record accurate position for each GPR data trace. We applied our 3DGPR system at Dangoyama Kofun, a burial mound, in Fukushima prefecture, Japan. A lot of trees and bushes covered the survey area and cannot be cut. We acquired six datasets in two days in this area. We successfully imaged the boundary between the original surface of the burial mound and the covering soil clearly at about 0.3 m depth around the mound. Furthermore, we found some strong reflections appeared at 0.8 m depth in the top area of the mound, which can be from the artificial parts of the burial mound. The results would greatly aid the following archaeological excavation. In addition, we compared the non-interpolated depth slice with interpolated depth slice at same position and demonstrated that the interpolation process on threedimensional GPR data may introduce some artifacts which might mislead our interpretation.