Method for enhancing resolution of earth formation elastic-wave velocities by isolating a wave event and matching it for all receiver combinations on an acoustic-array logging tool

#This invention generally relates to a method and apparatus for measuring and processing a characteristic of subsurface earth formations penetrated by a borehole. More specifically this invention relates to a method and apparatus for measuring and processing an acoustic characteristic such as slowness of subsurface sonic waves after these waves traverse earth formations adjoining a borehole or passing through a portion of the subsurface.

#Sonic wave information is used by the oil industry to examine and evaluate the earth's subsurface in the exploration and evaluation of valuable mineral deposits. Sonic waves are generated and recorded in oil well logging. This is called sonic or acoustic logging. The sonic wave measurement taken in well boreholes is typically the formation compressional slowness (the reciprocal of velocity). However, many different acoustic wave types may be measured, for example shear waves or Stonely waves. In the uncased well borehole the compressional head wave or direct wave is the first arrival of the compressional waveforms; the compressional slowness may be derived by measuring the first time of arrival of energy at two acoustic sensors or receivers located a known distance apart. The method does not work well in the presence of noise. In boreholes with a casing or liner, the tube or casing wave also interferes with the detection of the acoustic waves associated with subsurface earth formations.

#Acoustic logging is performed in order to resolve the slowness or velocity structure of subsurface earth formations. The subsurface earth formation information accuracy or resolution that is possible is directly related to both the acquisition as well as the processing of the acquired data.

#FIG. 1 Illustrates various measurement scales provided by overlapping sub-arrays of different apertures.

#FIG. 2 Illustrates a waveform matching configuration diagram according to the present invention.

#FIG. 3 Shows an example of windowing for wave event isolation.

#The present invention enhances the resolution of earth formation compressional and shear-wave slowness (or velocity) profiles measured by an array acoustic tool. The enhanced resolution scale ranges from the conventional resolution of, by way of example 3.5 ft, (the array aperture), to 0.5 ft, (the inter-array receiver spacing). The vertical resolution in time (defined as half the width of a feature) is governed by the depth sampling rate of the logging tool and the slowness measurement scale. With the standard sampling rate of two samples per foot and a minimum measurement scale of 0.5 ft., a one foot vertical resolution can be achieved. While using overlapping sub-arrays of reduced aperture is necessary for the resolution enhancement, the decrease in data redundancy and increase in noise contamination poses a problem. The present invention solves the problem. In this invention, the wave event of interest is windowed or isolated using the wave's traveltime information. Then the wave event is matched at two arbitrary receivers in the sub-array by time-shifting the wave data using a trial slowness value. Finally, the waveform matching is performed for all pairs of receivers allowed by the sub-array aperture, so as to maximize the redundancy of information in the wave data. When the waveform mismatch residue is at the minimum for all overlapping sub-arrays, one obtains the slowness with the resolution of the sub-array aperture.

#Enhancing resolution of slowness estimates from an array acoustic tool is accomplished by overlapping sub-arrays across the same depth interval whose thickness is equal to the sub-array aperture. At a chosen depth of investigation, the acoustic source on the tool is activated and a recording array (typically eight to twelve receivers) records waveform data. This procedure is repeated while the tool is pulled up a distance equal to one inter-array receiver spacing (typically 0.5 ft, but this may vary). Consequently, the receiver arrays at successive source locations are overlapped. The concepts of previous workers (Hsu and Chang, 1987; Tang et al., 1994) were to use redundant information in overlapping arrays to improve both the vertical resolution and the accuracy of the formation acoustic slowness estimation. While overlapping arrays are employed in the present invention also, the present invention employs direct wave matching which is a different and much more accurate inversion than the Hsu and Chang multiple shot semblance or the Tang et al. phase matching. Additionally, the inversion of the present invention maximizes the redundant information available thereby allowing more calculations to increase accuracy, whereas the previous methods, by their nature, do not.

#FIG. 1 shows all seven possible sub-array configurations for an eight-receiver array-acoustic tool. The apertures of the sub-arrays range from 3.5 to 0.5 ft. The present invention for high-resolution elastic wave slowness determination has three important aspects. The first is the use of waveform matching for determining acoustic slowness across the sub-array. For any given receiver index, n, in the sub-array, the waveform at another receiver, m, can be shifted in time to substantially match with the waveform at receiver n, as