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In borehole seismic experiments the presence of the borehole has a significant effect on observations. Unfortunately, including boreholes explicitly in modeling schemes excludes the use of some methods (e.g., frequency-wavenumber) and adds prohibitively to the cost of others (e.g., finite difference). To overcome this problem, the authors use the concept of an effective source/receiver array to replace the explicit representation of the borehole by a distributed seismic source/receiver. This method mimics the presence of the borehole at seismic frequencies under a wide variety of conditions without adding a significant computational cost. It includes the effects of dispersive and attenuative tube wave propagation, the generation of secondary sources at interfaces and caliper changes, and the generation of conical waves in low-velocity layers. Comparison with a finite-difference scheme with an explicit borehole representation validates the approach. The modeling method applied to a continuity logging geometry demonstrates that the presence of guided waves does not uniquely imply bed connectivity. Results for a single-well imaging geometry emphasize the dominance of the tube wave in the hydrophone synthetics and demonstrates the necessity of using clamped geophones for single-well experiments. The concept of an effective source/receiver array is an efficient way of including borehole phenomena inmore » seismic modeling methods at minimal extra computational cost.« less

Recent studies have reported age-related deficits in visual acuity and changes in retinal morphology in pigeons. The experiment reported here was designed to determine the effects of age on intensity difference thresholds in pigeons. Six subjects, age 2–17 yr, were trained to discriminate between two stimuli that differed in luminance. When this training was complete, the subjects were presented with a series of stimulus comparisons ranging from 0.08 to 0.43 log unit. Threshold was calculated by determining the luminance difference that corresponded to 75% correct. These data were pooled with 56 intensity-difference thresholds that had been collected from pigeons of various ages over a 20 yr period using the same procedure. A regression analysis that was performed on the pooled data set gave the result b = 0.0038, d.f. = 61 P < 0.05, r2 = 0.066 which indicates that age accounted for approx. 7% of the variance in intensity-difference threshold. This finding, although statistically significant, indicates that as pigeons age their ability to perform this type of non-spatial discrimination task is not greatly impaired. This finding suggests that deficits associated with a spatial visual task in pigeons, such as visual acuity, are task specific and are not due to a global performance deficit.

We have developed a numerical technique for modeling subkilohertz seismic data acquired with a borehole hydrophone, a borehole volume source, or both, in the case of cross well data. Data acquired with such devices contain tube wave related phenomena that exist because of the borehole itself. This new technique avoids modeling the source and receiver boreholes directly, and yet models the tube wave related effects. This is done The computed line source mechanism is now used in a pre existing modeling code, overiding the normal choices of : buried point force or a buried monopole. The modeling cod1 must compute as output the wavefield at all points along the trajectory of a hypothetical receiver well.

Acoustic wave propagation in a fluid‐filled borehole is affected by the type of rock which surrounds the hole. More specifically, the slowness dispersion of the various body‐wave and borehole modes depends to some extent on the properties of the rock. We have developed a technique for estimating the dispersion relations from data acquired by full‐waveform digital sonic array well‐logging tools. The technique is an extension of earlier work and is based on a variation of the well‐known Prony method of exponential modeling to estimate the spatial wavenumbers at each temporal frequency. This variation, known as the forward‐backward method of linear prediction, models the spatial propagation by purely real‐valued wavenumbers. The Prony exponential model is derived from the physics of borehole acoustics under the assumption that the formation does not vary in the axial or azimuthal dimensions across the aperture of the receiver array, but can vary arbitrarily in the radial dimension. The exponential model fits the arrivals of body waves (i.e., head waves) well, because the body waves are dominated by a pole rather than a branch point. Examples of this processing applied to synthetic waveforms, laboratory scale‐model data, and field data illustrate the power of the technique and verify its ability to recover dispersion relations from sonic array data. The interpretation of the estimated dispersion in terms of rock properties is not discussed.

A subsurface point body force can be realized using a body force on the axis of a small‐radius borehole drilled perpendicular to the force. This observation, originally made by Kitsunezaki (1980), is having a significant impact in shear‐wave logging, and may potentially affect downhole seismic sources. Applying the principle of reciprocity to this finding, a horizontal vibration sensor on the axis of a vertical hole will be unaffected by the existence of the hole at low frequencies. Numerical methods determine both the frequency below which, and the offset beyond which, borehole‐related effects are negligible. If the shear wavelength is greater than ten times the diameter of the hole and if the measurement is made at least one shear wavelength from the point force, then the borehole effects will be minimal.

Acoustic well logging methods historically have been based on the excitation and reception of axisymmetric wave phenomena in a fluid‐filled wellbore. We consider the reception of nonsymmetric wave phenomena excited by acoustic multipole sources. The first three orders of multipoles are the monopole, dipole, and quadrupole, and we examine these particular sources in detail. Existing sonic tools make use of a monopole source, while more recently, both dipole and quadrupole sources have been explored. An exact frequency‐wavenumber domain representation of the acoustic field in the borehole due to a multipole source is formulated and numerical methods are used to compute synthetic space‐time domain waveforms. We consider wideband monopole, dipole, and quadrupole excitations with center frequencies of 1, 4, and 12 kHz, and treat both slow and fast formation models. Finally, we derive low‐frequency, far‐field asymptotic expressions for the monopole, dipole, and quadrupole waveforms. At frequencies such that the shear wavelength is on the order of the borehole diameter or less, the difference between the monopole, dipole, and quadrupole waveforms is primarily in the nature of the surface wave mode which they excite: the monopole excites a Stoneley, or tube mode; the dipole excites a flexural mode; and the quadrupole excites a screw mode. By comparison, the compressional and shear head waves and the trapped waveguide modes do not change as much as the order of the multipole is changed. At low frequencies, where the shear wavelength is much longer than the diameter of the hole, the monopole excites a dominating tube mode, while the dipole and quadrupole excites dominating shear waves. Low‐frequency asymptotic expressions for the waveforms agree well with the numerically computed waveforms.

Poisson’s ratio) the less attenuation there will be due to shear mode conversion. In fact, the mode conversion will not occur and the trapped mode will not attenuate if the formation is soft like a fluid (shear speed is zero and Poisson’s ratio is 0.5). On the other hand, the trapped compressional mode will be highly attenuative if the formation rock is more rigid (shear is faster and the Poisson’s ratio is lower). For example, in Figure 3 we show waveforms for a case with formation Poisson’s ratio of 0.2 (shear velocity: 1.51 km/s). The trapped compressional mode is highly attenuative (it leaks into formation shear). In addition, a Stoneley wave is present in the late time because the hole is more rigid. Since the waveforms in Figures I and 3 are quite different, we conclude that the shale Poisson’s ratio must be higher than .2 in the shale formation of Figure 1. A Poisson’s ratio of 0.48 is not likely to occur naturally in shale formations. One possible explanation why the shale appears so soft is that shale alteration has occurred after the drilling. Even though shale is usually not very permeable to mud filtrate, water taken up by the shale can gradually progress away from the well wall. As a result a cylindrical layer of very soft material will form. To show that a shale alteration can also cause the dispersive mode seen in Figures 1 and 2, we show, in Figure 4, numerically generated waveforms of a borehole with a shale altered zone. The Poisson’s ratio in the altered zone is 0.48 and in the unaltered zone is 0.2. The depth of alteration from the well wall is 6 inches. In Figure 4, the low frequency dispersive mode also travels with little attenuation. This indicates that shale alteration can also give waveforms similar to those in Figure 1. In conclusion, sonic waveforms may carry in-situ information about shale and shale alteration. The information can be contained in dispersive modes that have modal attenuation highly sensitive to the rigidity of the formation rocks. In the presentation, we show the dispersion and attenuation of these modes as a function of frequency for various hole rigidities.

This paper describes new developments of a wireline tool for stress measurements. The tool was initially designed for open-hole tests and its application was limited to very low permeability formations (up to 50 μd). Since then the capabilities of the tool have been extended to handle both impermeable formations and permeable formations (currently up to 400 md). New testing and interpretation procedures make it possible to obtain reliable measurements of the minimum horizontal stress and also the maximum horizontal stress in an open-hole environment or to conduct tests in a cased hole environment. These new developments make detailed stress measurements possible in a wide range of formations, providing input to numerous applications: design of hydraulic stimulations, stability of deviated wells, screenless completions for sand control and enhanced recovery programs.