Full Waveform Acoustic Logs Research Articles

Inversion of borehole guided wave amplitudes for formation shear wave attenuation values

Formation shear wave attenuation (Qβ−1) and borehole fluid attenuation (Qƒ−1) values are estimated from the guided wave arrivals in full waveform acoustic logs. The use of guided waves avoids the problem of head wave geometrical spreading losses. Attenuation estimates are obtained from the inversion of guided wave spectral ratios using analytic partition coefficient expressions. The guided wave amplitude ratios are measured at uniformly spaced frequencies within a selected frequency window containing the pseudo‐Rayleigh and Stoneley waves. Inversion of synthetic data from an open hole geometry results in Q estimates within 2–3% of the actual values. Inversion of cased hole synthetic data gives fluid Q and cement layer Q values within 2–3% of the actual values, but the formation shear wave Q estimate is in error by about 25% due to difficulty in identifying the pseudo‐Rayleigh wave cutoff frequency from the amplitude spectra. Synthetic data results also indicate that the Q estimates are very sensitive to errors in any of the parameters that affect the partition coefficient calculations, such as the borehole fluid velocity or borehole radius. Application of the method to real data recorded in open boreholes provides reasonable results, but noise increases the variance of the Q estimates.

Fracture characterization by means of attenuation and generation of tube waves in fractured crystalline rock at Mirror Lake, New Hampshire

Results are presented from experiments carried out in conjunction with the U. S. Geological Survey at the Hubbard Brook Experimental Forest near Mirror Lake, New Hampshire. The study focuses on our ability to obtain orientation and transmissivity estimates of naturally occurring fractures. The collected data set includes a four‐offset hydrophone vertical seismic profile, full waveform acoustic logs at 5, 15, and 34 kHz, borehole televiewer, temperature, resistivity, and self‐potential logs, and borehole‐to‐borehole pump test data. Borehole televiewer and other geophysical logs indicate that permeable fractures intersect the Mirror Lake boreholes at numerous depths, but less than half of these fractures appear to have significant permeability beyond the annulus of drilling disturbance on the basis of acoustic waveform log analysis. The vertical seismic profiling (VSP) data indicate a single major permeable fracture near a depth of 44 m, corresponding to one of the most permeable fractures identified in the acoustic waveform log analysis. VSP data also indicate a somewhat less permeable fracture at 220 m and possible fractures at depths of 103 and 135 m; all correspond to major permeable fractures in the acoustic waveform data set. Pump test data confirm the presence of a hydraulic connection between the Mirror Lake boreholes through a shallow dipping zone of permeability at 44 m in depth. Effective fracture apertures calculated from modeled transmissivities correspond to those estimated for the largest fractures indicated on acoustic waveform logs but are over an order of magnitude larger than effective apertures calculated from tube waves in the VSP data set. This discrepancy is attributed to the effect of fracture stiffness. A new model is presented to account for the mechanical strength of asperities in resisting fracture closure during the passage of seismic waves during the generation of VSPs.

Application of an acoustic model to determine i n s i t u permeability of a borehole

A simple acoustic model has been developed to study the attenuation of a tube wave due to fluid flow from a borehole into its surrounding porous formation. It is found that attenuation increases with increasing porosity, with increasing frequency, and with decreasing borehole radius. Comparison with field data indicates that this attenuation mechanism can be significant for formations with a permeability exceeding about 100 millidarcy (md). Utilizing an appropriate correction for intrinsic attenuation, it is also demonstrated that tube wave attenuation data obtained through full waveform acoustic logs can be used to estimate in situ formation permeability.

Synthetic full waveform acoustic logs in cased boreholes : Tubman, K M; Cheng, C H; Toksoz, M N GeophysicsV49, N7, July 1984, P1051–1059

Synthetic full waveform acoustic logs in cased boreholes : Tubman, K M; Cheng, C H; Toksoz, M N GeophysicsV49, N7, July 1984, P1051–1059

Synthetic full waveform acoustic logs in cased boreholes

A general expression is derived for the dispersion relations and the impulse response of a radially layered borehole. The model geometry consists of a central fluid cylinder surrounded by an arbitrary number of solid annuli. A Thomson‐Haskell type propagator matrix is used to relate stresses and displacements across the layers. Although the model is completely general, the geometries considered here are restricted to those of a cased hole. Layers of steel, cement, and formation surround the innermost fluid layer. Synthetic microseismograms containing all body and interface waves are calculated for a variety of model parameters. Formation body wave arrivals are relatively unaffected by the presence of a casing. They may, however, be hard to identify cement velocities are close to or larger than those of the formation. The Stoneley and pseudo‐Rayleigh wave arrivals are strongly influenced by the casing parameters. They respond to the combined effects of the steel, the cement, and the formation.

Reply [to “Comment on ‘Determination of in situ attenuation from full waveform acoustic logs’ by C. H. Cheng, M. N. Toksöz, and M. E. Willis”

Reply [to “Comment on ‘Determination of in situ attenuation from full waveform acoustic logs’ by C. H. Cheng, M. N. Toksöz, and M. E. Willis”

Comment on “Determination of in situ attenuation from full waveform acoustic logs” by C. H. Cheng, M. N. Toksöz, and M. E. Willis

Comment on “Determination of in situ attenuation from full waveform acoustic logs” by C. H. Cheng, M. N. Toksöz, and M. E. Willis

Automatic P and S velocity determination from full waveform digital acoustic logs

Automatic methods of determining P and S velocities from full waveform acoustic logs are studied and compared. The suggested P‐wave method is an event detector which is based on threshold detection in a window near the previous picks and fine adjustment by a semblance correlation. The moveouts found by the correlation process are used to find common source P velocities as well as effective borehole compensated (BHC) P velocities. Compensated velocities are derived from waveforms from complementing tool positions which compare favorably to velocities from standard BHC sonic logs. We call the most reliable method for S waves the P‐correlated S‐method. It consists of correlating the P waveform with the rest of the record to find the S arrival. The S waveform is then correlated with the next record to determine the S velocity. This method is compared with published techniques and proves more stable and reliable. The method does not necessitate a distinct S‐wave arrival and can utilize the existence of the reflected conical wave whose phase velocity is controlled by the S velocity of the formation.

Determination of in situ attenuation from full waveform acoustic logs : Chuen Hon Cheng; Nafi Toksoz, M; Willis, M E J Geophys Res, V87, NB7, 10 July 1982, P5477–5484

Determination of in situ attenuation from full waveform acoustic logs : Chuen Hon Cheng; Nafi Toksoz, M; Willis, M E J Geophys Res, V87, NB7, 10 July 1982, P5477–5484

Determination of in situ attenuation from full waveform acoustic logs

Methods for the determination of in situ P and S wave attenuation from full waveform acoustic logs are developed. For P waves, the peak amplitude ratios of the refracted P waves from two different receivers can be used with geometrical spreading taken into account. For S waves, owing to the contamination by the guided waves, its attenuation cannot be determined directly. Instead, S wave attenuation is determined from the attenuation of the guided waves using the partition coefficients (normalized partial derivatives of the phase velocity with respect to the body wave velocities). Analytical forms of these partition coefficients are presented here, along with examples for a number of different rock formations (granite, limestone, sandstone and soft sediments). The results show that in high velocity rocks, the fluid attenuation controls the guided wave attenuation except near the cutoff frequency of the pseudo‐Rayleigh wave. For low velocity rock formations, especially in the case where the S wave velocity is lower than the fluid velocity, the S wave attenuation is the main contributor to the guided wave attenuation. Synthetic microseigmogram calculated with the measured body wave attenuation agrees well with the actual microseismograms.