Full Waveform (Acoustic Logging)

Key Takeaways

• Shear wave slowness from full waveform processing is irreplaceable for geomechanical modeling — shear wave slowness (Dts, in microseconds per foot or meter) combined with compressional slowness (Dtc) and bulk density allows calculation of the elastic moduli (Young's modulus, shear modulus, bulk modulus) and Poisson's ratio that characterize how rock deforms under stress; these parameters are the inputs for wellbore stability analysis (predicting safe mud weight windows to prevent borehole breakout and tensile fractures), hydraulic fracture design (predicting fracture height containment and fracture compliance), and reservoir geomechanics (modeling how reservoir pressure changes affect stress state and compaction during production); there is no equivalent logging measurement that provides shear velocity directly — the full waveform sonic is the only non-invasive borehole measurement that contains this information, making it indispensable for any well requiring mechanical earth model construction.
• Stoneley wave analysis provides formation permeability estimation in permeable formations — the Stoneley wave (or tube wave) travels along the borehole wall at a speed that depends on formation stiffness and fluid properties, and in permeable formations it partially converts to formation pore fluid flow as it passes, causing measurable attenuation and slowness changes relative to an impermeable formation; the magnitude of Stoneley wave attenuation (loss of amplitude) and slowness increase (compared to a modeled impermeable baseline) provides a qualitative-to-semiquantitative indicator of formation permeability and fracture openness; in naturally fractured carbonates and tight gas sandstones, Stoneley wave analysis can identify permeable fracture zones and open fractures that resist detection by resistivity and image logs, providing flow unit information for reservoir characterization that is particularly valuable when core is unavailable for permeability measurement.
• Slowness-time coherence (STC) processing is the standard method for extracting arrival times from full waveform arrays — the full waveform tool records the pressure wave at each of multiple receivers (typically 8-13 receivers in a 3-6 foot array) at digitizing rates of 100-250 kHz; STC processing searches through all possible slowness values and window start times, computing the coherence (correlation) of the waveform across all receivers at each slowness-time combination; compressional, shear, and Stoneley arrivals appear as distinct coherence peaks in the slowness-time plane, and their slowness values are read at the peak coherence for each wave type; the STC output is displayed as a slowness-time plane image, allowing log analysts to visually verify that the picked slowness values correspond to actual coherent wave arrivals rather than noise peaks or cycle-skipping artifacts that would produce incorrect slowness values.
• Cross-dipole acoustic tools measure shear wave anisotropy that reveals stress orientation and natural fractures — in a vertical borehole, standard monopole acoustic tools generate compressional and Stoneley waves but cannot effectively excite shear waves in slow formations (where shear velocity is lower than the borehole fluid compressional velocity); dipole acoustic sources generate flexural modes that allow shear wave measurement in slow formations and, when fired in two orthogonal directions (cross-dipole configuration), measure shear wave velocities in two perpendicular planes; if the formation is anisotropic (as in a stress-anisotropic environment or a formation with aligned natural fractures), the fast and slow shear wave velocities differ, and the azimuth of fast shear wave propagation corresponds to the maximum horizontal stress direction or the fracture strike; this information is directly applicable to well planning (orienting horizontal wells parallel to minimum stress to maximize fracture width), completion design (identifying natural fracture orientation for engineered stimulation), and regional stress mapping for field development planning.
• Full waveform data quality is highly sensitive to borehole condition, tool centralization, and mud properties — acoustic waves travel through the borehole fluid to reach the formation, and any irregularity in the borehole (rugosity, washouts, tool decentralization) creates scattered noise that masks the formation arrivals and degrades waveform quality; in highly rugose boreholes (often encountered in unconsolidated sands, swelling shale, or reactive formations), the acoustic signal may be dominated by borehole guided modes and pipe noise rather than formation arrivals, making it impossible to extract reliable formation slownesses even from full waveform data; mud type and density also affect acoustic propagation — heavy barite-weighted mud attenuates compressional waves more than lighter muds, and oil-based mud changes the acoustic coupling between the tool and the formation; quality control of full waveform data requires review of the raw waveform display to confirm that the identified arrivals are coherent formation waves rather than borehole artifacts, and remedial acquisition (pulling the tool slowly or making repeat passes) may be required in problem zones.