Stoneley Wave

A Stoneley wave is a large-amplitude interface wave generated by a sonic logging tool within a fluid-filled borehole. It propagates along the boundary between the borehole fluid and the surrounding formation, traveling at roughly 90 percent of the compressional velocity of the borehole fluid. Because Stoneley waves are sensitive to formation permeability, shear velocity, and the presence of natural fractures, they are among the most diagnostically valuable signals recorded by modern acoustic logging tools.

Physical Characteristics and Propagation

Stoneley waves are sometimes called tube waves because they propagate as a guided mode along the cylindrical borehole, causing pressure oscillations within the fluid column coupled with elastic deformation of the formation wall. Their frequency range in wireline sonic logging typically falls between 1 and 3 kHz, which is lower than the compressional and shear head waves recorded by the same tools. In logging-while-drilling (LWD) environments, the frequency content shifts further downward due to the larger drill collar geometry and the different acoustic coupling conditions. The particle motion combines fluid pressure variation with radial displacement of the borehole wall, making Stoneley waves a composite measure of both fluid and formation mechanical properties.

Relationship to Formation Permeability

When a Stoneley wave pressure pulse travels past a permeable zone, fluid is driven in and out of the pore throats lining the borehole wall. This fluid exchange dissipates acoustic energy, producing a measurable reduction in Stoneley wave amplitude and an increase in slowness relative to a tight, impermeable formation. The magnitude of attenuation scales with formation permeability according to the poroelastic framework described by Biot theory. Zones with high effective permeability appear as pronounced attenuation anomalies on Stoneley amplitude logs, allowing engineers to identify productive intervals without cutting core samples. This relationship is the physical basis for Stoneley permeability estimation, a distinct application discussed in dedicated workflows.

Fracture Detection and Reflection Analysis

Natural fractures that intersect the borehole act as acoustic discontinuities for Stoneley waves. A portion of the incident Stoneley wave is reflected at each fracture, and the reflected arrival travels back up the borehole where it can be recorded by the tool's receivers. Stoneley wave reflection coefficients correlate with fracture aperture and hydraulic conductivity, providing a means to identify and characterize natural fractures independently of resistivity or image log interpretation. Processing of Stoneley reflection data involves separating upgoing and downgoing wavefields, then mapping reflection events to depth. This fracture identification capability is particularly valuable in carbonate reservoirs and naturally fractured tight gas formations where open fractures dominate flow behavior.

Shear Velocity in Slow Formations

In formations where the shear wave velocity is slower than the compressional velocity of the borehole fluid, refracted shear head waves cannot be generated and the standard monopole sonic measurement cannot directly record shear slowness. Stoneley waves offer an alternative path to shear velocity in these so-called slow formations. Dispersion analysis of Stoneley wave slowness as a function of frequency, a technique sometimes called Stoneley dispersion inversion or QSTD processing, extracts the formation shear slowness from the dispersive behavior of the tube wave at low frequencies. This extends the applicability of acoustic formation evaluation into unconsolidated sands, shallow offshore sediments, and other slow-formation environments where dipole shear measurements may also be difficult to record cleanly.

Key Logging Tools and Acquisition

The Schlumberger DSI (Dipole Shear Imager) and its successor tools record both dipole flexural waves and monopole Stoneley waves in a single pass. Baker Atlas XMAC tools provide similar capability. These instruments use monopole transmitters to generate Stoneley energy and arrays of receivers to record the waveforms at multiple offsets, enabling slowness and amplitude to be determined as a function of depth using semblance processing or waveform inversion. LWD acoustic tools from multiple vendors also record Stoneley arrivals, though the annular geometry and drill string noise require additional processing to isolate the tube wave signal. Careful borehole condition assessment, including caliper data and mud properties, is necessary to interpret Stoneley logs accurately because borehole rugosity and mudcake can mask or distort the signals.

Key Takeaways

• Stoneley waves are guided interface waves traveling at roughly 90 percent of borehole fluid velocity, sensitive to both formation and fluid properties at the borehole wall.
• Amplitude attenuation and slowness increase in permeable zones because pore fluid exchange dissipates the wave's energy, enabling continuous permeability profiling along the wellbore.
• Natural fractures reflect Stoneley waves, and analysis of upgoing and downgoing reflections identifies fracture locations and provides a qualitative estimate of aperture.
• Dispersion analysis of Stoneley slowness gives shear velocity in slow formations where conventional refracted shear arrivals cannot be measured, extending formation evaluation capability to challenging lithologies.