Borehole Televiewer: Definition, Acoustic Imaging, and Fracture Log

A borehole televiewer (BHTV) is an ultrasonic wireline logging instrument that produces a continuous, oriented, 360-degree image of the borehole wall by rotating a piezoelectric acoustic transducer at 2 to 6 revolutions per second while the tool is pulled uphole at a controlled rate. As the transducer spins, it emits short, high-frequency pulses (typically 200 to 500 kHz) that travel outward through the borehole fluid, reflect off the formation face, and return to the same transducer. The tool simultaneously records two physical measurements for every azimuthal sample: the two-way travel time (TWT) from emission to reception, which maps variations in borehole radius and therefore yields a continuous acoustic caliper image; and the amplitude of the returning echo, which encodes the acoustic reflectivity of the surface encountered. When these two data streams are unrolled, they generate paired amplitude and radius images that allow geoscientists and drilling engineers to identify open fractures, stress-induced breakouts, bedding planes, vugs, and, in cased wellbores, internal corrosion or casing damage. The instrument is alternatively marketed as an ultrasonic borehole televiewer (ULTC), ultrasonic imager, or under trade names such as Schlumberger CAST-V, Halliburton CBIL (Circumferential Borehole Imaging Log), and Baker Hughes STAR imager.

Key Takeaways

• The borehole televiewer emits ultrasonic pulses from a rotating transducer and records two-way travel time and reflected amplitude, producing paired radius and reflectivity images of the entire borehole wall circumference at centimetre-scale resolution.
• Unlike resistivity image logs such as the FMI or EMI, the BHTV operates in any conductive or non-conductive wellbore fluid including oil-based muds (OBM) and synthetic-based muds (SBM), where resistivity pads cannot establish adequate electrical contact.
• Planar features such as fractures, bedding contacts, and faults appear as sinusoids on the unrolled image; the azimuth of the sinusoid minimum gives the dip direction and the amplitude of the sinusoid gives the true dip angle, allowing direct structural and fracture analysis without deviation corrections.
• In-situ horizontal stress orientation is determined from borehole breakouts (compressive spalling elongating the borehole in the minimum horizontal stress direction) and drilling-induced tensile fractures (narrow, vertical, diametrically opposed cracks aligned with maximum horizontal stress), both of which are unambiguously visible on BHTV amplitude images.
• In cased wells, the high-frequency pulse penetrates the fluid column but reflects from the metal casing inner wall rather than formation, making the tool valuable for detecting internal corrosion pitting, scaling, perforations, and mechanical deformation without requiring a full workover.

How the Borehole Televiewer Works

The core measurement element is a single piezoelectric transducer housed in a pressure-rated tool body approximately 43 to 89 mm (1.7 to 3.5 in) in diameter. A precision motor rotates the transducer continuously while a magnetic compass or three-axis accelerometer and magnetometer package records the tool's azimuthal orientation relative to magnetic north at each firing instant, so every pixel in the resulting image can be assigned both a borehole azimuth and a measured depth. The transducer fires pulses at rates of 100 to 300 firings per revolution, and the combination of firing rate and rotation speed determines the lateral sampling density around the borehole circumference. At a typical logging speed of 3 to 5 m/min (10 to 16 ft/min) and 4 revolutions per second, the tool achieves approximately 2 to 5 mm (0.08 to 0.20 in) azimuthal resolution and 1 to 3 mm (0.04 to 0.12 in) vertical sample spacing, sufficient to detect fracture apertures as small as 1 mm (0.04 in) in the amplitude image, though true aperture is smeared by the acoustic beam width.

The two-way travel time measurement is converted to radius using the acoustic velocity of the borehole fluid. In water-based mud (WBM) the velocity is approximately 1,500 m/s (4,920 ft/s), while in OBM it typically ranges from 1,350 to 1,450 m/s (4,430 to 4,760 ft/s) and must be calibrated against a known diameter or a dedicated caliper run to avoid systematic radius errors. The resulting radius image functions as a high-resolution acoustic caliper log, revealing ovalization caused by stress-induced breakouts, washouts, and any borehole spiraling induced by the drillstring or mud motor. The amplitude image records how much acoustic energy returns from the wall. Hard, dense formations (tight carbonates, crystalline basement) produce high-amplitude returns. Soft or rugose formations, open fractures filled with mud filtrate, and vuggy porosity all produce low-amplitude patches because energy scatters or is absorbed before returning cleanly to the transducer. This makes the amplitude image particularly powerful for distinguishing open fractures (dark patches, low amplitude, also visible as widened-borehole anomalies in the radius image) from closed or mineralized fractures (subtle amplitude contrast with minimal radius change).

Centralizers are mandatory for valid BHTV data. Because the transducer fires from a fixed point inside the tool body, any lateral displacement of the tool from the borehole axis introduces a systematic error in the TWT-to-radius conversion: the near side of the borehole appears closer and the far side appears more distant, creating a false sinusoidal radius artifact that can mask or mimic structural features. Most logging contractors require at least two bow-spring centralizers or rigid centralizers placed within 3 m (10 ft) of the tool head, and in deviated wells additional centralizers are placed at intervals up the drillstring. The tool will not function in gas-filled boreholes because acoustic pulses cannot propagate across the gas-fluid interface with sufficient energy to reflect from the formation and return; this is a fundamental physical limitation distinguishing BHTV from photographic or optical borehole imagers (OBI), which also require a clear fluid column but can function in air-filled holes under certain configurations.

Measured Parameters and Data Products

The primary deliverables from a borehole televiewer run are two oriented image logs displayed as unwrapped cylinders with north at both edges of the image track and south in the centre, plus structural picks and interpreted feature tables. The amplitude image is the primary geological interpretive product, providing a visual map of reflectivity contrasts analogous to a photographic image of the borehole wall. The radius (TWT) image acts as a quality-control check and a high-resolution caliper, flagging eccentric tool position, key-seating, and borehole enlargement zones. Interpreters pick sinusoidal features by fitting sine curves to linear features visible in both images and compute the following from the geometry of each sinusoid: true dip angle (0 to 90 degrees), dip azimuth (0 to 360 degrees), feature type classification (natural open fracture, drilling-induced fracture, sedimentary bed contact, stylolite, vug), and, for stress analysis, the orientations of breakout long axes and tensile crack pairs.

Fracture intensity indices (P10: fracture count per metre of core or log, P32: fracture area per unit volume) can be estimated from BHTV picks after applying a geometric correction for the probability of a randomly oriented fracture intersecting the borehole. These parameters feed directly into discrete fracture network (DFN) models used for reservoir characterization, hydraulic fracture simulation, and well planning. The minimum and maximum horizontal stress orientations, Shmin and SHmax, are extracted from the borehole failure analysis and are critical inputs for mud weight optimisation in nearby wells and for design of oriented perforations in completion programmes.

Borehole Televiewer Versus Resistivity Image Logs

The two principal borehole imaging technologies are acoustic (BHTV) and resistivity-pad-based (FMI, XRMI, EMI, OBMI). Each has distinct strengths that determine which tool is run in a given programme. Resistivity imagers derive their images from the contrast in electrical conductivity between formation rock, pore fluids, and conductive mud filtrate that has invaded the near-wellbore zone. They require intimate electrical contact between the tool pads and the borehole wall and a conductive mud system (WBM or brine). They deliver extremely high visual resolution (approximately 0.2 to 0.5 mm) in smooth boreholes and provide direct petrophysical information about fluid-filled versus cemented features. However, they cannot operate in OBM or SBM environments because the non-conductive hydrocarbon-based fluid breaks the electrical circuit. The BHTV has no such restriction: it operates identically in WBM, OBM, SBM, completion brines, and diesel-based spacers, making it the default imaging choice in the deepwater Gulf of Mexico, the North Sea, and any other basin where OBM is used for wellbore stability or lubricity.

In highly rugose or washed-out intervals where resistivity pad contact is intermittent, the BHTV also performs better because the acoustic pulse travels across the standoff from tool to wall rather than requiring physical contact. The tradeoff is that BHTV amplitude resolution is lower than FMI resolution in good boreholes, and the BHTV cannot discriminate fluid types by resistivity contrast. Best practice in well characterisation programmes often combines both tools: FMI in the WBM upper hole sections for maximum geological resolution, BHTV in the OBM reservoir sections for structural and stress analysis where resistivity imaging is unavailable.