Borehole Optical Televiewer (OTV)

Basic Concept

Optical televiewers (OTV) provide high-resolution, magnetically oriented, color images of the borehole wall. These images are used to map rock types and orientations of planar features such as fractures, joints, lithologic contacts, and bedding that intersect the borehole. Most OTV tools use a digital camera directed upward or downward on a conical mirror to collect 360-degree images of the borehole wall. The tools provide continuous images through air- or water-filled boreholes.

The digital, azimuthally oriented image of the borehole wall is split (typically on north), unrolled, flattened, and inspected for structural features. The OTV log represents a map of the borehole wall. The image can be processed and interpreted to identify borehole irregularities and sinusoidally shaped features representing planar features that intersect the borehole (Williams and Johnson, 2004). The mid-point depth and orientation (strike and dip) of the planar features can be determined in processing.

The detection and characterization of fractures are important for geologic and fracture network characterization, contaminant investigations, and water-resource evaluations. The location of fractures within a borehole can be used to plan flowmeter, discrete hydraulic tests, and sampling strategies by directing the placement of tools, packers, and or pumps. In addition, the results can be used to help interpret other borehole logs including acoustic or resistivity image logs and/or surface geophysical surveys.

Theory

Early borehole optical tools were limited, but they have evolved to combine video camera logging and acoustic televiewer (ATV) technology to generate high resolution images of the side walls of the borehole (Prensky, 1999). Optical imaging technology was expedited with the advent of charged coupled device (CCD) cameras and complementary metal-oxide semi-conductor (CMOS) sensors. These cameras digitize the intensity of the color spectrum in red, green, and blue. The camera lens is directed toward a conical mirror (Galliot and others, 2007), and measurements are collected with 180, 270, 360, 720, to more than 1800 increments azimuthally. Depth measurements are collected at high vertical resolution (0.01-0.02 ft increments) producing high-resolution images.

Typically, deviation logs are collected concurrently with OTV data. A three-axis fluxgate magnetometer and three-axis accelerometer are used to determine the azimuthal direction and inclination. Image logs are collected with the tool centered in the borehole. The orientation of the borehole deviation and magnetic declination can be applied to the apparent orientations of interpreted features to obtain corrected orientations of the planar features. (Williams and Johnson, 2004)

OTV images display real-color pixels with depth along the y (left) axis and azimuthal direction the x-axis (figure 1). Planar features that intersect the borehole appear as sinusoids on the magnetically oriented images, and their orientation can be computed. The strike is normal to the azimuth of the lowest point (dip azimuth) on the sinusoid and is expressed with respect to magnetic or true north. In addition, the apparent dip can be computed as the sine of the amplitude normalized by the diameter. Thus, the depth and orientation of planar features such as fractures, bedding, and lithologic contacts can be mapped.

Figure 1: Acoustic and optical televiewer data, and interpretation of features shown in three types of structure plots, including a projection plot that shows the trace of the feature on the image, a tadpole plot that shows strike, dip, and depth of features, and a stereographic projection that shows the strike and dip of features. From Johnson and others, 2016

With digital data the processing and interpretation of images can be done manually with post-processing software or using automated structural algorithms (e.g. Wang and others, 2016). Advanced 2- and 3-D imaging software packages permit the corrections for magnetic declination, borehole deviation, borehole diameter, and decentralization. In addition, structural interpretations can be represented in sine wave overlays on the images, tadpole plots, rose diagrams, and stereographic projections.

Geophysical logs of core analysis, gamma, OTV image, travel time, amplitude, and caliper

Figure 2. Selected borehole geophysical logs and rock types in borehole MW-14. ATV data shown in logs 1) “Amplitude-mean”, 2) “Travel Time”, 3) “Amplitude”, 4) “Amplitude-median”, 5) “Approx. Amplitude”, and 6) “Acoustic caliper max”. (Johnson and others, 2011)

Applications

Optical imaging can be used for qualitative and quantitative interpretation of lithology, structure, bedding, breakout and stress analysis, thin-bed detection, referencing or correcting core, casing inspection, and determination of vuggy porosity. These investigations have been used for water-resources, contaminant, and or geologic mapping investigations. The most widespread use of OTV tools has been in open-hole fractured rock applications (Paillet and others, 1990; Williams and Johnson, 2004). Rock types and weathering can be mapped directly from the images figure 2 (Johnson and others, 2011; Holbrook and others, 2019). Analysis of fracture orientations, break-out zones and pop-out structures can be used to determine the in-situ stress fields (Kingdon and others, 2016). The methods used in stress-field analysis of acoustic image processing (Zoback and others, 1985) also can be applied to OTV logs.

Some specialized applications include using OTV logs to estimate vuggy porosity and the presence of gas or fluid filled voids. Cunningham and others (2009) introduced a method for quantifying vuggy porosity in karstic limestone. In addition, englacial ice and debris structures can be mapped in coreholes in glaciers (Roberson and Hubbard, 2010; Hubbard and others, 2013).

Borehole images can be directly compared to core and can be used to orient core samples. OTV data can often image the fracture zones where there is missing rock known as “core loss” in the core samples (Rahiman, 2018). Advanced software programs provide the user to generate virtual core where optical televiewer logs can be wrapped on a cylinder, the caliper log, or an azimuthal acoustic caliper log.

One critical limitation to OTV logging in a fluid-filled borehole is that it requires sufficiently clear fluid and lighting to illuminate the borehole wall. Without variable illumination and/or shutter speed, the lighting conditions may not permit the detection of features of interest (Williams and Johnson, 2004). Another limitation is if the tool is not centered, the images can have shadows and will be distorted. In addition, borehole images can get “disoriented” in the presence of magnetic minerals and steel casing; however, this artifact can sometimes be removed in post processing.

Typical applications for optical borehole imaging.

Mapping geologic structures including fractures, bedding planes, lithologic contacts, foliation, weathering surfaces and regolith etc
Supplement core, identify core loss, orient core
Characterize thin bedding, foliation, joints, and fractures.
Map fracture depth and orientation
Map rock units and contacts
Geotechnical applications – RQD, estimating rock properties
Casing inspection
Mapping layers within glacial ice
Break out zone analysis for determining stress fields

Examples/Case studies

Williams, J.H., and Johnson, C.D., 2004, Acoustic and optical borehole-wall imaging for fractured-rock aquifer studies: Journal of Applied Geophysics, v. 55, no. 1-2, p. 151-159. https://doi.org/10.1016/j.jappgeo.2003.06.009

Imaging with acoustic and optical televiewers results in continuous and oriented 360j views of the borehole wall from which the character, relation, and orientation of lithologic and structural planar features can be defined for studies of fractured-rock aquifers. Fractures are more clearly defined under a wider range of conditions on acoustic images than on optical images including dark-colored rocks, cloudy borehole water, and coated borehole walls. However, optical images allow for the direct viewing of the character of and relation between lithology, fractures, foliation, and bedding. The most powerful approach is the combined application of acoustic and optical imaging with integrated interpretation. Imaging of the borehole wall provides information useful for the collection and interpretation of flowmeter and other geophysical logs, core samples, and hydraulic and water-quality data from packer testing and monitoring.

Kingdon, A., Fellgett, M.W.,Williams, J.D.O. ,2016, Use of borehole imaging to improve understanding of the in-situ stress orientation of Central and Northern England and its implications for unconventional hydrocarbon resources, Marine and Petroleum Geology, v. 73,20 p. https://doi.org/10.1016/j.marpetgeo.2016.02.012.

Abstract: New interest in the potential for shale gas in the United Kingdom (UK) has led to renewed exploration for hydrocarbons in the Carboniferous age Bowland–Hodder shales under Central and Northern England. Following an incidence of induced seismicity from hydraulic fracturing during 2010 at Preese Hall, Lancashire, the publicly available databases quantifying the in-situ stress orientation of the United Kingdom have shown to be inadequate for safe planning and regulation of hydraulic fracturing. This paper therefore reappraises the in-situ stress orientation for central and northern England based wholly on new interpretations of high-resolution borehole imaging for stress indicators including borehole breakouts and drilling-induced tensile fractures. These analyses confirm the expected north northwest – south southeast orientation of maximum horizontal in-situ stress identified from previous studies (e.g. Evans and Brereton, 1990). The dual-caliper data generated by Evans and Brereton (1990) yields a mean SHmax orientation of 149.87° with a circular standard deviation of 66.9°. However the use of borehole imaging without incorporation of results from older dual-caliper logging tools very significantly decreases the associated uncertainty with a mean SHmax orientation of 150.9° with a circular standard deviation of 13.1°. The use of high-resolution borehole imaging is thus shown to produce a more reliable assessment of in-situ stress orientation. The authors therefore recommend that the higher resolution of such imaging tools should therefore be treated as a de-facto standard for assessment of in-situ stress orientation prior to rock testing. Use of borehole imaging should be formally instituted into best practice or future regulations for assessment of in-situ stress orientation prior to any hydraulic fracturing operations in the UK.

Holbrook, W. S., Marcon, V., Bacon, A.R., Brantley,S.L., Carr, B.J., Flinchum, B.A., Richter, D.D. & Riebe, C.S., 2019, Links between physical and chemical weathering inferred from a 65-m-deep borehole through Earth’s critical zone, Scientific Reports volume 9, Article number: 4495, https://www.nature.com/articles/s41598-019-40819-9/

As bedrock weathers to regolith – defined here as weathered rock, saprolite, and soil – porosity grows, guides fluid flow, and liberates nutrients from minerals. Though vital to terrestrial life, the processes that transform bedrock into soil are poorly understood, especially in deep regolith, where direct observations are difficult. A 65-m-deep borehole in the Calhoun Critical Zone Observatory, South Carolina, provides unusual access to a complete weathering profile in an Appalachian granitoid. Co-located geophysical and geochemical datasets in the borehole show a remarkably consistent picture of linked chemical and physical weathering processes, acting over a 38-m-thick regolith divided into three layers: soil; porous, highly weathered saprolite; and weathered, fractured bedrock. The data document that major minerals (plagioclase and biotite) commence to weather at 38 m depth, 20 m below the base of saprolite, in deep, weathered rock where physical, chemical and optical properties abruptly change. The transition from saprolite to weathered bedrock is more gradational, over a depth range of 11–18 m. Chemical weathering increases steadily upward in the weathered bedrock, with intervals of more intense weathering along fractures, documenting the combined influence of time, reactive fluid transport, and the opening of fractures as rock is exhumed and transformed near Earth’s surface.

Rahiman, T.I.H., 2018, Digital imaging of geotechnical boreholes: benefits and risks, Australian Geomechanics Journal, v. 53, no. 3, p. 153-161.

In recent years, the use of borehole televiewers has become common in geotechnical site investigations in Australia, particularly those involving engineering structures in rock. Borehole televiewers can rapidly and efficiently generate vast lengths of high resolution digital imagery of borehole walls and borehole survey data. These extraordinary geo-referenced images of the subsurface present several major advantages for ascertaining in situ geotechnical information over conventional geotechnical core logging. The mystery of core loss zones can be resolved. Rock fabric directions do not require relatively unreliable oriented core measurements. Deviated boreholes can now be correctly placed in 3D ground models. Borehole principal stress directions can be assessed and the painstaking task of logging rock defects at mm scale can be done patiently onscreen. As with any ground test procedure, there are potential bad practices, hidden sources of error and some drawbacks in the results of televiewer data. Poor results can be due to the wrong choice of tools, overlooking of tool calibration checks, and lack of care with tool centralisation. Major sources of inaccuracies can arise from neglecting corrections of borehole diameter and trajectory variations. Quality issues and erroneous readings of borehole geometry aside, the televiewer images present factual and unbiased depictions of the boreholes. Yet analysis of the images to extract information of geotechnical interest can be subjective, requires input from an engineering geologist or geotechnical engineer and verification with rock core data. © 2018 Australian Geomechanics Society.

Cunningham, K.J., Carlson, J.I, and Hurley, N.F, 2004, New method for quantification of vuggy porosity from digital optical borehole images as applied to the karstic Pleistocene limestone of the Biscayne aquifer, southeastern Florida. J. Appl. Geophys., 55:77–99, doi:10.1016/j.jappgeo.2003.06.006.

Vuggy porosity is gas- or fluid-filled openings in rock matrix that are large enough to be seen with the unaided eye. Well-connected vugs can form major conduits for flow of ground water, especially in carbonate rocks. This paper presents a new method for quantification of vuggy porosity calculated from digital borehole images collected from 47 test coreholes that penetrate the karstic Pleistocene limestone of the Biscayne aquifer, southeastern Florida. Basically, the method interprets vugs and background based on the grayscale color of each in digital borehole images and calculates a percentage of vuggy porosity. Development of the method was complicated because environmental conditions created an uneven grayscale contrast in the borehole images that makes it difficult to distinguish vugs from background. The irregular contrast was produced by unbalanced illumination of the borehole wall, which was a result of eccentering of the borehole-image logging tool. Experimentation showed that a simple, single grayscale threshold would not realistically differentiate between the grayscale contrast of vugs and background. Therefore, an equation was developed for an effective subtraction of the changing grayscale contrast, due to uneven illumination, to produce a grayscale threshold that successfully identifies vugs. In the equation, a moving average calculated around the circumference of the borehole and expressed as the background grayscale intensity is defined as a baseline from which to identify a grayscale threshold for vugs. A constant was derived empirically by calibration with vuggy porosity values derived from digital images of slabbed-core samples and used to make the subtraction from the background baseline to derive the vug grayscale threshold as a function of azimuth. The method should be effective in estimating vuggy porosity in any carbonate aquifer.

Roberson, S. and Hubbard, B., 2010, Application of borehole optical televiewing to investigating the 3-D structure of glaciers: implications for the formation of longitudinal debris ridges, midre Lovénbreen, Svalbard, Journal of Glaciology, Volume 56, Issue 195, pp. 143 - 156, DOI: https://doi.org/10.3189/002214310791190802

Digital optical televiewing (OPTV) of hot-water-drilled boreholes is evaluated as a technique for the investigation of englacial ice and debris structures on the basis of six boreholes drilled in the terminus region of midre Lovénbreen, Svalbard. The resulting OPTV logs successfully reveal several visually distinctive englacial ice properties and deformation structures (e.g. oblique englacial fractures imaged here for the first time). Combining these OPTV logs with surface mapping has resulted in the identification of eight separate structural elements, several of which can be interpolated onto 3-D grids at a node spacing of 1 m vertically and 10 m horizontally. Basally derived englacial sediment layers are also found to be intercalated with primary stratification, elevated into near-vertical planes around a central fold axis by large-scale lateral folding. The analysis also allows supraglacial longitudinal debris ridges to be subclassified into two types: a previously described (type-I) form, which are the exposed fold axes of large-scale lateral folds, and a new (type-II) form experiencing secondary deformation by small-scale horizontal folding in association with vertical displacements across arcuate shear planes in response to longitudinally compressive stresses near the glacier terminus. Although using boreholes to investigate glacier structure is limited (e.g. by parallelism with vertical planes), applying OPTV to multiple boreholes at midre Lovénbreen has successfully revealed a range of 3-D structural elements at high spatial resolution. As such, interpolating between multiple OPTV logs overcomes many of the problems associated with interpretations made solely on the basis of surface-based structural mapping, and combining the two techniques represents a powerful glaciological tool.

Hubbard, Bryn & Malone, Terry. (2013). Optical-televiewer-based logging of the uppermost 630 m of the NEEM deep ice borehole, Greenland. Annals of Glaciology. 54. 83-89. 10.3189/2013AoG64A201.

We report on the application of optical televiewing (OPTV) to the uppermost 630 m of the North Greenland Eemian Ice Drilling (NEEM) deep ice borehole, Greenland. The resulting log reveals numerous natural and drilling-related properties, including the integrity of the borehole casing and its joints, the presence of drill-tooth scoring on the ice wall of the borehole and the presence of regularly repeated layering, interpreted to be annual, to a depth approaching 200 m. A second OPTV log was acquired from a nearby shallow borehole. With the exception of the uppermost ∼10 m, this log shows a gradual decrease in luminosity with depth, interpreted as a decrease in light scattering with firnification. This shallow log also clearly images annual layers, allowing the construction of an age–depth scale. Comparing this with an independent core-based scale reveals that the OPTV record yields an age of 1724 at the deepest common point of both scales (80 m), 13 years older than the core-based record at 1737. However, all of this deviation accrues in the uppermost ∼30 m of the OPTV record where highly reflective snow saturates the luminosity of the borehole image, an artefact that can be reduced by further adaptation of the OPTV system.

References

Gaillot, P., Brewer, T., Pezard, P, and EnChao, Y., 2007, Contribution of borehole digital imagery in core-log-seismic integration, Technical Developments, Scientific Drilling, No. 5, doi:10.2204/iodp.sd.5.07.2007

Johnson, C.D., Mondazzi, R.A., and Joesten, P.K., 2011, Borehole geophysical investigation of a Formerly Used Defense Site, Machiasport, Maine, 2003–2006: U.S. Geological Survey Scientific Investigations Report 2009–5120, 333 p., at http://pubs.usgs.gov/sir/2009/5120/.

Johnson, C.D., Kiel, K.F., Joesten, P.K., and Pappas, K.L., 2016, Characterization of fractures and flow zones in a contaminated crystalline-rock aquifer in the Tylerville section of Haddam, Connecticut: U.S. Geological Survey Data Series 1020, 40 p., http://dx.doi.org/10.3133/ds1020.

Hubbard, Bryn & Malone, Terry., 2013, Optical-televiewer-based logging of the uppermost 630 m of the NEEM deep ice borehole, Greenland. Annals of Glaciology. 54. 83-89. 10.3189/2013AoG64A201.

Prensky, S. E., 1999, Advances in borehole imaging technology and applications, in Lovell, M. A., Williamson, G. and Harvey, P.K. (eds) Borehole Imaging: applications and case histories, Geological Society, London Special Publications, 159, 1-42. 1-86239-04306/99.

Paillet, F.L., Barton, C., Luthi, D., Rambow, F., and Zemanek, J., 1990, Borehole imaging and its application in well logging – A review, Borehole Imaging, edited by F. Paillet et al., chap. 1, pp.1-23, Society of Professional Well Log Analysts.

Wang, C., Zou, X., Han, Z, Wang, Y., and Wang, J, 2016, An automatic recognition and parameter extraction method for structural planes in borehole image, Journal of Applied Geophysics, v. 135, p. 135-143, https://doi.org/10.1016/j.jappgeo.2016.10.005.

Zoback, M.D, Moos, D., Mastin, L., and Anderson, R.N., 1985, Well bore breakout and in situ stress, Journal of Geophysics Review, v 90, p5525-5530.