Borehole Logging And Borehole Logs - Facts or Interpretation?

Borehole logs are normally the primary requirement of a GI, not the construction of boreholes per se. Unfortunately, the value of the fieldworks and the time and effort in undertaking them can often obscure the technical objectives of the investigation.

Information comes at a cost, so boreholes should be designed with an end-purpose in mind to maximise the benefit and ensure that appropriate information for project designers is obtained in a cost-effective manner. For geotechnical investigations, this generally includes the geological sequence of material types, their strength, stiffness/compressibility, permeability and geochemical composition.

Geological descriptions of the ground require suitable samples for inspection, and when assessing material properties, these must be representative of in situ (‘undisturbed') conditions. These descriptions may be supplemented by simple field tests – notably the Standard Penetration Test (SPT) – to provide an assessment of the state of soils in the ground, particularly non-cohesive coarse soils (sands and gravels) which cannot be recovered in an undisturbed condition. Other potential sources of information include observations of drill rig behaviour, other field tests during drilling, tests in installations, geotechnical laboratory testing and geophysical in-hole surveys. 

Driller's log

The driller's log contains a summary of the plant and equipment used, dates, times, depths and diameters, usually with a very brief description of strata based on arisings/cuttings. There may also be some comments about any difficulties or conditions encountered during the drilling, such as water entries or odours.

Even with this minimal level of detail, ground conditions are already open to an element of interpretation. While the driller has the opportunity to view cuttings/arisings brought to the surface, the log requires this to be conveyed as descriptions of the strata, providing depths of stratum boundaries and an assessment of what the potentially highly disturbed material brought to the surface represents in terms of strata in the ground.

Driller's logs are submitted during the drilling works but are conventionally not included in the report. The driller's descriptions are also not included as part of the engineer's log, the assumption being that these will be superseded by a more detailed description. While the driller's description may be simple, it should still follow the basic approach used by all involved in soil and rock description. Training is important and may need to be formation-specific for some projects, helping drillers to understand the peculiarities of particular geological materials to ensure high quality samples and core are recovered. The latest edition of the AGS publication, Electronic Transfer of Geotechnical and Geoenvironmental Data (AGS4.1), includes a group for drillers' descriptions – DLOG. This allows drillers' descriptions to be transferred in the digital data, which suggests that these might start to be seen in the reports.

Drilling parameters

During drilling, the driller has the opportunity to note observations which may be indicative of ground conditions, proving useful for the borehole log. This can range from slow progress/hard boring (including chiselling) – which may be indicative of coarser particle 'obstructions' and/or stronger/denser ground – to blowing sands and unusually rapid penetration during rotary drilling. Rotary rigs are fitted with basic instrumentation (for example, to monitor bit and pump pressure), but other parameters are often inferred only in a qualitative manner from the driller's observations. These are chiefly used as an aid during drilling and to maximise the penetration rate rather than to provide any information on the ground conditions.

To make use of the potentially valuable information that can be obtained from the actual borehole construction, rigs can be fitted with drilling parameter recording (DPR) instrumentation. In 2019, SOCOTEC purchased the latest Jean Lutz ‘DIALOG Version 5’ system for rotary drilling rigs, which can be adapted to suit multiple applications and display real-time information both digitally and graphically. The system records a range of parameters, including rotation speed, borehole depth, down thrust pressure, drill head rotation torque, drilling fluid pressure, penetration rate and holdback pressure, with optional expansion for other parameters. Once data has been collected from the drilling rig’s calibrated sensors, it is downloaded and inputted into the Jean Lutz EXEPF processing software for reporting.

The benefit of using DPR depends on the ground conditions and the type of drilling required. In the example below, the drilling parameters from a rotary coring project indicate fairly consistent ground conditions over the depth range shown, but the sudden increase in drilling rate corresponded to a zone of core loss. This confirmed that much weaker ground was present, rather than leaving the logging geologist uncertain as to whether the loss was related to the ground conditions or the drilling process itself. In variable ground conditions, DPR data can provide useful information to confirm stratigraphy in terms of material types and properties with depth in a similar way to a Cone Penetration Test (CPT).

Engineer/geologist's description

Descriptions of samples and rotary drilled core are carried out by engineers/geologists in accordance with the industry recognised standard, BS 5930. The latest edition issued in 2020 is compliant with the latest Eurocode related standards for soil and rock description (BS EN ISO 14688 and 14689) released in 2018. BS 5930 contains guidance and additional descriptive terminology to enhance these.

However, there are aspects of the description that, although referred to in BS 5930, are not mandatory, but may be required within a project specification. For example, the description of soil and rock colours is normally based on a simple scheme, but recent projects carried out by SOCOTEC have specified using the Munsell colour system, which quantifies the colour.

Noting the presence of fossils is a normal part of rock logging but does not usually extend to actual identification. However, in some geological units, such as the Chalk, fossil types are particular to specific horizons and are used as markers. Correlating the details of the geology between boreholes is often only possible where fossil types are identified, which requires meticulous and time-consuming logging. This may be crucial to long linear projects (including recent railway and tunnelling projects carried out by SOCOTEC) where an understanding of the structural geology along the route is essential to put all the engineering information into context.

The performance of rock in engineering is largely controlled by the fracture state. Conventional logging of core includes describing the spacing, orientation and surface condition of fractures. For some projects, a more detailed discontinuity description may be specified, including separate descriptions of every fracture present for type, set number, dip, roughness, planarity, joint roughness coefficient (JRC), surface appearance, aperture, infill and wall weathering.

A further step in the logging process may be adding formation-specific weathering grades to the strata, as used for chalk and Mercia mudstone. Weathering classifications provide a useful means to modelling the ground for design purposes.

An enhanced descriptive approach often serves to assign some quantitative terms to the description to better facilitate data analysis, rather than purely qualitative terminology. However, in some cases, such as for detailed fracture logging, it may not be realistic to present all the information on the borehole log due to layout and space limitations, and this may be reported separately.

Laboratory testing

Geotechnical laboratory testing of samples and core is usually carried out as part of the ground investigation. Testing serves two purposes, providing:

• Confirmation and quantification of description from inspection, e.g particle size distribution (PSD) and rock strength (from UCS)
• Material properties for design purposes, e.g strength, compressibility and permeability.

The results of some laboratory tests can be used to influence elements of soil and rock description. However, the fundamental distinction between fine and coarse soils (traditionally cohesive and non-cohesive/granular) relies upon the logger's assessment of the soil behaviour, rather than being a function of PSD or plasticity testing, again highlighting the skilled interpretation that is required. Laboratory data is not included on borehole logs in traditional onshore investigations, although it may be on logs for offshore drilling.

Field testing

Borehole field (or in situ) testing has advantages over laboratory testing in avoiding issues relating to sample disturbance and analysing a larger volume of the ground. The downside is that the actual material is not visible and factors such as external stresses and drainage cannot be controlled (unlike in the laboratory setting). However, field testing may be the only viable method for materials that cannot be recovered in undisturbed state, e.g coarse soils (sands and gravels) and extremely weak rocks.

The Standard Penetration Test (SPT) is unusual in that the test results define relative density, which is an integral part of the description of coarse soils (e.g loose, dense). All other parts of the description are based on direct inspection - the results of field tests used to assess shear strength, for example, (such as field vane, pressuremeter and SPT) are not incorporated into the log descriptions. However, good practice dictates that the results from field and laboratory tests are reviewed against the corresponding borehole logs to ensure that they are commensurate with the expected properties for the ground conditions described. This may require a higher level of expertise to be able to interpret the results, particularly for more advanced tests.

Surveying the hole

In completed boreholes, the geological profile encountered can be surveyed using 'geophysical' wireline and logging methods. There are a range of tools to suit different purposes, including natural gamma, resistivity, density and fluid temperature. A televiewer log provides an 'unwrapped' image of the borehole wall and, while conveying limited information on material type and no information on strength, is most effective for visualising discontinuities/bedding features. A major benefit of the televiewer log is its ability to interpret the dip angle and direction of a discontinuity. Moreover, highly fractured zones in the ground are likely to be much less disturbed in the borehole wall than in the recovered core.

The image below shows the typical sinusoidal forms of planar dipping discontinuities from which the dip and dip direction can be assessed. The highly fractured zone in the first 1.5m of the hole is noteworthy, as the spacing and orientation of fractures are clearly visible. In comparison, the recovered core (seen in the photograph) is in a disturbed state, with evidence of rock pieces being rotated during drilling. Reliable assessment of the in situ ground conditions without the benefit of the televiewer log in this instance would not be possible, and the core would likely be logged as non-intact and disturbed.

Additional information from geophysical logging can enhance the information obtained from logging the core, but should this be part of the borehole log? From a practical aspect, there may be a substantial disconnect between the borehole logging, which may be carried out as the borehole is advanced, and the availability of the geophysical data. In this example, the results from the wireline logging carried out on completion of the borehole were unavailable for a matter of weeks after this section of core had been logged. Revisiting the borehole log, if not the core itself, would not be an insignificant task. While reconciling the fracture information from the two sources is sensible, the project specification should recognise if this is required and how it should be carried out so that suitable arrangements can be implemented. There is also the question again of how much information should be included on the borehole log, and how should information obtained from sources other than drilling and logging be identified on the log?

Digitalisation and beyond

In most ground investigation reports, the borehole log still remains in a format recognisable to that of 50 years ago, but with the ongoing development of data digitalisation in GI since the 1980s, the role of the borehole log as a self-contained entity is becoming less clear. Data is now routinely managed in digital format - collected on site using mobile devices, transferred as AGS digital data files, collated and stored in databases. The borehole log now represents a 'snapshot' of data held in the database, a representation that can be tailored to include or exclude whatever is required, without affecting the information provided in the overall project database. Arguably, the data itself has now become the primary deliverable rather than the GI report and the borehole logs, and could the use of BIM and the development of interpreted data transfer formats such as AGSi actually replace borehole logs in the future?