As with many logging-data end users, I have always been aware of the variable spatial resolutions of logging tools. I am also aware of the swathe of variables that can influence the resolution of logging data during the data collection phase. These include the source-sensor arrangement on the tool string, sampling rate, logging speed, penetrative strength of any radioactive sources, borehole conditions such as hole diameter and mud density, and the appropriateness of the tools for investigating the specific downhole lithologies. However, for the next few minutes I would like to focus solely on the tools themselves and what exactly can be measured when all other conditions are ideal.
Logging tool technology utilizes many forms of investigative method – from acoustic pulses to magnetic fields, passing through a range of different nuclear source types on the way. By consequence, individual tools measure different areas of the downhole environment. In other words, they exhibit different spatial resolutions.
Spatial (or measurement) resolution in logging tools can be separated into vertical resolution and depth of investigation; with vertical resolution detailing the ability of a tool to resolve changes in tool response parallel to the tool axis (Schlumberger); and depth of investigation detailing tool response perpendicular to tool axis. In other words, depth of investigation tells us how far into a geological formation (i.e. beyond the borehole walls) a tools response can be influenced.
The vertical resolution and depth of investigation of any given tool are often linked, as most physical fields are spherical (Clark et al. 1988). However, this is not always the case. The sonic tool, for example, produces acoustic waves that do not propagate spherically but rather with a “field” shape closer to that of an ellipsoid (Rider and Kennedy, 2011 – strongly recommended reading for a summary of acoustic wave propagation characteristics within a formation). A good rule of thumb in modern sonic tools is that the depth of investigation in inches, is the same as the transmitter receiver spacing in feet.
The vertical depth of any given logging tool is often controlled by, or at least heavily influenced by, its transmitter-receiver spacing. However, this is only part of the story. While a tool may have excellent qualitative spatial/vertical resolution, it may still exhibit poor quantitative resolution. Quantitative tool resolution can also be termed minimum bed thickness; defined as the minimum thickness of geological bed for which a tool can/will detect true values.
Most minimum bed thicknesses are calculated through tool testing; as the minimum thickness often relies as much on the formation being tested as it does the tool itself. This creates a situation where certain lithologies can be read for true values at finer resolutions than other lithologies.
For a good example of how vertical resolution of a tool can be lithology-dependant take a look at electrical resistivity tools. Resistivity tools can have extensive vertical resolutions, but can also be subject to shoulder bed effects. This is where the beds either side of a bed of interest influence the tools response. The influence of a shoulder bed is magnified with increasing contrast between the measurement values of the two beds. Furthermore, studies by Clarke et al. (1988) show that shoulder bed effects can be greater if the shouldering beds are of lower resistivity (more electrically conductive) relative to the bed of interest.
|Altered from Clarke et al. 1988|
In addition to vertical resolution, measurement depth of investigation can also be lithology-dependant. A good example here comes from the porosity tool where non-porous beds can be seen to a greater depth than a porous bed as water contained in pores intercepts neutrons emitted from the source. This effect can be significant, with 0% porosity beds penetrated by 60 cm, 10% porosity beds by 34 cm, and 30% porosity beds by only 16.5 cm (Rider and Kennedy, 2011).
There are few cases where a logging tools minimum bed thickness is in fact finer than its vertical resolution. Theys (1999) showed that the photoelectric factor output of the density tool often accurately identified beds 10-20 cm thick, even though the transmitter-receiver spacing for the tool was 36 cm and the minimum bed thickness for the overall density measurement was closer to 40 cm.
Due to this lithology control (in addition to the environmental controls omitted in this blog) it can prove challenging to report on the true vertical resolution of a given logging tool. See the image below for some rough areas of investigation of common logging tools.
|Approximate to-scale graphical representation of common logging tool areas of investigation|
In order to find out what is truly going on down a borehole, a data user needs to understand both what is being measured, and how – more reason to interrogate multiple data types – core, seismic, and log data to name a few.
Failing the ability to do this, one can always use certain log data to quality control other data since most tools utilise different investigative technologies and thus present different limitations. There are also inversion methods and other tools that data users can utilize to increase tool accuracy and shrink minimum bed thickness detection windows. A good summary of some methods can be found in this paper by Sanchez-Ramirez et al. (2009).
As I see it, the only way round this issue is (as with any trades person) to know your tools as there is nearly always a difference between bed detectability and minimum bed thickness.
Clark, B., Liiling, M.G., Jundt, J., Ross, M. and Best, D., 1988, January. A dual depth resistivity measurement for FEWD. In SPWLA 29th Annual Logging Symposium. Society of Petrophysicists and Well-Log Analysts.
Rider, M. and Kennedy, M., 2011. The geological interpretation of well logs: Rider-French Consulting Ltd.
Sanchez-Ramirez, J.A., Torres-Verdín, C., Wang, G.L., Mendoza, A., Wolf, D., Liu, Z. and Schell, G., 2009, January. Field examples of the combined petrophysical inversion of gamma-ray, density, and resistivity logs acquired in thinly-bedded clastic rock formations. In SPWLA 50th Annual Logging Symposium. Society of Petrophysicists and Well-Log Analysts.