In broad terms, the ability of a logging tool to detect and resolve a feature. For cases involving vertical wells with beds sitting normal to the well bore, vertical resolution is the thinnest bed (or lamination if less than 1 cm) thickness that a sensor can measure. In reality, resolution is under the control of a large number of variables that include:
- Physics of the measurement, (e.g. resistivity versus nuclear),
- Contrast in physical properties between adjacent beds, (e.g. density, resistivity, Pe or gamma-ray emissions),
- Frequency of the measurement, including rotational sampling,
- Volume of investigation,
- Tool response,
- Detector spacing,
- Logging speed,
- Inclination of a feature to the well bore,
- Nature of the feature imaged, e.g. highly conductive features such as mm thick pyrite-rich bands are usually readily detectable on resistivity images, whereas they may not be detected with a BHTV device.
Vertical bed resolutions can range from a few mm to feet depending on the tool type and the logging situation it is found in. Logging companies detail resolutions for varying tool types. For borehole image interpretation the size of pixels on a screen is a ready measure of practical detection and resolution. A difficult area to detail that is covered in Theys (1999).
THEYS, P. 1999. Log Data Acquisition and Quality Control, Second Edition. Éditions Technip, Paris, 453.