1. n. 
A formation layer above or below the layer being measured by a logging tool. The term is used in particular in resistivity logging to describe the layers above and below a reservoir. Some resistivity tools, such as induction and laterolog devices, can sense beds located tens of feet from the measure point and can be significantly affected by shoulder beds even when the reservoir is thick. The term is more commonly used for vertical wells, and is derived from the typical picture of resistivity log response across a reservoir: a high resistivity reservoir (the head) with two low-resistivity shales above and below (the shoulders). The term also may be used in horizontal wells, although in that context the term surrounding bed is more common. The term adjacent bed is used in both cases.
Synonyms: adjacent bed