Good question and not one I've really given thought to before. This might be the sort of thing you're interested in:
Some searching show's that there's ISO19123:2005 - "Geographic information -- Schema for coverage geometry and functions" - needless to say, being an ISO standard it's decidedly not "Open" - they cost a small fortune to access - http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=40121
ISO 19123:2005 defines a conceptual schema for the spatial
characteristics of coverages. Coverages support mapping from a
spatial, temporal or spatiotemporal domain to feature attribute values
where feature attribute types are common to all geographic positions
within the domain. A coverage domain consists of a collection of
direct positions in a coordinate space that may be defined in terms of
up to three spatial dimensions as well as a temporal dimension.
There's also the GML 3.2.1 Application Schema for Coverages - http://www.ogcnetwork.net/node/1592 which:
has been developed jointly by the GML, WCS, and SWE groups, defines
OGC's unified coverage model. GML serves both as conceptual model,
ensuring a concisely defined semantics, and as one of many possible
encoding formats. This way, coverages can be transported across
different services, such as WCS, WCPS, WPS, and SOS.
As you can guess, being GML based, they're both rather verbose unlike WKT.