a disclosed formation monitoring system includes a casing that defines an annular space within a borehole . an array of electromagnetic field sensors is positioned in the annular space and configured to communicate with the surface via a fiberoptic cable . a computer coupled to the fiberoptic cable receives said measurements and responsively derives the location of any fluid fronts in the vicinity such as a an approaching flood front to enable corrective action before breakthrough . a disclosed formation monitoring method includes : injecting a first fluid into a reservoir formation ; producing a second fluid from the reservoir formation via a casing in a borehole ; collecting electromagnetic field measurements with an array of fiberoptic sensors in an annular space between said casing and said borehole , said array communicating measurements to a surface interface via one or more fiberoptic cables ; and operating on said measurements to locate a front between the first and second fluids .