A sensor comprises a sensor resonator member 110 configured to resonate at a resonant frequency when exposed to a radio frequency electromagnetic field of a corresponding frequency, and a capacitive element 140 coupled to the sensor resonator member, the capacitive element comprising first and second capacitor plates 144 configured to be deposited in a granular material such that a portion of the granular material interposed between the plates forms a dielectric layer 142 of the capacitive element, wherein the sensor is configured such that the resonant frequency is dependent on a capacitance of the capacitive element. The member 110 comprises inductive element 120 and capacitor 130. The sensor may be located near buried pipes to locate leaks such that changes in moisture content of the material 142 affect the resonant frequency.
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