A seismic data acquisition system, comprising: one or more sources for generating seismic waves; and one or more sensor units (100, 206) for recording seismic waves generated by the sources, the one or more sensor units (100, 206) having a first seismic sensor for measuring a vertical wavefield of the seismic waves, and a second seismic sensor for measuring a gradient of the vertical wavefield; characterised in a housing, the first seismic sensor and the second seismic sensor being disposed in the housing, wherein the second sensor is a MEMS-based rotation sensor (10,30,60,80); and wherein the MEMS-based rotation sensor (10, 30, 60, 80) comprises: a substrate; a central anchor (12, 38, 64, 84) disposed on the substrate; a proof mass (14, 36, 62, 82) coupled to the central anchor via a plurality of flexural springs (16, 39, 66, 86), the proof mass (14, 36, 62, 82) having a first electrode (18, 32) coupled to and extending from the proof mass; a second electrode (20, 34) fixed to the substrate, one of the first and second electrodes (18, 20, 32, 34) being configured to receive an actuation signal, and another of the first and second electrodes (18, 20, 32, 34) being configured to generate an electrical signal having an amplitude corresponding with a degree of angular movement of the first electrode (18, 32) relative to the second electrode (20, 34); and closed loop circuitry configured to receive the electrical signal and provide the actuation signal, and at least one of the first and second electrodes (18, 20, 32, 34) including a beam-shape wherein a length of the electrodes is greater than a width of the proof mass (14, 36, 62, 82). one or more sources for generating seismic waves; and one or more sensor units (100, 206) for recording seismic waves generated by the sources, the one or more sensor units (100, 206) having a first seismic sensor for measuring a vertical wavefield of the seismic waves, and a second seismic sensor for measuring a gradient of the vertical wavefield; characterised in a housing, the first seismic sensor and the second seismic sensor being disposed in the housing, wherein the second sensor is a MEMS-based rotation sensor (10,30,60,80); and wherein the MEMS-based rotation sensor (10, 30, 60, 80) comprises: a substrate; a central anchor (12, 38, 64, 84) disposed on the substrate; a proof mass (14, 36, 62, 82) coupled to the central anchor via a plurality of flexural springs (16, 39, 66, 86), the proof mass (14, 36, 62, 82) having a first electrode (18, 32) coupled to and extending from the proof mass; a second electrode (20, 34) fixed to the substrate, one of the first and second electrodes (18, 20, 32, 34) being configured to receive an actuation signal, and another of the first and second electrodes (18, 20, 32, 34) being configured to generate an electrical signal having an amplitude corresponding with a degree of angular movement of the first electrode (18, 32) relative to the second electrode (20, 34); and closed loop circuitry configured to receive the electrical signal and provide the actuation signal, and at least one of the first and second electrodes (18, 20, 32, 34) including a beam-shape wherein a length of the electrodes is greater than a width of the proof mass (14, 36, 62, 82). a substrate; a central anchor (12, 38, 64, 84) disposed on the substrate; a proof mass (14, 36, 62, 82) coupled to the central anchor via a plurality of flexural springs (16, 39, 66, 86), the proof mass (14, 36, 62, 82) having a first electrode (18, 32) coupled to and extending from the proof mass; a second electrode (20, 34) fixed to the substrate, one of the first and second electrodes (18, 20, 32, 34) being configured to receive an actuation signal, and another of the first and second electrodes (18, 20, 32, 34) being configured to generate an electrical signal having an amplitude corresponding with a degree of angular movement of the first electrode (18, 32) relative to the second electrode (20, 34); and closed loop circuitry configured to receive the electrical signal and provide the actuation signal, and at least one of the first and second electrodes (18, 20, 32, 34) including a beam-shape wherein a length of the electrodes is greater than a width of the proof mass (14, 36, 62, 82).