BACKGROUND

Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A seismic survey typically involves deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations creating pressure changes and vibrations along their way. Changes in elastic properties of the geological formation scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy only one type of sensors or both. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.

Historically, seismic data acquisition along a surface has been accomplished by placing seismic sources and sensors along a straight line. In such a configuration, it is assumed that the reflection points in the ground are located in a two-dimensional plane delimited by the transverse line and the vertical axis. This is often referred to as a two-dimensional seismic survey. However, three-dimensional seismic surveys are often preferred in order to obtain better signal quality and to improve the space and the time resolution. One of the drawbacks of three-dimensional surveys is the requirement of a large amount of sensors, which necessitates a large deployment crew. This results in increased costs and decreased efficiency. Accordingly, improved seismic sensors allowing for sparse sampling and thus less deployment of sensors without compromising data quality are desired.

BRIEF SUMMARY

The present disclosure is directed to a MEMS-based rotation sensor for use in seismic data acquisition and sensor units having same. A sensor unit for land-based seismic data acquisition includes a particle motion sensor for measuring a vertical wavefield in which the vertical wavefield has a horizontal gradient. The sensor unit further includes a first MEMS-based rotational accelerometer for measuring an x-component of the horizontal gradient and a second MEMS-based rotational accelerometer positioned orthogonally to the first MEMS-based rotational accelerometer. The second MEMS-based rotational accelerometer measures a y-component of the horizontal gradient. At least one of the first and second MEMS-based rotational accelerometers includes a substrate, an anchor disposed on the substrate, and a proof mass coupled to the anchor via a plurality of flexural springs. The proof mass has a first electrode coupled to and extending therefrom. A second electrode is fixed to the substrate, and one of the first and second electrodes is configured to receive an actuation signal, and another of the first and second electrodes is configured to generate an electrical signal having an amplitude corresponding with a degree of angular movement of the first electrode relative to the second electrode. The MEMS-based rotation sensor further includes closed loop circuitry configured to receive the electrical signal and provide the actuation signal.

A seismic data acquisition system is described having one or more sources for generating seismic waves and one or more sensor units for recording seismic waves generated by the sources. The one or more sensor units include 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.

A method for performing seismic data acquisition is also described. The method includes generating seismic waves using one or more sources, and recording seismic waves generated by the sources using one or more sensor units. The one or more sensor units include 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.

