Seismic sensors measure an artificial seismic response wave reflected by different layers of the subsoil after initiating a predetermined artificial seismic interrogation wave (ground shaking) on the surface of the terrain by a source controlled by an operator. Examples of seismic sensors include geophones or accelerometers having sufficient sensitivity for measuring the response wave reflected in the ground. Following ground shaking, each seismic sensor acquires seismic data. The seismic data may be digitized and transmitted to a base station for later processing. The transmission is conventionally accomplished by wire link (for example a cable) or by radio link.
To conduct petroleum prospecting over a large tract of land, which may measure several kilometers by several kilometers, an operator distributes modules over the land. Cartography of the subsoil corresponding to the land is then possible from the seismic data measured at each node and is exploited to identify the potential presence of petroleum.
A seismic acquisition device of cellular type may be used by dividing the land into cells, such that each cell includes an access node to the cell and geophone units. The geophone units transmit digital data via wireless link, for example, at 2.4 GHz to their cell's access node, and the access nodes transmit data to a central control unit. However, cellular transmission of seismic data in this arrangement has a relatively high cost of installation of cellular hardware. Additionally, without cables to transfer power to the seismic sensors each seismic sensor requires an onboard battery.
When the cellular network transmits data in a radio frequency band requiring authorization of use administrative steps may slow down the process of deployment of a prospecting mission. Alternatively, the acquisition modules may transmit in a free band such as, for example, in the 2.4 to 2.48 GHz band or in the 5.4 to 5.8 GHz band. But, antennas operable on these bands have low gain and low height, which may hinder operation when the acquisition module is located in a zone where data transmission conditions are difficult. For example, when the antennas of data acquisition modules are covered by excessive grass height or when there is an obstacle on the communication path between two antennas.
In another solution, acquisition modules may transmit wireless data to an operator carrying a monitor in proximity to the sensor to download recorded data from the module. This arrangement is disadvantageous because the operator must physically move to each module to receive the data.
When installing, operating, or transporting data acquisition modules, the data acquisition modules may be subjected to aggressive external forces. For example, in order to install a data acquisition module on terrain, a user grips the module and force a part of it in the ground or on the ground. Antennas are fragile and often break if large force is exerted on them. Additionally, the data acquisition module may be subjected to numerous aggressive external forces prior to being deployed on terrain. For example, the data acquisition modules may be unloaded from a truck or helicopter and piled on the ground so that personnel can distribute them to different positions on the ground. These external forces present an additional complication to the design of a data acquisition network, and in particular to wireless data acquisition modules.
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