CityShark, a station dedicated to ambient noise (microtremor) recording, was designed and built jointly by the French Institute for Research for Development (IRD) and LEAS, a French company dedicated to geophysical data acquisition.
Because portable seismic stations are designed to be installed for long time periods (days to months) and to detect earthquakes, operators using them for microtremor recording face several obstacles that considerably slow and complicate the data acquisition process. Seismological stations are not easy to use in experiments for which time of recording is on the order of tens of minutes (site studies) or for repetitive recording of several minutes at interval times on the order of hours (building response studies).
Running seismological stations requires setting a series of parameters ( e.g., STA, LTA, trigger level, pre-event memory), usually by communicating with the station with the help of an external computer. The data recorded are then downloaded using a portable computer or by manipulation of a removable high-capacity storage medium. In order to maximize the station autonomy, data are often compressed and power consumption is minimized. The use of a GPS receiver is needed for time accuracy. All the equipment must be packaged in a rugged box to be able to function in extreme environments ( e.g., rain, sun, snow, wind, and dust). Finally, these stations need to be installed and run by well trained people. These stations are made to record earthquakes and avoid recording ambient noise, i.e., the opposite goal of microtremor recording. Hence, the use of portable seismological stations for microtremor recording considerably slows data acquisition, and data processing is needlessly complicated.
Running microtremor experiments for site studies in Pujili, Ecuador (Gueguen et al., 1998) and Nazca, Peru (Chatelain et al., 1997) and encountering the kind of problems mentioned above led us to the idea of building …