GNGTS 2013 - Atti del 32° Convegno Nazionale

Calibration of a new uncoupled acoustic sensor for ground motion detection A. Vesnaver 1,2 , E. Poggiagliolmi 3 , F. Brunetti 1 , D. Zuliani 1 1 OGS - Istituto Nazionale di Oceanografia e di Geofisica Sperimentale, Trieste, Italy 2 King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia 3 Entec Integrated Technologies, London, United Kingdom Introduction. A standard geophone for seismic surveys is coupled to the ground by a spike. This device records the ground motion with respect to an inertial mass attached to a coil in a magnetic field, both suspended by a spring. Geophones have been used for decades in the oil and gas exploration industry and they are currently being employed in seismic recording systems with a number of channels exceeding 200,000. Even larger systems, totaling 1 million receivers, have been announced to be commercially available in the coming months. The number of geophones equals the channel number only in the uncommon case of single- sensor recording: more often, a small array of geophones is connected to each channel (e.g., a dozen). Consequently, the total number of geophones to be planted and retrieved during these surveys exceeds already 1 million. The ensuing field operations require crews composed of hundreds of people, which result in high costs and demanding logistics. In desert areas (as in the Middle East and North Africa, or Arctic) mobilizing people and instruments is the main problem, while in urban areas planting spikes into the ground may be unfeasible, because it may damage roads, yards and building. Uncoupled sensors are much less invasive and, if properly miniaturized, their weight and volume may be significantly smaller than standard geophones based on a moving coil. Removing the coupling of sensors to the ground not only reduces the required man-power significantly, but also improves the signal quality. The coupling with the ground may be very poor in the sandy deserts of North Africa and Middle East, or in the permafrost of Canada and Siberia, where a major share of hydrocarbon reserves are located. For these reasons, we have been developing an uncoupled acoustic sensor that should provide better signal quality at lower costs of surveys on land (Poggiagliolmi et al. , 2012, 2013). In this paper, we present the basic principles and the calibration results obtained in the laboratory to compare the performance of the new device with that one of standard geophones for land seismic surveys. Uncoupled sensor description. Fig. 1 shows the basic principle of the proposed uncoupled sensor. A piezoelectric transmitter emits a continuous acoustic carrier frequency, which impinges on the ground surface where it is Doppler modulated by the ground’s vibrations. The back-scattered signal is recorded by a piezoelectric receiver, and the resulting Doppler frequency shift is used to measure the ground displacement or velocity. A few prototypes of the new sensors have been made and tested both in the field and the lab. Field tests were carried out for different ground types: glass panes, cotto tiles, fine and coarse gravel, sand, leaves, pine needles, among others (Poggiagliolmi et al. , 2012, 2013), with very encouraging results. Quite surprisingly, the energy of the back- scattered signal is not decreasing so much with the roughness of the target surface; instead, the main effect observed is a less sensitive dependence on the dip angle of such a surface with respect to the source. For very flat surfaces (as glass panes or cotto tiles) such angular dependence is much higher. The back-scattered signal is strong in two Fig. 1 – General scheme for an acoustic uncoupled sensor. 77 GNGTS 2013 S essione 3.1