GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 75 in the substratum; 3) the shallow-water enhances the effect of noise due to propellers or other natural and artificial causes. However, the economic and societal importance of such environments call for the develop- ment of new technologies and scientific methods that could open their study to a wider spec- trum of users, on one side simplifying and better standardizing the survey procedures; on the other side, reducing the costs. The ongoing research and developments in the field of robotics could represent an interest- ing opportunity. In fact, the relatively recent availability of miniaturized although accurate sen- sors, such as gyroscopes, GPS receivers, compass, motion sensors, etc., as well as innovative “open” hardware architectures ( Arduino , Raspberry , etc.) facilitate the design and the develop- ment of Unmanned Surface Vehicles (USV), that could operate in extremely shallow, difficultly accessed environments, such as small rivers, channels, lakes and coastal lagoons. ���� �� ��� This is the case of SWAP (Shallow-WAter Prospector) developed by ISMAR-CNR and Consorzio Proam- biente (Tecnopoli dell’Emilia Romagna) a small, highly flexible vehicle that could be employed in a variety of subaqueous environments. Parallel to the development of the USV, it is also im- portant to test new methods and instruments for the geophysical prospection in such a peculiar conditions, which could be employed on board small-size platforms. We present some example of geophysical surveys carried out with different instruments and techniques in shallow water environments, including a coastal lagoon, a river stream, an artificial channel and a lake. Methods. The Unmanned Surface Vehicle. SWAP (Fig. 1) is an USV based on a trimaran framework, i.e., a multihulled boat consisting of a main hull and two smaller outrigger hulls, attached to the main hull with lateral bars. The main hulls hosts the batteries, that supply two brushless engines, controlled by a hybrid speed controller driven by an Arduino Due board. The maximum speed reached by SWAP is 3.5-4 knots during a test session carried out in a small lake. Quality check of the data is obtained via sub-sampling of the collected time-series, which are packed and transmitted to a host computer onshore through radio transmission. Although the architecture of the system is modular and could be easily expanded to other USB-based digital sensors, three basic devices were installed on each prototypes: 1) a DGPS receiver; 2) a high- resolution single beam echosounder; and 3) a chirp- sonar subbottom profiler. These are considered “basic sensors” that will be likely employed in most of the survey carried out in shallow-water areas. The vehicle is equipped with a differential GPS system that provide and accuracy of < 1 m in the x-y positioning. NMEA-183 format strings are transmitted to the host computer onshore to monitor continuously the position of the vehicle and errors in the execution of the mission- related profiles. A HMC5883L magnetometer, an ADXL345 accelerometer, and a ITG3200 gyroscope, interfaced to the ARDUINO-DUE board constitute the basic system for inertial navigation and correction of geophysical data for pitch, roll, heave and heading. Geophysical sensors. Ecosounder. A vertical incidence 200 kHz pinger has been mounted onboard of SWAP. Such sensor is particularly suitable for shallow-water environments because is characterized by high operating frequency, narrow beam width (8°, conical), a short pulse length (350 µs). This configuration lead to a minimum depth range around 0.5 m. However, Fig. 1 – The Unmanned Surface Vehicle SWAP (Shallow-Water Prospector), designed and built at ISMAR-CNR in cooperation with Consorzio Proambiente (Tecnopoli dell’Emilia Romagna).