In situ real-time optical sensing device for three-dimensional water chemistry surveillance

In situ water chemistry instruments offer many advantages over existing lab-based techniques such as flexible deployment options (e.g. autonomous vehicles, buoys), enhanced spatial and temporal data resolution, and quicker turnaround time from deployment to measurement results. Understanding the biogeochemical processes of large water bodies requires access to spatiotemporal variability of water chemistry information that is not feasibly obtained with the collection of water samples. With the advance of technology, robotic environmental sensing in general has increasingly become viable and has been attracting a swath of developments in the past decade (Bogue 2011; Dunbabin & Marques 2012). Researchers have deployed surface crafts to support water quality research in various degree of robotic autonomy (Casper et al. 2009; Dunbabin et al. 2009; Koprowski et al. 2013) with limited flexibility in vertical measurement. A practical approach is to deploy a water chemistry sensor aboard an autonomous underwater vehicle (AUV), and make use of the mobility of the platform to obtain real-time data for the generation of three-dimensional (3-D) water chemistry maps. The use of AUVs (Ellison & Slocum 2008; Hemond et al. 2008; Godin et al. 2011) for environmental monitoring has been demonstrated using customized instrument targeted for specific applications. In this paper, an optical sensing device (LEDIF) capable of detecting a wide range of contaminants and natural substances in real-time is instrumented aboard an AUV named STARFISH for 3-D chemical mapping. LEDIF is an optical sensor developed by Ng, Hemond, and Senft-Grupp at Center for Environmental Sensing and Modeling of Singapore-MIT Alliance for Research and Technology, for sensing a range of non-volatile substances in the freshwater and marine environments. STARFISH AUV is an autonomous platform developed at Acoustic Research Laboratory of National University of Singapore to support research in the area of environmental sensing and robotic intelligence.

The aim of this work is to test both systems, functioning as one instrument, to perform 3-D chemical mapping at a reservoir. The compound of interest, chlorophyll a, is chosen because it is a pigment produced by algae and is commonly regarded as a proxy for algal biomass, a key parameter of aquatic systems. Turbidity, another important metric, is also measured at the same time to determine the cloudiness of the water. The objectives are to complete long range (over 3 km, covering 1,000 × 400 m) surface and dive missions to verify LEDIF-STARFISH's capability in detecting spatiotemporal variation in chlorophyll a concentration, identify short-term spatial changes with additional surveys on the same day, and compare results with missions on later days. Results are reported in the form of 3-D water chemistry maps, providing insight into spatial distribution of plankton biomass in both the horizontal and vertical space of the reservoir.

A compact modularized real-time sensor (LEDIF) was integrated into an AUV (STARFISH) for in situ sensing of water chemistry. A doppler velocity log (DVL) was also employed to improve underwater location tracking. The collected data was used to generate 3-D maps of chlorophyll a and turbidity of the reservoir and assess the extent of spatial and temporal variability of the signals.

STARFISH, abbreviated from Small Team of Autonomous Robotic FISH, are low cost AUVs designed to support scientific research in cooperative robotics and environmental sensing down to maximum depth of 100 m (Koay et al. 2011). STARFISH AUVs are comprised of cylindrical sections with distinct functionalities mated together to form a fully sealed torpedo shape vehicle. It is a highly modular system with well-defined electrical, mechanical, and software interfaces that allow straightforward addition of scientific payloads independently developed by collaborators.

As STARFISH travels, water is drawn into the through hull liquid manifold of LEDIF for measurements. When the AUV surfaces, the collected data is communicated over WiFi to a laptop and used to generate targeted water chemistry maps with a simple level set method. To achieve significant spatial coverage in a short time, multiple AUVs can be deployed for concurrent mapping of different areas at different depths. While aboard the STARFISH, LEDIF can operate in several modes (e.g., surface chemical mapping).

The results demonstrate the repeated ability to deploy LEDIF aboard the STARFISH AUV and convert independent chemical measurements into 3-D, time-varying concentration maps. For these missions, with LEDIF operating in passive mode, concentration peaks and variations were easily identifiable. However, features such as precise ‘hot spot’ delineation, were not feasible. Those abilities require the next level of LEDIF and STARFISH integration to enable adaptive sampling and route planning, and identify the need for continuing research with this instrument and mobile platform. It shall be reiterated that out of LEDIF-STARFISH mission path line, the water chemistry maps were obtained using simple level set method.

An in situ real-time optical sensor (LEDIF) with fluorescence, absorbance, and scattering measurement capabilities is deployed aboard a STARFISH AUV for 3-D topographic chemical mapping. The instrument is capable of observing a variety of important compounds found in natural waters. This project provides a powerful tool to understand the spatiotemporal variability of water quality in water bodies and water management systems, and can help improve strategic planning, economical value, and control of regulated water systems. When coupled with adaptable formation control of multiple AUVs, it can be used to improve water safety and security of large consumable water sources. It also provided an unprecedented insight into the phytoplankton biomass spatiotemporal distribution at a local reservoir.