## 3 Data Acquisition and Manual Habitat Mapping  
The NMPZ has been mapped in a multi-platform manner (Figure 1), encompassing acoustic systems ranging from SSS and MBES to a chirp Sub-Bottom Profiler (SBP). An extensive supplementary visual seafloor inspection survey, using a towed underwater camera, offered the means for ground-truthing and validation of the mapped seafloor components. During the coupled SSS-MBES survey, 36 survey lines, with a total length of 262 km, were carried out using an Edgetech 4200 SP dual-frequency SSS, transmitting simultaneously at 100 and 400 kHz and a dual-head MBES Elac Nautic Seabeam 1185, transmitting at 180 kHz, covering a total area of 84 km\({}^{2}\) (see Figure 3). To meet MBES survey requirements, vessel motion was acquired using an SMC IMU-108 MRU system while sound velocity (SV) in the water column was recorded using Valeport MiniSVS as a keel SV probe and Valeport MIDAS SVP as an SV profiler, realizing down-casts on a frequent basis. The MBES data was acquired through Hypack 2016, while Elac's HydroStar software was used as the inertial beam-forming, navigation and attitude system. A Real Time Kinematics (RTK) GPS  
Figure 3: Flow diagram about the object-based feature extraction method, starting from the swath sonar datasets (left column), followed by the feature extraction stage (middle column) showing representative examples of features organized per feature extraction method and resulting in the segment averaged corresponding features (right column). was used to obtain a 10 cm lateral positioning accuracy. To avoid changes in source levels, pulse lengths, receiver gains and directivity patterns, all above parameters were set constant throughout the survey. Automatic Gain Control (AGC) and the Time Varied Gain (TVG) were turned off. Pulse length and frequency were constant through the swath, with no sector dependence, and automatic adjustment of the pulse length as a function of depth was set off to 1 ms. The swath coverage sector was changing between 130\({}^{\circ}\) and 150\({}^{\circ}\) depending on depth (130\({}^{\circ}\) at areas deeper than 50 m) with an along- and across-ship beam of 1.5\({}^{\circ}\), eventually producing 106 or 126 equiangular soundings per swath respectively, corresponding to data point densities ranging from 0.4 to 3.2 points/m. Adapting to the needs of a time-limited field-work schedule and to the inherent low coverage of MBES in shallow waters, as it is proportional to depth, full coverage data was only feasible through the 100 kHz SSS dataset (see Figure 3), whose slant range was set to 200 m. The lower the frequency the lower the sound attenuation in the water column and thus low frequency SSS can achieve full coverage seabed surveys in a fraction of time than with higher frequency SSS. Given the above, the effective slant range of the 400 kHz SSS was found to be 100 m, thus leaving systematic data gaps between the successive survey-lines, corresponding to about 50% of the surveyed area (see Figure 3). The equivalent mean data coverage of the MBES was about 70%, ranging from <30% shallower than \(-\)20 m and >90% deeper than \(-\)30 m. A 20-km long ground-truth survey (see Figure 1) was concurrently run using a SeaViewer 950 analog towed camera with an attached GoPro to validate acoustic bottom types. The ground-truth survey was planned so that all distinct seafloor types, having been recognized by expert interpreters, were visually inspected (see Figure 4). Due to the lack of an underwater positioning system, adequate positioning of the underwater imagery was performed at post-processing using landmarks, collecting salient bottom features, evident both in the video and in the backscatter mosaic, calculating the layback of the tow-camera to the vessel and accounting for the mid-distances using standard spline interpolation. Careful interpretation of all the available marine geoacoustical and ground-truth datasets led to a detailed manual habitat map of the NMPZ, on the basis of the Natura habitat classification scheme (Figure 4).