In China, the lack of high-resolution topographical data from small target areas limits our understanding of deep sea phenomena. Here, the high-resolution topography of the PACMANUS hydrothermal field and the DESMOS hydrothermal field were measured using a remotely operated underwater vehicle (ROV) with a multibeam system. A composition positioning system on the ROV provided high positioning accuracy (greater than 0.5 m). The data show that four knolls develop in the PACMANUS field on the crest of the Pual ridge. The DESMOS caldera is approximately 250 m deep in the center with an E–W diameter of approximately 1 km and an N–S diameter of approximately 2 km. This survey is the first trial in China using ROV data to measure the deep-sea high-resolution topography of small-scale target areas. Validation of the survey method proved it to be applicable, which will promote the development of deep sea research in China.

Seamounts, hydrothermal/cold spring systems, and mid-ocean ridges in the deep sea serve as natural laboratories for the study of deep earth processes, geodynamics, and deep-sea biology and ecology. The absence of high-resolution topographical data from those areas restricts our understanding of the phenomena in the deep sea. Near-bottom observations in the deep sea is lacking due to the limits of deep-sea detection technology. The Manus basin, a fast-spreading back-arc basin in the Bismarck Sea [1], has several famous active hydrothermal fields in the eastern portion (Vienna Woods [2], PACMANUS [3, 4], DESMOS [5], and the SuSu knolls [6]). Many researchers have studied the hydrothermal processes and geochemical characteristics in this area [7–10]. In contrast, research studies that focus on the high-resolution topography and geomorphology of the hydrothermal fields are rare [4]. Moreover, data obtained by multibeam systems on board a ship cannot satisfy the requirements for high-resolution studies of a small target area due to their low resolution.

The ROV “FAXIAN” with a multibeam system (Kongsberg EM2040) and a composite positioning system was first used to survey the hydrothermal fields in the Manus Basin in June 2015. The composite positioning system provided long baseline (LBL) navigation and positioning during measurements (Fig. 1). The accuracy of the measured distances between the ROV and the LBL beacons was greater than 1.8 cm (Fig. 1a). The accuracy of the ROV positioning was greater than 0.5 m (Fig. 1c). These led to a much higher resolution of the bathymetry data (better than 2 m) than that obtained by a multibeam system on board a ship. Here, the raw bathymetric data obtained were processed using CARIS software. The processed data were then made into images by Surfer software with a mesh grid of 6 m.

Fig. 1
figure 1

Schematic explanation of the multibeam survey of deep-sea small-scale target areas with a ROV and the evaluation of the positioning accuracy of the ROV. a Measured distances between the ROV and Beacon 1. b Relative positions of the ROV and beacons to the support vessel. c Measured positions of the ROV to the support vessel

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Based on the high-resolution data, we describe the topographical details of the PACMANUS and DESMOS hydrothermal fields. The results show that several ridges appear in the eastern Manus basin, of which the Pual ridge is the largest (Fig. 2a). The PACMANUS field is located on the crest of the Pual ridge, where four knolls can be clearly identified (Fig. 2b). The locations of the knolls coincide with the hydrothermal plumes discovered previously [4]. The DESMOS field is a caldera that is approximately 250 m deep in the center with an E–W diameter of approximately 1 km and a N–S diameter of approximately 2 km (Fig. 2c). The seafloor is much steeper on the inner side of the circular fracture. Two highlands occur in the northern and the southern flanks of the caldera. Further observation indicated that pillow lava, sulfide talus, breccia, anhydrite, outcrops, and sediment all appeared in the DESMOS field (Fig. S1 online).

Fig. 2
figure 2

a Measured topography of the study area in the eastern Manus basin (with a mesh grid of 35 m). b and c show high-resolution images of the PACMANUS and the DESMOS hydrothermal fields, respectively (with a mesh grid of 6 m). Sites a-f, shown in white, denote the locations of the video images of deposits on the seafloor captured by the ROV that are presented in Fig. S1 online

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This ROV survey with a multibeam system is the first trial in China to measure the deep-sea high-resolution topography of small-scale target areas. The data support additional detailed research on these particular hydrothermal systems. The survey method that we deployed was tested and was proven to be applicable for promoting the development of deep sea research in China.