5:15 PM - 6:45 PM
[SCG48-P25] Cross-line laser orientation method: Advancing seafloor sampling with Shinkai 6500
Keywords:Shinkai 6500, Oriented sample, Paleomagnetism, Line laser
Paleomagnetism plays a crucial role in unraveling plate motions and the transport mechanisms of volcanic products. Despite its significance, the studies of seafloor deposits have not progressed, primarily due to the difficulty of orienting seafloor outcrops. Several researchers have attempted to develop orientation methods using a manned submersible and remotely operated vehicle (ROV). Methods by Hurst et al. (1994) and Ueda (2012) focused on planar surfaces like dike margins, fault planes, and bedding planes, while Yamazaki et al. (2021) collected oriented cores using an ROV-based coring apparatus. However, these methods often require specialized equipment and are limited to outcrops with planar surfaces.
In response, we have developed a rapid and straightforward method for collecting oriented samples on the seafloor using the cross-line laser on the submersible Shinkai 6500. Unlike other methods, ours is not restricted to rocks with a smooth surface; it is equally applicable to rocks with rough surfaces. Additionally, we have successfully applied this method to the collection of oriented sediment cores. In this presentation, we show our experiences with this method, as implemented during the YK23-16S cruise in September 2023.
The cross-line laser, consisting of horizontal and vertical line lasers, was installed on the pan-tilt unit of Shinkai 6500. The relative position of the irradiated cross-line laser to Shinkai 6500 is recorded every second as position of the pan-tilt unit. The heading direction, pitch angle, and roll angle of the inertial navigation system (INS) of Shinkai 6500 are also recorded every second. The absolute laser irradiation orientation can be estimated from these positions of both Shinkai 6500 and the pan-tilt unit. Additionally, the orientation (strike and dip) of a plane perpendicular to the cross-line laser can be calculated. The sample can then be accurately restored to its original orientation based on the trajectory of the two orthogonal lasers to the sample and the orientation of a plane perpendicular to the two line lasers.
When collecting an oriented rock, the cross-line laser's center is focused on the target rock. The photographs of the rock irradiated by the cross-line laser are captured using both a single-lens reflex camera through the center window and the No. 2 camera attached to the pan-tilt unit of Shinkai 6500 to provide images from various angles, necessary for reconstructing the trajectory of the cross-line laser. The sampling operations are also recorded by movies with two external cameras. To find records of the positions of Shinkai 6500 and the pan-tilt unit, time when the photograph was taken must be precisely recorded. The target rock is collected using manipulators. In the case of collecting an oriented sediment, a push corer made of polycarbonate (100 mm in diameter and 500 mm in length) is used. The corer is inserted into the sediment using a manipulator, and the cross-line laser is irradiated to the inserted push corer, ensuring that the center of the laser focuses on the target. Both oriented rock and sediment core samples were collected within 10 minutes of starting the sampling.
After obtaining the rock samples, they are irradiated with a cross-line laser on shipboard. The laser path is accurately aligned with the in-situ irradiation position, referencing the photographs and movies taken during sampling. The trajectory of the laser is then meticulously marked on the rock surface using a paint marker, with an arrow symbol indicating the upward direction. The precise restoration of the rock sample to its original orientation is achieved based on both the traced trajectory of the laser and the orientation of a plane perpendicular to the two line lasers irradiated when the sample was collected. Results of paleomagnetic experiments of the oriented samples have been reported by Shibuya et al. (2024, JpGU) and Kawamura et al. (2024, JpGU).
In response, we have developed a rapid and straightforward method for collecting oriented samples on the seafloor using the cross-line laser on the submersible Shinkai 6500. Unlike other methods, ours is not restricted to rocks with a smooth surface; it is equally applicable to rocks with rough surfaces. Additionally, we have successfully applied this method to the collection of oriented sediment cores. In this presentation, we show our experiences with this method, as implemented during the YK23-16S cruise in September 2023.
The cross-line laser, consisting of horizontal and vertical line lasers, was installed on the pan-tilt unit of Shinkai 6500. The relative position of the irradiated cross-line laser to Shinkai 6500 is recorded every second as position of the pan-tilt unit. The heading direction, pitch angle, and roll angle of the inertial navigation system (INS) of Shinkai 6500 are also recorded every second. The absolute laser irradiation orientation can be estimated from these positions of both Shinkai 6500 and the pan-tilt unit. Additionally, the orientation (strike and dip) of a plane perpendicular to the cross-line laser can be calculated. The sample can then be accurately restored to its original orientation based on the trajectory of the two orthogonal lasers to the sample and the orientation of a plane perpendicular to the two line lasers.
When collecting an oriented rock, the cross-line laser's center is focused on the target rock. The photographs of the rock irradiated by the cross-line laser are captured using both a single-lens reflex camera through the center window and the No. 2 camera attached to the pan-tilt unit of Shinkai 6500 to provide images from various angles, necessary for reconstructing the trajectory of the cross-line laser. The sampling operations are also recorded by movies with two external cameras. To find records of the positions of Shinkai 6500 and the pan-tilt unit, time when the photograph was taken must be precisely recorded. The target rock is collected using manipulators. In the case of collecting an oriented sediment, a push corer made of polycarbonate (100 mm in diameter and 500 mm in length) is used. The corer is inserted into the sediment using a manipulator, and the cross-line laser is irradiated to the inserted push corer, ensuring that the center of the laser focuses on the target. Both oriented rock and sediment core samples were collected within 10 minutes of starting the sampling.
After obtaining the rock samples, they are irradiated with a cross-line laser on shipboard. The laser path is accurately aligned with the in-situ irradiation position, referencing the photographs and movies taken during sampling. The trajectory of the laser is then meticulously marked on the rock surface using a paint marker, with an arrow symbol indicating the upward direction. The precise restoration of the rock sample to its original orientation is achieved based on both the traced trajectory of the laser and the orientation of a plane perpendicular to the two line lasers irradiated when the sample was collected. Results of paleomagnetic experiments of the oriented samples have been reported by Shibuya et al. (2024, JpGU) and Kawamura et al. (2024, JpGU).