1. A system for measuring a mechanical property of a sea floor sediment at full ocean depth, comprising an overwater monitoring unit and an underwater measurement device, wherein the underwater measurement device comprises an observation platform and a measuring mechanism carried on the observation platform;the observation platform comprises a frame-type body and a floating body, a wing panel, a height measuring device, a floating ball cabin, a leveling mechanism, a counterweight, a release mechanism, and an underwater acoustic communication device mounted on the frame-type body; wherein
the height measuring device is used to detect a height of the underwater measurement device away from the sea floor;
the floating ball cabin is in a shape of a floating ball, for sealing a circuit system while providing buoyancy; the circuit system communicates with the overwater monitoring unit through the underwater acoustic communication device, for uploading the height of the underwater measurement device away from the sea floor and measurement data of the mechanical property;
four wing panels are mounted on the frame-type body, which are distributed around the frame-type body and are located below the floating body, and a distance between the wing panel and bottom of the frame-type body is ? of the total height of the frame-type body; each wing panel is designed as a streamlined wing surface, with an inner side hinged with the frame-type body; each wing panel is provided with two slow-descent cylinders which are hinged on left and right sides of a bottom surface of the wing panel, and one end of the slow-descent cylinder is hinged to the frame-type body, and the other end is hinged to a bottom surface of the wing panel; when the height of the underwater measurement device away from the sea floor reaches a set height, the overwater monitoring unit issues a slow-descent instruction to control the wing panel to open outward relative to the frame-type body, and to adjust an extension length of a piston rod of the slow-descent cylinder according to a submerged depth of the underwater measurement device, so that an opening angle of the wing panel is adjusted and the underwater measurement device is controlled to carry out multi-speed slow-descent until the measurement device lands on the ocean floor stably;
the leveling mechanism comprises multiple leveling legs and multiple leveling cylinders; the leveling legs are located on the bottom of the frame-type body, and each leveling leg is connected to a leveling cylinder; the floating ball cabin is provided therein with an attitude sensor, for detecting an attitude of the frame-type body, and generating attitude data to be sent to the circuit system; when the frame-type body reaches the sea floor, the circuit system controls the leveling cylinder to drive the leveling leg to extend or retract according to the received attitude data, so as to adjust the frame-type body, making the frame-type body stand horizontally on the sea floor;
when a measurement operation performed by the underwater measurement device is completed, the overwater monitoring unit issues a jettisoning instruction to control the release mechanism to discard the counterweight, and control the wing panel to close, so that the underwater measurement device ascends to the surface under the buoyancy of the floating body; the release mechanism comprises a release cylinder, a fixed pulley, a cable and a hook; the fixed pulley is mounted on the frame-type body, and is wound with the cable; one end of the cable is connected to the release cylinder, and the other end is connected to the hook; in a default state, the hook extends into a lifting hole of the counterweight to hook the counterweight, so that the weight of the observation platform is increased, causing the underwater measurement device to descend to the sea floor by itself; when the underwater measurement device is recovered, the circuit system controls the release cylinder to pull down the cable, which causes the hook to rotate under self-weight and detach from the lifting hole of the counterweight, thereby separating the counterweight from the frame-type body;
a hydraulic station is mounted at a center position of an orifice plate on the bottom of the frame-type body, and communicates with the slow-descent cylinder, the leveling cylinder, and the release cylinder respectively through different oil tubes; each oil tube connected to each cylinder is provided with a solenoid valve; when a certain cylinder needs to be controlled to work, the solenoid valve on the oil tube connected to the controlled cylinder can be opened by the circuit system, then the hydraulic station is controlled to supply or pump the oil to the controlled cylinder to control a piston rod of the controlled cylinder to extend or retract;
the measuring mechanism comprises one or more of a cone penetration measuring mechanism, a spherical penetration measuring mechanism, and a vane shear measuring mechanism for measuring the mechanical property of the sea floor sediment, and/or a sampling mechanism for collecting a sample of the sea floor sediment.