Near-bottom Survey Research Articles

Depth-based pseudo-terrain-following navigation for cruising AUVs

This paper presents an approach of depth-based pseudo-terrain-following navigation for underactuated cruising AUVs subjected to near-bottom survey missions. Based on the waypoint-based path planning, our approach provides a set of optimal waypoints by following which a cruising AUV accomplishes an effectual near-bottom flight avoiding bottom collision. In deriving our waypoints, we solve an optimization problem. The problem formulation employs the performance index as a metric for the deviation between the AUV flight trajectory and the along-track altitude envelope of a terrain. An inequality constraint of the minimum allowable altitude is incorporated into our optimization. Moreover, considering the motion-constrained characteristics of a dynamic system, a state–space model describing the motion of a specific vehicle is also incorporated into the optimization. As the solution procedure a GDS-based search algorithm is used. By applying the algorithm, the optimal reference depths of the waypoints predefined within a horizontal plane are obtained. Being a depth-based control approach providing superior angular motion stability, our pseudo-terrain-following navigation is particularly suited to acoustic bottom mapping and imaging. The feasibility and efficacy of our approach are demonstrated through the application of optimal waypoints to an actual near-bottom survey mission using our cruising AUV.

Near-bottom survey of hydrothermal sulfide on an extremely magmatic accretion segment along an ultraslow-spreading ridge

An extremely magmatic accretion could exist along an ultraslow-spreading ridge that has been approved and surveyed, but less attention has been paid to its associated hydrothermal system, as it is supposed to be similar to those of fast/medium-spreading ridges. Here using an autonomous underwater vehicle and a human-occupied vehicle, a systematic investigation was conducted on the hydrothermal system of an extremely magmatic accretion segment along the Southwest Indian Ridge (50°28′ E). A collapsed chimney, microbial mat, and diffused fluid were observed in the old Duanqiao hydrothermal field (DQF), which was earlier a weakly active hydrothermal area. Notably, evident hydrothermal anomalies, including oxidation–reduction potential (ORP), turbidity, and methane, were detected above the new DQF, which lies southwest of the old DQF. Based on this investigation, we concluded that ORP is one of the most effective parameters available to identify vent location by analyzing each type of detected hydrothermal anomaly and the near-bottom magnetic inversion can image the suspected hydrothermal fluid circulation pathways. In general, we believe that the new DQF is controlled by a magmatic chamber different from those found on fast/medium-spreading ridges, and could be a large hydrothermal system.

Navigation Strategies of a Cruising AUV for Near-Bottom Survey of a Steep Terrain

Strategies of AUV navigation for achieving the near-bottom survey of a steep terrain are addressed. Unlike a flat terrain, a steep terrain can disrupt an altitude-based bottom-following dive of a cruising AUV. During its near-bottom dive over a steep terrain, a cruising AUV possibly experience longitudinal motion instability accompanied by severely fluctuating heave and pitch which may lead to bottom collision of the vehicle. Based on dive simulations, it is shown that the lost bottom lock of a sonar altimeter and the altitude overestimation are two major sources of the longitudinal motion instability. Underwater acoustic physics interacting with the sea water and bottom, as well as the coupled 3-d.o.f. longitudinal vehicle dynamics are incorporated into our simulation model. As alternative strategies for pursuing more reliable AUV-based near-bottom survey, the slope-following navigation and the pseudo bottom-following navigation have been investigated.

Genetic Algorithm inversion of geomagnetic vector data using a 2.5-dimensional magnetic structure model

We propose a new inversion method for vector magnetic field data, which uses the Genetic Algorithm in a space domain calculation to determine the best-fitting 2.5-dimensional (2.5-D) structure. This 2.5-D model is composed of magnetic boundaries with arbitrary strike and magnetic intensity. Two numerical formulas combine to express this model. One of them is a two-dimensional magnetic structure expression for a realistically shaped magnetic layer, and the other is a magnetization contrast expression for magnetic boundaries of variable strike. We use a Genetic Algorithm as the computational technique that supports optimum solutions for magnetization, magnetic strike, and boundary location. In practice, calculations are more accurate in the space domain instead of the more conventional frequency domain because it better preserves the short wavelength components and the true geometry between magnetic sources and observation points even for uneven survey track lines. The above leads to high resolution in the inferred magnetization without the need of upward continuation, which is particularly useful for inverting near-bottom survey data. The code is designed to use smaller storage and less computational time. Its application to synthetic data illustrates the power of resolution and precision in interpreting the fine scale processes of mid-ocean ridge accretion.

Structural asymmetry of the TAG Rift Valley: Evidence from a near‐bottom survey for episodic spreading

Comprehensive high‐resolution sidescan sonar imagery, swath bathymetry, and photographic data from the TAG rift valley (Mid‐Atlantic Ridge, 26°N) suggest significant asymmetry in crustal accretion. Discontinuities and asymmetry in structural and volcanic morphology and age distribution about the spreading axis suggest a recent episode of preferential accretion of crust to the east throughout the central 10 km of this spreading segment. Such episodes may contribute to the long‐term spreading asymmetry in the area.

Erosion and deposition on the eastern margin of the Bermuda Rise in the late Quaternary

A near-bottom survey has been made on the Eastward Scarp (32°50′N, 57°30′W) of the Bermuda Rise, which rises 1150 m above the 5500-m deep Sohm Abyssal Plain in the western North Atlantic. The survey reveals evidence of erosion and deposition at present and in the late Quaternary by the deeper levels of the westward flowing Gulf Stream Return Flow. Four distinct regions of increasing bed gradient show increasing sediment smoothing and scour in the transition from plateau to abyssal plain. Bedforms observed are current crescents, crag and tail, triangular ripples, elongate mounds, transverse mud ripples, lineations, and furrows ranging from 10 to 1 m or less in depth, decreasing generally with bed gradient. Measured near-bottom current speeds are up to 20 cm s −1. Temperature structure on the lower, steep, slopes suggests that detachment of bottom mixed layers may occur there. Extensive net erosion appears to be confined to the lower steep slopes of the scarp. Reflection profiles (4 kHz) show that there has been erosion in areas thinly draped with recent sediments and in areas that show development of small scarps. The distribution of subsurface acoustic characteristics of the region corresponds broadly to the areas characterized by bed gradient and distinct sedimentation conditions. Subsurface hyperbolae, possibly caused by buried furrows, show furrow persistence through several tens of metres of deposition. Erosion occurs up to the top of the scarp during episodes of presumed stronger currents, which may correspond with intensified circulation during glacials.

The oblique intersection of the Mid-Atlantic Ridge with Charlie-Gibbs transform fault

The junction angle between the western Charlie-Gibbs transform fault and the spreading axis of the Mid-Atlantic Ridge diverges by 40° from the orthogonal intersection assumed in many studies of plate boundaries. This has been established by a surface-ship reconnaissance and by mapping fault trends in a transponder-navigated deep-tow survey of the fracture valley 25 km from the intersection. One set of normal faults trends 325–330°, parallel to the obliquely spreading ridge axis, and another set trends 275°, parellel to the direction of relative plate motion. Although the near-bottom survey was in the theoretically inactive part of the fracture zone, beyond the transform fault section, there is evidence for recent motion on faults that cut the thick sediment fill of the fracture valley. Oblique spreading of a ridge axis near a transform fault may result from distortion of the regional stress field by a strike-slip couple. Tension parallel to the long axis of the strike-slip strain ellipse, which is responsible for oblique normal faulting in transform valleys, causes oblique dike injection and oblique faulting in the axial rift valley. These effects extend further from transfrom fault intersections on slow-spreading ridges than on fast-spreading rises.

A new undersea geological survey tool: ANGUS

A towed, near-bottom survey system termed ANGUS (Acoustically-Navigated Geological Undersea Surveyor) has been developed for geological and geophysical studies in the median rift valley of the Mid-Atlantic Ridge. System communication is solely acoustic and incorporates a relay transponder attached to either a camera fish, rock dredge, corer, or heat probe device with a shipboard transducer and a seafloor transponder array. The devices can be located with a precision of 5 to 10 m along a simultaneously collected high-resolution bathymtric profile. Automatic digital computer processing of the acoustic navigational information permits real-time monitoring of the path of the device on shipboard graphic display units. Approximately 60,000 high-quality photographs, 70 rock dredges, and 100 heat-flow observations have been collected and related to seafloor features with relief as small as 20 to 30 m. The basic camera fish allows for the addition of other geophysical instruments. Also, expendable sonobuoys have been positioned precisely to locate microearthquakes.

A near-bottom survey of lineated abyssal hills in the equatorial pacific

A 250 km2 area of abyssal hills in the vicinity of 14°N, 126°W (between the Clarion and Clipperton Fracture Zones in the Equatorial Pacific) was surveyed in detail using an instrument package towed close to the deep sea floor, the MPL Deep Tow device. Both topography and near bottom magnetic field are lineated perpendicular to the major fracture zones. Except for a few localized depressions, the sediment surface is generally smooth and of low relief with maximum elevation differences of 200 m and slopes of six degrees. Several small graben-like troughs and depressions were observed, most of them near the crest of one abyssal hill. The largest trough is two kilometers long, 250 m wide and 50 m deep with steep sides (>30°). These troughs are tentatively interpreted as the result of tensional separation at the tops of the hills caused by down-slope creep and consolidation of the pelagic sediments.