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We report the first observations based on acoustic imaging of large‐scale structure and time variability of buoyant plumes emanating from black smoker‐type seafloor hot springs. Three‐dimensional plume reconstructions were made from a digital data set of acoustic backscattering information recorded on a prototype submersible‐mounted sonar system. The acoustic images of two adjacent black smokers depict volume and show zones of flow organization (meters to tens of meters) in the lower 40 m of the buoyant plumes. The two plumes coalesce, bend in the prevailing current, exhibit short‐term (minutes) variation in cross section, and rapid (seconds) turbulent eddy variations at small scales (< 1 m). The plume imaging system is being developed for determination of plume dynamics, flux determinations when combined with chemical and thermal measurements, and long‐term monitoring of the activity of seafloor hydrothermal fields.

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During the November 1990 dive series of the US Navy DSV Turtle, plumes emanating from high-temperature black smoker-type vents were imaged using a prototype sonar system mounted on the submersible. The study focused on two plumes from adjacent sources that were scanned horizontally in arcuate sectors at increasing angular increments from the vent orifices up to 90°. Multiple data sets were recorded at horizontal ranges of 3–70 m from the base of the plumes and sonar ranges to 200 m with submersible stationary on the seafloor. Computer graphics were used to reconstruct the plumes in cross section and in 3D, revealing coherent plume images up to 100 m above the seafloor. Multiple plume cross sections recorded at the same level show changes in shape and distribution of suspended particulate matter on a time scale of seconds. Whole images show coalescing of the buoyant plumes and deflection by the prevailing current. The work demonstrates the value of acoustic imaging of plumes for initial detection and for characterization of plume dynamics.

Seafloor imaging for the purpose of seafloor characterizations and fine-scale feature identilication has traditionally been performed using towed sidescan systems. In the past 5 years, advancements have been made in using existing hull-mounted multibeam bathymetric systems as imaging systems. These hull-mounted systems are optimized for bathymetry, whereas the towed systems have been optimized for imagery.Two separate surveys were performed using both the University of Hawaii, Hawaii Institute of Geophysics (HIG) Acoustic Wide-Angle Imaging Instrument, Mapping Researcher 1 (HAWAII MR1 or MR1) system and the Navy's hull-mounted multibeam Sonar Array Survey System (SASS) over the same area. The MR1 operates at 11 kHz (port) and 12 kHz (starboard), transmitting and receiving on two rows of elements mounted on each side of the tow body. It has a variable pulse length ranging from 0.5-10 msec, and a sample rate of 1 kHz. In contrast, the SASS system has a hull-mounted transmit array with a multiple element receive array, pulse lengths from 3-7 mwc, and a sample rate of 333

ABSTFACTMultibeam bathymetric systems offer t o r e v o l u t i o nize our understanding of the ocean bottom and i t s major s t r u c t u r a l f e a t u r e s by providing highljr detailed neasurelcents of the bottom. F u r t h e r , multibean measurements yield bottom depth 2s a function of both spztiai coordinates (i.e. alongt r a c k 2nd a c r o s s -t r a c k ) . The depth resolution is s u f f i c i e n t l y f i n e t h a t c o n v e n t i o n a l c o n t o u r c h a r t s vould never incorporate a l l the information availa b l e i n t h e a u l t i b e a n data. As a n a l t e r n a t i v e t o s,Jch de'erminlszic presentations, i: can be suggeste d t h a t s t a t i s t L C e l c h a r s c t e r i z a t i m s ' De developed f o r t o p o g r a p h i c v a r i a b i l i t y a t h o r i z o n t a l s c a l e s which a r e small i n comparisor. t o t h e o v e r a l l dimensions of the feature of i n t e r e s t . Ve here discuss the stlldy of roagh topography on several seamounts by means of multibean bathymetry. The bottom topography I s analyzed in terms of numerous samples of 1503 m x 1600 n extent. A. primary quantity of intere s t i s t h e rms value of t h e r e s i d u a l :opogra?hy. The AwAt;erc of v a r i a b i l i t y f o r t h e rzs values i s 3es.ribed for several seanoucts and shown t o have c e r t a i n o v e r a l l similerities. INTROCLJCTIOPi Multibeam bathymetric swath napping systems such as SeabPam a r d S.A.S.S. o f f e r t o r e v o l u t i o n i z e our understanding of the ocean bottom and i t s major s t r u c t :~r a l f e a t u r e s by p r w i d i n g h i g h l y d e t a i l e d measurements of t h e bottom. P r i o r t o t h e 6 e v e l o pment of these systems xost af the bathynetric i n f o r m t i o n e x i s t e d i n t h e form of wide beam echo-sounder surveys from which only botton information i n t h e a l o n g t r a c k d i r e c t i o n o f t h e s:lmejr could be readily extracted. MLtibeam systems, however yield bottom depth as a function of b o t h t h e a l o n g t r a c k a n d c r o s s t r a c k d i r e c t i o n s of a silrvef and as a r e s u l t p r o v i d e a h i g h l y d e t a i l e d two-dimensional description of the ocean floor. h r t h e r m o r e , t h e d e p t h reso1u:ion of modern multibeam systems i s s u f f i c i e n t l y f i n e t h a t c o n t o u r c h a r t s et conventional scales of p r e s e n t a t i o n w i l l probably never incorporate a l l of t h e i n f o r m t i o n a v a i l a b l e f r o n t h e system. A s a n a l t e r n a t i v e t o such Presentations, it can be suggested that s t a t i s t i c a l c h a r a c t e r i z a t i o n s ought t o be developed f o r t o p o g r a p h i c v a r i a b i l i t y a t h o r i z o n t a l s c a l e s whlch a r e small i n comparison t o t h e o v e r a l l dimensions of t h e l a r g e s c a l e f e a t u r e s of i n t e r e s t . Thus the information available from multibeam bathyaetric systems can be divided into two c h a r a c t e r i z a t i o n s , one d e t e r m i n i s...

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