NEW METHODS IN HYDROGRAPHIC SURVEY

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1 International Hydrographic Review, M onaco, L V I (2 ), J u ly NEW METHODS IN HYDROGRAPHIC SURVEY by Anthony G. STEPH E N SO N Gardline H ydrographic Surveys, Inc. P a p e r first p u b lish e d in Sea Technology, V o l. 19, No. 3, M arch 1978, A r lin g to n, V a, U.S.A., and rep rin ted by kin d perm ission o f the E ditor. An ambitious engineering project to lay pipelines across the Straits of Florida from W e st Palm Beach to the northwestern tip o f Grand Bahama Island required a detailed survey of the Baham ian Trench and o f the very steep fa ll-off into it from the Grand Bahama Island. The initial approach was to obtain a very perfu nctory know ledge of the general trend o f the slope off the bank from Freeport to the northwest tip of the Grand Bahama Island and northw estw ard for 8 miles to M em ory Rock. These lines w ere to be used p rim arily if the optim um area showed no prom ise o f even slopes off the bank (fig. 1). Follow in g the coarse survey the plan was to run sounding lines perpendicular to the contours at approxim ately quarter m ile spacing and compare the records to observe gradient changes. These lines w ould be supported by side scan sonar lines parallel to the contours to develop a sonic picture of slope variations, outcrops of rock or coral, or cliffs. Should areas o f lesser gradient or sm oother surface be located by this tool, detailed bathym etry was to be undertaken in the areas o f interest. On the basis o f all this inform ation a route w ould be selected and used to take a visual look at the conditions of the seabed. The horizontal control fo r the survey was com plicated due to, firstly, the w ide area over w hich control was required and, secondly, the lack of suitable land areas north of the island. T o control the survey by the conventional range/range method w ould have required lengthy search for very old triangulation monuments and a subsequent land survey to develop suitable points on w hich to locate the shore stations. Though arduous and time-consuming, this could have been achieved along the southerly coast o f the island, but was not possible north o f the island w ithout using a sm all boat to gain access to the sm all islets. These problem s w ere recognized prior to m obilization and it was decided to use a H ydrobar, a range/azim uth positioning system that was origin ally developed fo r the control o f hydrographic surveys in narrow waterw ays, in order to reduce the large amount o f shore control required w hen using

2 range/range positioning. Hydrobar is a semi-automated system in which the angle measured by the theodolite is transmitted to a receiver onboard the vessel. The observer first zeroes the instrument on a reference object of known bearing and then, as he tracks, the bearing of the vessel is transmitted to it, either continuously twice a second or on command, as he presses a pushbutton at the m om ent of the fix. Range is measured independently, by any o f the common m icrowave systems, to a shore station placed over the same survey monument as the theodolite tripod, so that aboard the vessel both the range and bearing from a single point are available. One range of a Trisponder system was used fo r the survey. The bearing discrimination is O'O'.Ol better than 1 foot per m ile and the position lines always cut at 90, so that at short ranges the system gives better accuracy than any other electronic system. There was, however, considerable software manipulation required to achieve the inherent good accuracies. This was due to the fact that the range bearing and depth arrive at the receiver asynchronously and have to be m arried to the true tim e o f transmission. The method was ideally suited to the area to be surveyed in the Bahamas as the whole survey area could be controlled from one point. The H ydrobar system w as used in conjunction with Hydrocarta, a

fu lly automated hydrographic data acquisition and plotting system. F ollowing each day s work, it was essential to plot up the data so that decisions could be made on the area to survey the next day.

3 fu lly automated hydrographic data acquisition and plotting system. F ollowing each day s work, it was essential to plot up the data so that decisions could be made on the area to survey the next day. The Hydrocarta system accepts most positioning systems including the range/azimuth Hydrobar; the position values are read as often as they are available, typically once a second, and converted into x and y coordinates. These positional updates are used to plot the boat s position continuously on a previously prepared chart which may be on any scale. The Hydrocarta plotter is shown in fig. 2, and peripheral sounding and side scan equipment with it in fig. 3. F ig. 2. H ydrocarta plotter. F ig. 3. P lo tte r (upper righ t) w ith periph eral survey equipm ent.

4 T h e rem ote left/ rig h t disp lay gives guidance to the helm sm an, also bein g updated on each com pu ted postplot. T h e system is in terfaced to an echosounder throu gh a d ig itizer and records every depth, w ith tim e and position once a second, on m agnetic tape. A t any in terval o f tim e or distance a fix is generated, d raw in g a cross on the ch art and p rin tin g out the tim e, fix num ber, range, angle, x -y position and depth. Sim ultaneously, event m arks are draw n au tom a tica lly on the echosounder and side scan sonar analogu e records. A t the end o f the day, a fter the observed tide readings are available, an o fflin e p rogram reduces the depths to chart datum, selects soundings and plots them in their tru e position to any scale, w ith options as to the fo rm o f ed itin g requ ired and the size o f soundings. Since every depth is available to the com pu ter at this stage, it is able to distingu ish betw een bottom echoes and false echoes w ith high re lia b ility and the resulting chart has a greater accu racy than can be obtain ed b y hand. F ig. 4. H eal tim e tra ck chart, hand an n ota ted, Crosses are k eyed to a u tom a tic p rin tou t o f survey con trol and depth data. W h ile m obiliza tion o f the equ ipm ent w as bein g undertaken onboard a ch artered boat (fig. 5), a search fo r an established con trol m onum ent was m ade in the Baham as. A m ark w as located at Settlem ent P o in t that had the advantage o f seeing all o f the area o f p rio rity, but it w as on ly ju st above sea level. T o obtain a greater lin e-of-sigh t, a w ooden towter 16 feet high w as built over the m ark and com pleted p rior to the survey vessel s a rriva l in F reep o rt (see fig. 6). T h e sophisticated com bin ation o f position in g and data collection, h ow ever, w as o f little avail w hen out o f visib ility range. Th u s the in itia l reconnaissance lines run into the coast, w o rk in g north from the south o f the area, w ere con trolled by sextant, radar and azim uth circle, p lottin g

5 F ig. 5. Chartered ship u tilized in survey. S g m m 1'ic,. <>. Survey control tow er. by hand on the largest scale h ydrographic chart until approxim ately 5 m iles a w ay fro m the control station, w hen the vessel becam e visible. T h is first day o f sounding ended at the com m ercial dock o f the Grand Baham a H otel at W e s t End. T h e next day the w ork continued north at one m ile and 1/2 m ile intervals to 8 m iles north o f Settlem ent P oin t. These long distances m ade apparent the variable visib ility conditions from day to day. T h e observer on the tow er could only see the boat about 5 m iles on the first clay, though range and azim uth transm issions w ere received 17 m iles aw ay, but on the second day he had no trouble in seeing the boat at 8 miles.

6 W h ile the boat returned to dock each evening, the day s soundings w ere plotted up and processed, along w ith the side scan runs. It became very apparent that the in itial dropoff o f the bank was almost sheer from 150 to 270 feet in most o f the survey area but there was evidence o f areas o f smoother slopes. These areas were then detail sounded w ith lines 50 meters (164 feet) apart. The side scan sonar was less effective over the slope area than w here the bottom was flat, but it did show clearly w here the cliffs w ere to be found. The project to this stage included nearly 100 survey lines and nearly 50 m iles o f side scan lines, and had taken eight days on site. A ll in form a tion had been plotted and contoured. Th e final stage for this investigatory survey was to bring in the subm ersible to take video pictures o f the areas o f interest down to about 1,000 feet. The survey vessel returned to M iam i to m obilize the submersible w h ile the survey data tapes w ere taken to the office in Houston fo r preparation o f the final charts. The client required both a conventional hydrographic chart and crosssections from w hich he could compare the bottom slope on different lines. It was difficult to do this on the echosounder records because variations in the current, w ind and boat speed changed the horizontal scale. It was decided to draw up true to scale sections o f each sounding line to obtain a realistic understanding o f the degree of slope. Th is w as done directly from the data tapes through another offline program, designed fo r dredging surveys, that takes the position and depths recorded and plots the profile to any com bination o f vertical and horizontal scales (see figs 7 and 8). M ore than eighty sections w ere developed and drawn in India ink on plastic film. The general trend o f the slopes could easily be seen but it was also apparent that erroneous digitization of the depth had occurred in some areas. This w as due to tw o factors. Firstly, although as can be seen in fig. 9 the outboard mount fo r the echosounder transducer was gim baled to reduce errors as the boat rolled, the narrow 7 beam o f the transducer was sweeping up and dow n the steep slope as the boat pitched, causing a ragged seabed appearance, and secondly the side lobe o f the sonic beam from the transducer was periodically digitized causing a sudden reduction in depth, though still follow in g the seabed profile. This was not so evident in the bathym etric charts, due to the editing process and the sm aller scale. Th e section, how ever, drawn at a scale 1 : plotted all soundings returned, 4 per second, showing all the im perfections actually received. Fortunately, the human eye can do its own editing and draw a line through the serrations o f the sw inging transducer beam and perceive the true trend below the side lobe effect. W h en laid in order on top o f each other, the sections can be seen successively through the plastic film and yield a three dim ensional effect. In this w ay, it was noted that even in locations that appeared to have little or no sudden drop, profiles close by showed evidence o f steeper slopes. It was thus proven that though some areas had smooth slope conditions, th ey w ere very localized.

7 T o CN o ya -5 ja o T3 aes 45 «S b C '5 o J5 F ig. 8. Profile of optimum slope conditions. 0^eïïsssasiÿs!8sss!csffiss = is?

I* io.!). Outboard mount for echosounder transducer. Fi g. 10. A portion of a chart drawn by Hydrocarta. Scale is 1/5 000. Depths i n fathom s and tenths o f fathoms.

8 I* io.!). Outboard mount for echosounder transducer. Fi g. 10. A portion of a chart drawn by Hydrocarta. Scale is 1/ Depths i n fathom s and tenths o f fathoms. W h i l e th is fa c t w a s b e in g o b s e rv e d in H o u s to n b y th e d r a w n sections it w a s a ls o b e in g p ro v e n b y th e s u b m e r s ib le in th e B a h a m a s. D iv in g in the selected a re a s, it re c o rd e d sm o oth to steep v a ria t io n s in the b o ttom slo p e. T h e s u b m e r s ib le also n o te d c o ra l o u tc ro p s o f u p to 10 feet in lo c a lized are a s.

9 The bathymetry and positioning was undertaken by two hydrographic surveyors onboard one vessel and the Hydrobar station was manned by a local surveyor. Other personnel onboard in charge of the sub-bottom profiling and side scan sonar were under a separate contract. No draftsman or hand plotting by the surveyors was required. The preliminary charts for each day s work took approximately one hour to draw up automatically (fig. 10). This survey operation showed that modern survey equipment, with the capability of providing a variety of data presentations for onsite decision-making, adds a new dimension to site surveys. It also showed, however, that a high degree of professional judgment, with awareness of each piece of equipment, is also necessary if the best use is to be made of the information obtained. A QUESTION OF VALUES The in crease in m an-m ade disasters on a w o r ld w id e scale h ighlights that tech n ology has ra c e d ah ead of com m on sense. T h e w o r d te ch n o logy, as used today p a rtic u la rly in p o litic a l corrid ors, cam e fro m the 17th century Greek sign ify in g art, shape and construction. W h a t skilled c arp e n ter w o u ld send a tru ck c a rry in g tons o f volatile explosives d o w n a coun try lane to save a $ 15 toll at the expense o f 200 lives? W h o is there to evaluate the total lack of com m on sense su rro u n d in g m ovem ents of sim ila rly dan gerous substances all over the globe? W e have becom e spectators of tragic events that w ill jeopardize the existence of our great gran d ch ild ren. L o o k at the values in the oceans, w h e re m illions o f tons of h y d ro c a rb o n s are c a rrie d from p lace to p lace and w h e re the w o r d b o n a n z a is used fo r o il discoveries. The extraction of fossil fuels is pressed fo rw a r d at a ra p id pace by profligate waste, both in u n prod u ctive energy expen d itu re an d in pollution caused b y so-called accid en ts". A c c id e n t is defin ed as an event w ith out ap paren t cau se, but the causes o f most catastrophes at sea are too hum an and all too apparent. This repetition and p ro life ra tion w ill continue if the la w s of m aritim e countries rem ain unchanged. T h e gam e of roulette p layed by vessels c a rry in g lethal cargoes w ill soon re ach a crescendo if n oth in g is done to change the present chaos. M arine in su ran ce k n o w s it too w ell, and a study o f ship casualties becom es a ch illin g chore. (C ontinued on page 40)

10 A QUESTION OF VALUES (contin ued from page 39) The u ltra -ra p id b u ild in g o f ultra-large tankers w a s not fo llo w e d by ultra-efficient train in g of men to m an them. T h is p au city of good seam en is com pou n ded b y boredom at sea w h e re m any duties trad itio n ally p e rfo rm e d b y men are n o w autom ated. I f one adds to this lam entable chain o f events that only a sm all portion of w o r ld w a te rs have been su rveyed since the days o f leadline, one begin s to com prehend the urgency o f early action. T h e skill o f a m aster a n d his officers is ju d g e d by p eople w ith m in im al k n o w le d g e o f the seas an d w h o rely on in su ran ce policies to keep them fin an cially buoyant. T h e w e ll-m a n a g e d fleets w ith exp erien ced m asters and c re w suffer com petition from ill-trained foolish ones. Both on la n d an d at sea transportation of dangerous chem icals sh ould be p rerou ted and escorted by police, Coast G u a rd or m ilitary w h e n in congested w a te rs into their ports o f destination. So it is a question of values of o rd er o f m agnitude. H o w m any m ore tankers c a rry in g oil are to sp e w their cargoes into the a lre a d y polluted oceans fo r w a n t of o rd in a ry com m onsense precautions? H o w lon g must h y d ro g ra p h e rs fa c in g a m am m oth task contem plate grossly un seam anlike n avigatio n al practices costing billions, w h en they are starved of m illions to do their vital w ork? W e have only a short tim e to learn our values. Excerpts fro m a contribution by James D a w s o n, a London Insurance Broker, to Sea Technology, September 1978, b y kind perm ission o f the Editor.

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