COMPUTER PRODUCED PROFILES OF MICROTOPOGRAPHY AS A SUPPLEMENT TO CONTOUR MAPS by P aul J. G rim <*», George H. K e l l e r, and R obert J. B a r d a y <**> A tlantic O ceanographic and M eteorological L aboratories Miami, F lorida ABSTRACT The principal products of deep sea bathym etric surveys are contour m aps depicting the relatively larger featu res of the floor. U nfortunately, however, these m aps can n o t display m ost of the sm all scale fe atu res (m icrotopography) th a t are recorded on fath o g ram s. K now ledge of this m icro topography and some m eans of displaying it are increasingly im p o rtan t as m an develops a greater need for utilization of the sea floor. Since fu tu re surveys are likely to be autom ated for data reduction, an econom ical and quickly produced supplem ent to contour m aps can be com puter produced profiles th a t clearly disp lay the details of the bottom top ograph y. Illu s tra tions based on a recent narrow -beam echo sounder survey clearly point out the benefit from presenting bathym etric data in other th an m ap form. INTRODUCTION Man is rap id ly developing his technology to the p o in t w here exp loitation of the sea floor will be a com m on occurrence in all depths of the oceans. One of the tools m ost needed for this will be an accurate knowledge of the undersea topography. T his m ust be know n if m an is to lay pipelines and cables, construct bottom m ounted stru ctu re s or conduct sea floor m ining activities. However, bathym etric contour m aps of large portions of the deep ocean do not show the exact n atu re of the sea floor because these areas are often characterized by sm all scale features (here referred to as m icrotopography) having lateral dim ensions m uch less th an the sep aratio n of the survey track lin es. A know ledge of th e n a tu re of (*) Currently at the National Geophysical Data Center of NOAA s Environmental Data Service. (**) Currently at the Department of Oceanography, Oregon State University, Corvallis, Oregon.
this m icrotopography is the very inform ation th at will be m ost valuable for som e aspects of m a n s u tilizatio n of the sea floor. The purpose of this paper is to point out th at in m any oceanic areas, b ath y m etric profiles show ing the m icrotop ograp hy are valuable su pplem ents to contour m aps. W ith the use of com puters and autom atic X-Y plotters, these profiles can be produced rapidly and econom ically since it is likely th a t m ost deep sea soundings collected in the fu tu re will be digitized for co m pu ter processing. The profile presentation of bathym etry is p articularly m eaningful if, w hen conducting the survey, a narrow beam echo sounder is used. Such a sounding system obtains a m uch greater degree of detail than is norm ally possible w ith a conventional wide beam sounder (see, for exam ple, K r a u s e and K a n a e v, 1970) th u s providing a very close approxim ation of the sea floor relief even in the ro u ghest terrain s. EXAMPLES FROM A SURVEY SOUTH OF PANAMA AND COSTA RICA To develop the argum ent presented in this paper, bathym etric data selected from a recent deep-sea survey are used. These data serve to illu stra te : ( 1 ) the ch aracter of typical m icrotopography as show n by narrow beam echo sounder fathogram s, (2 ) the difficulty or near im possibility of show ing in a m eaningful m an n er the n atu re of the sea floor in certain areas w ith contour m aps based on the trackline spacing th at will likely be used in fu tu re deep sea surveys of significantly large regions of the ocean floor and (3) how the m icrotopography can be show n w ith com puterproduced profiles quickly and econom ically if digital m ethods are used for red u cin g the survey data. The survey was conducted in the equatorial Pacific Ocean (figure 1) in A ugust 1969 by the N ational Ocean Survey ship Oceanographer. A pproxim ately 11 200 kilom etres ( 6 000 nautical m iles) of trackline data w ere obtained w ith satellite navigational control centered on the P anam a fractu re zone, a seism ically active fau lt zone trending north -so u th between 82 and 83 W (M o l n a r and S y k e s, 1969). The basic survey p attern was a grid w ith tracklines oriented north-south and east-w est at one q u arter degree in terv als (about every 28 kilom etres, or 15 n au tica l m iles). B athym etric and m agnetic resu lts of this survey, including d ata reduction techniques, have been discussed by G r im (1970a). All depths have been corrected for v ariatio n s of sound velocity using M atthew s tables (1939). The trackline density of this survey is about the sam e as that which has been used in the past, and proposed for future system atic surveys in the deep ocean. See, for exam ple, the coverage of NOAA s north Pacific SEAMAP survey given by R yan and G r im (1968).
F i g. 1. Survey area. The stippled part of the area is characterized by extremely rugged topography. Computer produced bathymetric profiles are given for tracklines A and B in figures 5 and 6. NARROW BEAM ECHO SOUNDER FATHOGRAMS D epth m easurem ents w ere obtained w ith a gyro-stabilized n arro w beam echo sounder (20 000 hertz w ith a 3 beam w idth betw een 3 d B points and 6 between 15 db points) and recorded on a precision b ath y m etric recorder. The records w ere digitized at 5-m inute intervals (about every 2.3 km, or 1.25 n. m iles) and at additional tim es (peaks, deeps, an d slope changes) judged necessary to show the details of the sea floor on the subsequently produced com puter plots. Plates 1 and 2 display portions of the survey fathogram s w hich reveal areas of highly irre g u la r relief. These fathogram s clearly show th a t the very ab ru p t changes in the topography are faithfully recorded w ith a narrow -beam sounder. W ith conventional w ide-beam echo sounding equipm ent the straig h tn ess of the scarps and the existence of the narro w deeps w ould not be seen. D iscussions of directional echo sounding an d how these resu lts com pare w ith those of wide beam sounders can be found in C o h e n (1959) and K r a u s e and K a n a e v (1970).
PROFILE A R [XJd Ni k! 1mi i 8 i s % <g' U à 1 '>*
PROFILE Portions of narrow beam echo sounder fathogram records corresponding to parts of profiles A and 15 in figures 5 and (i respectively. Vertical exaggeration about 35/1. Vertical black lines were made automatically every 5 minutes and at fix times and other events. The fathogram in plate 2 is printed in reverse so that its orientation is the same as profile B in figure (i.
LOSS OF TOPOGRAPHIC DETAIL ON A CONTOUR MAP In portions of the survey area it is difficult or im possible to contour accurately the sea floor because of the m icrotopography. Such a terrain is located in the southern p art of the survey area. The com plicated n atu re of the bath y m etry owing to such m icrotopography is clearly shown by the series of profiles in figure 2. E ach trackline plot represents the average depth of the soundings obtained along th at p articu lar segm ent. W ith one exception these averages lie betw een about 2 900 m etres and 3 300 m etres (the exception being the sh ort east-w est segm ent at 3.5 N betw een 83.5 W F ig. '2. Bathymetric profiles from southern part of survey area presented along tracklines (see figure 1). The blackened portions of the profiles represent depths shoaler than the average depth along each trackline segment. Vertical exaggeration for each profile is about 20/1. Relief of the microtopography is given by the 1 000-metre scale. The fathogram record between E and E' is shown in plate 2.
Fig. 3. Alternating ticks normal to the tracklines indicate locations of 100-metre contour lines. Dots show locations of bathym etric peaks. Deeps are not indicated (see inset and text for explanation). and 84 W w hich has an average d ep th of about 2 700 m etres). T h e p o r tions of each profile shoaler th a n the average d ep th are blackened. M ost of the depths w ith in the a rea of figure 2 are betw een 2 600 and 3 800 m etres. W est of 83 W the grain of the topography (i.e. the tren d of the ridges and troughs) is east-w est as seen by the correlation of some of the larger features across several north-south traverses. T his is consistent w ith the observation th at the topography along the north-south tracklines shows m ore peaks and deeps th an th a t along east-w est lines (i.e. the north-south lines have a sh o rter topographic w avelength ). However, m ost of the sm aller peaks an d deeps cannot be correlated across the n o rth-south lines showing th at they lose their identity (for exam ple, by bifurcating or m erging w ith other ridges and troughs) over the spacing of the tracklines.
F ig. 4. Generalized contour m ap (200-metre interval) of area shown in figures *2 and 3. Contours are also based on a single northeasterly trending trackline which is not shown here or in figures 2 and 3 for clarity of presentation. The m ost noticeable aspect of figure 2 is the ruggedness of the sea floor although it has about the sam e general depth throughout the area. A brupt changes in depths of hundreds of m etres w ith slopes of 15 to 2 0 " or m ore are com m on. It is difficult to evaluate the problem s in contouring such an area on the basis of the profiles. F igure 3 shows in m ap form how com plex the area is for one attem pting to contour it w ith a 1 0 0 -m etre contour interval. T his figure w as m ade by using all of the depths digitized from the sounding profiles. The intersections of the tracklines w ith the 100-metre isolines (i.e., w here 1 0 0 -m etre contour lines would cross) are indicated by alternating tick m ark s norm al to the track (this does not necessarily im ply th at the contours tren d norm al to the trackline). M inim um depths (peaks seen on the fathogram s) are indicated by dots along the trackline. F or clarity of p resen tatio n the values of the individual contour crossings at each tick are not given an d th e deeps th a t exist betw een the peaks are not shown.
T his figure dem onstrates the problem s in showing the actual sea floor relief in th is area w ith a contour m ap. If such deep sea areas are to be contoured, the tracklines will have to be spaced m uch closer than those of this survey and an appropriate m ap scale used. In spite of these difficulties, bathym etric contour m aps in area sim ilar to this are essential to provide some indication of the general depths. Such a m ap is given in figure 4. The contours of this m ap were draw n after subjectively sm oothing the depth profiles. By com paring this figure w ith the profiles in figure 2, some of the larger features are seen to be clearly show n by the contours, alth o u g h there is no in dication of the great am o u n t of m icro topography present in this area. It should be em phasized th at if this area w ere contoured by other w orkers using the sam e data, it is likely th a t the contour lines in m uch of the area would be draw n quite differently from the presen tatio n given here. COMPUTER PRODUCED PROFILES In figures 5 and 6 com puter-produced profiles from the survey are given as exam ples of how the profiles m ight be presented (although it is probably desirable to publish the profiles at a larger scale th an given in the pages of this review). These profiles were m ade by an offline X-Y plotter using a com puter program (G r im, 1970b) w hich accepts reduced bathym etric data as input and produces annotated plots of b athym etry at any scale and vertical exaggeration desired. The profiles w ere m ade on the plotter in ink and are suitable for publication w ith very little additional labeling. In the future one of the steps in producing contour m aps will probably be obtaining reduced data such as those used as in p u t to the above plotting program. T hus, the profiles can be con sidered as an econom ically p ro d u c ed spin off in the process leading to the p ro du ction of th e fin al m ap. A lthough m ost of the inicrotopography seen on the fathogram s can be displayed w ith com puter-produced profiles, some of the very sm allest features (those w ith relief on the order of m etres and lateral dim ensions on the order of tens of m etres) are difficult to show. If these features are deem ed im p o rtan t enough to present on the profiles, then care m ust be tak en, not only in scaling the depths at very sh o rt in terv als on the fa th o gram s but in the vertical exaggeration an d physical size of the resulting plots DISCUSSION Small scale bottom features are becom ing increasingly im p o rtan t as technological advances enable m an to utilize g reater and g re ater d ep th s in
-SDO F ig. 5. Computer produced bathymetric profile. See figure 1 for location. Vertical exaggeration 50/1. Plate 1 shows the fathogram record corresponding to bracketed part of the profile. SH3L3W wr HLd30
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his exploitation of the sea floor. P roperly annotated an d presented bathym etric profiles are essential in locating sites for various ocean engineering pro jects and for planning such operations as the m ining of m anganese nodules or other m ineral resources. The detail provided by such profiles is also required for search and rescue operations as well as various salvage program s. In addition to the probability th at such inform ation will be of great practical use in the near future, these sam e data are of significant in terest to the m arine geologist in studying geologic processes th at have affected the sea floor. Various features resulting from sedim entary processes show up very clearly on profiles. F or exam ple, the detail associated w ith su b m arine canyons, fan valleys, and deep sea channels th at exist on continental m argins and th eir adjacent abyssal plains can only be shown in such profiles. 1 he n atu re of sm all lineations on the sea floor such as sand w aves m ight be determ ined by com paring profiles based on results obtained from system atic survey tracklines. O ther bottom features seen on profiles w ould include block faulting, the sm oothing effect of sedim ent as a function of distance from a sedim ent source, and evidence for craters on seam ounts or the flattening of their sum m its. The idea of producing bathym etric profiles from deep sea tracklines is not new. T his has been a stan d ard technique for presenting b ath y m etric inform ation in scientific jo u rn als for years but at scales which often om it the detail needed to evaluate the m icrotopography. More recently, d ata reports from bathym etric and geophysical studies based on profiles produced by autom atic X-Y plotters have been published (e.g., Hayes et al., 1969; L o w r i e and E s c o w i t z, 1969). However, alm ost all the tracklines of such scientifically oriented studies are not spaced closely enough to allow contouring of the data. T his paper has show n th at even when surveys are system atic and detailed enough to generate bathym etric contour m aps, properly produced profiles are a valuable supplem ent to the m aps because only w ith such profiles can the m icrotopography be delineated. It is especially im p o rtan t th at the idea of publishing profiles be brought to the atten tio n of survey organizations such as hydrographic offices th at m ay not be research oriented but w ill probably conduct m any of the deep sea surveys in the future. The trad itio n al single product of a bathym etric survey m ade by such organizations is a contour m ap and the routine p ublishing of deep sea bathym etric profiles, such as suggested here, m ay be a novel concept. T h u s far only deep sea surveys have been considered. However, the ideas developed here can also be applied to surveys conducted on continental shelf and slope areas. Although such surveys are likely to have a trackline d ensity sufficient to perm it the construction of relatively accurate contour m aps, the n atu re of features w ith scales of tens or hundreds of m etres w ill still be difficult to show m ean in g fu lly on these m aps. C om puter produced profiles could essentially be published as data rep o rts (including trackline location charts) at the sam e tim e or at an earlier time than the contour m ap. These reports could be available m onths or even years before the m aps since the profiles are sim ply a straig h tfo rw ard m ethod of show ing the basic data and w ould require none
of the in terp retatio n needed in contouring the depths. T he physical size of the profile plots w ould depend on the roughness of the sea floor in the area surveyed, the accuracy of the navigational control, and the type and q uality of the fathogram records. For exam ple, the size of profiles from a survey conducted w ith a wide beam echo sounder w ith relatively poor control m ight be m uch sm aller th an the size of profiles based on a survey conducted w ith good control and a narrow -beam echo sounder. In general, both the profiles and the trackline location chart should be presented at a scale th at allows the positions of individual features to be determ ined w ith an accu racy con sistent w ith th a t of th e n avig ation al control. In the fu tu re an increasing num b er of bathym etric surveys will use advanced echo sounders and will be au to m ated for data reduction. The resu ltin g contour m aps of m any oceanic areas can be supplem ented by com puter produced profiles quickly and econom ically by using the reduced b a th y m etric data. Such profiles will d isp lay m uch of th e sea floor m icro topography w hich cannot be shown adequately on contour m aps but a know ledge of w hich m ay be very im p o rtan t as m an acquires the ability to utilize th e sea floor m ore fully. ACKNOWLEDGEMENTS W e w ish to th an k Dr. D.C. K r a u s e for his com m ents on this paper. REFERENCES C o h e n, P.M. : D irectional echo sounding on hydrographic surveys, International Hydrographic Review, Vol. 3 6 (1 ), p. 29-42, 1959. G r im, P.J. : B athym etric and m agnetic anom aly profiles from a survey south of P anam a and Costa Rica (USC&GSS Oceanographer, A ugust 1969), ESSA T echnical M em orandum, ERLTM-AOM L 9, 87 p., 1970a. G r im, P.J. : Com puter program for autom atic plotting of b ath y m etric and m agnetic anom aly profiles, ESSA T echnical M em orandum, ERLTM - AOML, 8, 31 p., 1970b. H a y e s, D.E., J.R. H e i r t z l e r, E.M. H e r r o n and W.C. P itm a n III : P relim inary rep o rt of Volume 21 USNS Eltanin cruises 22-27 Jan u a ry, 1966 - F ebruary, 1967, L am ont D oherty Surveys of the W orld Ocean (General E ditor, M aurice Ew ing), Tech. Rep. No. 2-CU-2-69, 130 p., 1969. K r a u s e, D.C. and V.F. K a na ev : N arrow -beam echo sounding in m arine geom orphology, International Hydrographic Review, Vol. 47 (1), p. 23-33, 1970.
L o w r i e, A. and E. E s c o w it z, editors : U.S. Naval O ceanographic Office global ocean floor analysis a n d research data series, Vol. 1, Kane cru ise 9, 971 p., 1969. M a t t h e w s, D.J. : Tables of the velocity of sound in pure w ater and sea w ater for use in echo sounding and sound ranging, B ritish A dm iralty H ydrographic D epartm ent No. 282, 1939 (second edition). M o l n a r, P. and L.R. S y k es : Tectonics of the C aribbean and Middle A m erica regions from focal m echanism s and seism icity, Geological Soc. of America Bulletin, Vol. 80 (9), pp. 1639-1684, 1969. R yan, T.V. and P.J. G r im : A new technique for echo sounding corrections, International Hydrographic Review, Vol. 45, (2), p. 41-58, 1968.