3b_Current Methods for Obtaining Logging and Spliting Marine Sediment Cores

8/20/2019 3b_Current Methods for Obtaining Logging and Spliting Marine Sediment Cores

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Current Methods for Obtaining Logging and Splitting

Marine Sediment Cores

P. P . E . W E A V E R a n d P . J . S C HU L T HE I S S

Insititue of Oceanographic Sciences DL) Brook Road, Wormley, Godalming, Surrey GU8 SUB, UK

(Received 27 April, 1989; accepted 1 September, 1989)

Key words: coring devices, box corer, gravity corer, piston corer,

giant piston corer, ' P' wave log, wholecore logging, kevlar, warp.

Abstract. The main types of deep-sea sediment coring devices are

described and their relative merits and drawbacks are discussed.

These devices include box corers, gravity corers, piston corers,

giant piston corers and vibrocorers. Recent utilisations of kevlar

and polyester coring warps are also discussed, since these are the

only warps capable of handling the large weights associated with

the larger devices. Recent developments in wholeeore logging,

including 'P' wave, density and magnetic sus~ptibility, are de-

scribed as are methods of subcoring and core splitting to obtain

the maximum amo unt o f detail on the split surfaces. T he

wholecore logs together with a good colour photograph of the

recently split sediment surface provide a lasting unambiguous

record of the core.

ntroduction

Our knowledge o f the deep-sea f loor is buil t up from

interpretations of a combination of imaging tech-

niques (geophysical mapping and profil ing) and by

direct sampling. A vast range of devices has been

developed for sampling the seabed, and these fall

into various categories, with each category being

aimed at either particular depth penetrations, partic-

ular sediment types or a specific use for the sample,

The deepest and most complete record of deep-sea

sediments and basement rocks has been provided by

the Ocean Dril l ing Program (ODP), using a combi-

nation of downhole hydraulic piston coring and

rotar y drilling. The drilling capabilities of the OD P

and its predecessor the Deep Sea Dril l ing Project

(DSDP) have been well documented elsewhere (see,

Storms, this volume) and are not considered further

in this chapter. Smaller and less expensive research

vessels than those used for drilling are capable of

* Present address: Sch ultheiss Geotek, Fern Cottage, Ma rley Lane,

Haslemere Surrey GU27 3RF, UK

Marine Geophysical Researches 12: 85-100, 1990.

9 1990 Kluwer Academic Publishers. Printed in the Netherlands.

using a variety of devices to sample the upper 20-

25 m of the sediment column. The largest o f these

ships are also capable of handfing the relatively new

giant piston corers which currently can sample to

depths of 35 m be low the sea floor, and in the futur e

may be able to sample as deep as 50 m.

There are two important considerations to be

taken into a ccount be fore taking a deep-sea core: (a)

how hard is the substrate?; and (b) what is the core

to be used for? In some circumstances i t may be

prudent or necessary to use more than one type of

corer to ensure that a complete record of a si te has

been obtained to the depth o f interest or to a de pth

limited by the available technology. This is often the

case if a good-quality core o f the sediment surface is

required together with a sediment record several

metres long. Scientists wishing to make physical

property measurements, and those interested in an

accurate record of the thicknesses of all lithological

units in a core, should be particularly careful and

aware of problems arising from core disturbance and

core mis-sampling. In some cases these are not easily

detected and may lead to incorrect interpretations.

Coring techniques have developed over the years

with a lot of ingenuity but without any signif icant

high technology . The success of a coring operation

has all too often been judged by the cr i ter ia of how

full or how long is it? . Attention has, more recently,

been focused on the problems associated with core

disturbance and ensuring the general quali ty and

representativeness o f the core rather than solely that

of maximizing the depth o f penetration. There has

been a move towards the design of more sophisti-

cated corers in recent years, often involving the

mounting of monitoring equipm ent on the corer ,

both to improve penetration and to produce a de-


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8 P P E WEAVER AND P J SCHULTHEISS

t a il e d r e c o rd o f t h a t p e n e t r a ti o n . A l t h o u g h s o m e

w o rk h a s b e e n c o m p l e t e d o n t h e d e s i g n a n d t e s t i n g

of th is type o f equ ipmen t ( see Park er a nd S i ll s, th is

vo lume) , there i s no opera t iona l co rer wh ich uses i t

rout inely .

S e v e ra l o t h e r m a j o r r e v ie w s o f c o r i n g e q u i p m e n t

( e .g . t h o s e o f B o u m a , 1 9 6 9 ; M o o re a n d H e a t h ,

1978 ; Lee and Clausner , 1979) and o f co re hand l ing

(B o u m a , 1 9 6 9 ) h a v e b e e n p u b l i s h e d i n t h e p a s t t w o

decades . Deta i l s o f severa l co rers no t men t ioned

in th i s chap ter wi l l be found in those papers .

Th i s c h a p t e r s u m m a r i z e s t h e m a i n t y p e s o f c o r i ng

equ ipmen t cu rren t ly in use , inc lud ing recen t ly

d e v e l o p e d g i a n t p i s t o n c o re r s , a s w e l l a s m o d e m

methods fo r hand l ing and descr ib ing co res . There i s

n o w a g r e a t e r e m p h a s i s o n w h o l e c o re l o g g in g a n d

h i g h -q u a l i t y c o re p h o t o g ra p h y , w h i c h c a n r e p l a c e

a n d i m p ro v e m u c h o f t h e l a b o r i o u s c o re d e s c r ip t i o n

w o rk f r e q u e n t l y c a r r i e d o u t b y i n e x p e r i e n c e d

personnel .

ox Corers

Th e m a i n a d v a n t a g e o f b o x c o re r s is t h a t t h e y o b -

ta in l a rge-vo lume co res o f su r face sed imen t wi th

m i n i m u m d i s tu rb a n c e . Th e y c o n s i s t o f a s q u a re b o x

(occas iona l ly a l a rge d iameter cy l inder) , a head-

weigh t and a spade- typ e l ever a rm (F ig . l a ) . S om e

des igns a l so inc lude a t r ipod suppor t f rame to en -

su re ver t i ca l co r ing (F ig . lb ) . The c ross -sec t iona l

a r e a o f t h e b o x m a y b e u p t o 0 .2 5 m 2, b u t t h e

length is general ly less than 1 .2 m. T he sp ade leve r

arm l i es ho r izon ta l ly du r ing dep loymen t and de-

scen t , bu t on recovery i t i s pu l led in to i t s ver t i ca l

p o s i ti o n , t h u s c l o s i n g th e b o t t o m o f t h e c o re b o x

pr io r to pu l l -ou t . In m any o f the designs the spade

lever a rm a l so c loses the top o f the box , thus ensu r-

i n g a n u n d i s t u rb e d s e d i m e n t -w a t e r i n t e r f a c e . S o m e

box co rers u t i li ze a sc is so r a r rangemen t o f two

spade l ever a rms (F ig . l a ) wh ich ensu res tha t the

box i s main ta ined in a ver t i ca l pos i t ion , thus e l imi -

na t ing the need fo r a t r ipod f rame. I t has become

apparen t , however , tha t s ing le-spade co rers a re o f -

t e n m u c h b e t t e r a t c o r i n g s a n d y s e d i m e n t s . M o s t

b o x c o re r s a c t i v a te t h e s p a d e c l o s u re b y m e a n s o f a

no- load re lease . Al tho ugh th i s sys tem i s s imple , i t

can l ead to p re- t r ipp ing , espec ia lly in rough seas .

The lOS box co re r (F ig . l a ) has an acous t i ca l ly

ac t iva ted re lease wh ich i s f i red a f te r the co rer has

penet ra ted the sed imen t . Al though th i s sys tem po-

ten t ia l ly a l lows the co rer to bounce on the sea f loo r ,

n o p ro b l e m s h a v e b e e n e x p e r i e n c e d t o d a t e .

Grav i ty Corers

These co rers a re very s imple , bu t var iab le in des ign ,

cons i s t ing o f a l a rge headweigh t , to w h ich i s a t -

t ached a b ar re l o f var iab le l eng th wi th a co re cu t te r

and ca tcher a t the lower end . The sh ip ' s warp i s

a t t ached d i rec t ly to the co re head , excep t in the

dev ice descr ibed by Hvors lev and S te t son (1946) , in

which the co rer i s t r iggered by a t r igger weigh t .

M o s t r o u n d b a r r e l c o r e rs o f t h is t y p e (F i g. l c )

u t i l i ze co re l iners , bu t those wi th long box-shaped

barre l s , such as the Kas ten lo t co rer (F ig . ld ) , do

n o t . M o s t i n c o rp o ra t e a f l a p v a l v e a t t h e t o p t o

a l l o w w a t e r t o e s c a p e d u r i n g c o r i n g b u t w h i c h i s

c losed dur ing pu l lou t , ascen t and rec overy to p re-

v e n t s e d i m e n t b e i n g w a s h e d o u t . P ro b l e m s w i t h

washou t occur par t i cu la r ly a t the sea su r face du r ing

re t r i eva l as a resu l t o f the lo ss o f buoyancy when

the co re i s l i f t ed f rom the water .

G ra v i t y c o re r s a r e i n e x p e n s i v e a n d e a s y t o u s e

a n d c a n t a k e h i g h -q u a li t y c o re s o f th e u p p e r f e w

met res o f the sea f loo r . They are , however , sub jec t

to two m ajo r sou rces o f e r ro r . These a re (1 ) mis-

sampl ing due to sed imen t p lugg ing in the co re bar -

re l , and (2 ) repene t ra t ion caused b y vert i ca l

osc i l la t ions o f the sh ip 's warp . Em ery and Die tz

(1 9 4 1 ) s h o w e d t h a t o p e n b a r r e l g r a v i t y c o re r s w e re

capab le o f t ak ing very shor tened sec t ions o f co re ,

a n d H v o r s l e v a n d S t e t s o n (1 9 4 6 ) s h o w e d h o w t h e

s e d i m e n t l a y e r s w o u l d b e s h o r t e n e d a h e a d o f t h e

corer . Weaver and Schu l the i s s (1983b) were ab le to

s h o w e v i de n c e o f g r a v i ty c o re r s b o u n c i n g o n th e

sea floo r , enab l ing m ul t ip le sampl ing o f the uppe r

s e d i m e n t s e c t i o n b y r e p e a t e d p e n e t r a t i o n s . Th e y

a l s o s h o w e d h o w e a c h s u c c e s s i v e r e p e n e t r a t i o n w a s

shor tened re la t ive to the p rev ious one , and how the

s h o r t e n i n g w a s m o re p ro n o u n c e d i n t h e s o f t e r m o re

p las t i c , sed imen t l ayers . Th is sugges t s tha t g rav i ty

corers can be un re l i ab le in sed imen ts o f mixed com -

pos i t ion , and p roduce increas ing ly un re l i ab le resu l t s

wi th increas ing pene t ra t ion . The l eng th o f co re tha t

c a n b e t a k e n w i t h a n o p e n b a r r e l g r a v i t y c o re r i s

l imi ted by the increas ing f r i c t ion be tween the co red

sed imen t and the in s ide o f the bar re l . At some s tage

the fo rce requ i red to move the co re up the bar re l :


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1 0

1 0 0 0 :

MARINE SEDIMENTCORE METHODS

b

87

e) d) e)

100

| . . m

1 0 0 ]

1

a

lOOcm

5 0

Fig. 1. Corers used for near-surface sedim ent samplin g: a) IOS box corer doub le spade) after Peters e t a l . 1980); b) box core with

single-spade and tripod frame see Bouma, 1969); e) hydrostatic gravity corer Richards and Keller, 1961); d) square barrel Kastenlot

core Kogl er, 1963); e) free-fall, pop- up boo me rang corer Moo re, 1961).

exceeds the fo rce requ i red fo r the barre l and co re to

ac t as a so l id rod fo r fu r ther pene t ra t ion . Th is i s

k n o wn a s p l u g g i n g a n d c o m m o n l y o c c u r s b e -

tween 3 and 6 m pen et ra t ion . In te rm i t ten t o r par t i a l

p lugg ing i s the cause o f co re shor ten ing . A mor e

deta i led s tudy o f thi s e f fec t has been m ade by

Parker and S i l l s ( th i s vo lume) in one sed imen t type

us ing a t echn ique to con t inuous ly moni to r the sed i -

men t su rface bo th ins ide and ou ts ide the co re barre l

d u r i n g p e n e t r a t i o n .


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8 8 P P E W E A V E R A N D P J S C H U L T H E I SS

G r a v i t y c o r e s a r e f r e q u e n t l y u s e d a s t r i g g e r

w e i g h t s f o r p i s t o n c o r e r s a n d o f t e n p r o d u c e a m o r e

r e p r e s e n t a t i v e r e c o r d o f t h e s u r f a c e s e d i m e n t t h a n

t h a t o b t a i n e d f r o m t h e p i s t o n c o r e . C a r e m u s t b e

t a k e n , h o w e v e r , t o e n s u r e t h a t n o r e p e n e t r a t i o n o f

t h e t r i g g e r c o r e r h a s o c c u r r e d s i n c e t h i s i s c o m m o n

d u r i n g p i s t o n c o r i n g ( M c C o y , 1 9 8 0 ) .

T h e K a s t e n l o t c o r e r ( F i g . l d ) b r i d g e s t h e g a p

b e t w e e n g r a v i t y a n d b o x c o r e r s s i n c e i t h a s a r e l a -

t i ve l y l a rge c ross - sec t i ona l a r ea (225 cm2), and ca n

t a k e c o r e s u p t o 6 m l o n g ( K o g l e r , 1 96 3) . T h e b o x

s p l it s l o n g i t u d i n a l l y i n t o t w o h a l v e s a n d s o i t m u s t

b e l a id h o r i z o n t a l l y t o o p e n t h e c o r e , w h i c h o f t e n

c a u s e s s o m e s l u m p i n g a n d d i s t u r b a n c e o f t h e s u r -

f a c e s e d i m e n t . T h i s c o r e r d o e s n o t u s e a l i n e r a n d i s

u s u a l l y s u b - s a m p l e d i m m e d i a t e l y a f t e r o p e n i n g . S e c -

t i o n s o f t h e c o r e d m a t e r i a l c a n b e s t o r e d i n 1 m -

l o n g b o x e s , w h i c h h a v e t h e s a m e c r o s s - s e c t i o n a s

t h e c o r e r b u t w i t h c l o s e d e n d s f o r l a t e r e x a m i n a -

t i o n . I t i s a l s o p o s s i b le t o r e m o v e o n e c o r n e r o f th e

c o r e i n t o a V - s h a p e d t r o u g h w h i c h c a n b e s t o r e d i n

a D t u b e a s a n a r c h i v e s e c t io n . T h e l a r g e c r o s s -

s e c t i o n a l a r e a a l l o w s t h i s c o r e r t o p r o v i d e h i g h -

q u a l i t y c o r e s w h i c h o f t e n s u f f e r l e s s d i s t u r b a n c e a n d

c o r e s h o r t e n i n g t h a n t h o s e t a k e n b y o t h e r g r a v i t y

c o r e r s . H o w e v e r , t h e K a s t e n l o t c o r e r i s s u s c e p t i b l e

t o r e p e n e t r a t i o n ( W e a v e r a n d S c h u l t h e i s s , 1 9 8 3 b ) .

T h e B o o m e r a n g c o r e r ( F i g . l e ) is a m o d i f i c a t i o n

o f t h e o p e n - b a r r e l g r a v i t y c o r e r w h i c h o p e r a t e s a s a

f r e e f a l l / p o p - u p c o r e r . I t c o n s i s t s o f a b a l l a s t s e c t i o n

compr i s i ng a s t ee l bar r e l , wei gh t , s t ee l f l oa t p ro t ec-

t i o n a n d l e a d p i l o t w e i g h t . T h e f l o a t s e c t i o n c o n s i s t s

o f a co re l i ner , va l ve r e l ease and t wo g l ass spheres .

T h e w h o l e s y s t e m i s r e l e a s e d f r o m t h e s h i p , a f t e r

w h i c h i t q u i c k l y r i g h ts i t s e l f a n d a t t a i n s a t e r m i n a l

v e l o c i ty o f a p p r o x i m a t e l y 7 m / s e c . A h o l l o w r u b b e r

b a l l p r e v e n t s r e le a s e o f t h e f l o a t p o r t i o n d u r i n g

d e p l o y m e n t , b u t t h i s i s c o m p r e s s e d a n d r e l e a s e d

d u r i n g d e s c e n t . W h e n t h e c o r e r i m p a c t s w i t h t h e

seabed t he p i l o t wei gh t s l i des up t he bar r e l , r e l eas -

i n g t h e f l o a t s e c t i o n w h i c h t h e n a s c e n d s , c l o s i n g t h e

v a l v e - re l e a s e a t t h e t o p o f t h e l i n e r a n d p u l l in g f r e e

t h e l i n e r t u b e w i t h t h e c o r e i n s i d e . T h e f l o a t s e c t i o n

a s c e n d s t h r o u g h t h e w a t e r c o l u m n a n d i s i d e n t i f i e d

a t t h e s u r f a c e b y a n e l e c t r o n i c f l a s h i n o n e o f t h e

s p h e r es . T h e a d v a n t a g e o f th i s s y s t e m is t h a t s e v e r a l

c o r e s c a n b e o b t a i n e d r a p i d l y b y d e p l o y i n g t h e m a s

a s e r i e s p r i o r t o r e c o v e r y . F u r t h e r m o r e , t h e c o r e s i t e

i s l i k e l y t o b e d i r e c t l y b e n e a t h t h e s h i p s p o s i t i o n ,

s i n c e t h e r o u n d - t r i p t i m e i s a b o u t 1 5 m i n / 1 0 0 0 m

w a t e r d e p t h a n d h e n c e t h e r e i s l i t t l e t i m e f o r t h e

e q u i p m e n t t o d r i f t d u r i n g t h e o p e r a t i o n . T h e d i s a d -

v a n t a g e o f t h e s y s t e m i s t h a t o n l y s h o r t c o r e s

( < 1 .2 m ) c a n b e o b t a i n e d a n d i t c a n s o m e t i m e s b e

d i f f i cu l t t o f i nd t he f l oa t i ng co re i n poo r v i s i b i li t y o r

r o u g h w e a t h e r .

Piston orers

T h e p i s t o n c o r e r ( F i g . 2 ) w a s f i r s t d e v e l o p e d b y

K u l l e n b e r g i n 1 9 4 7 t o o v e r c o m e t h e d e p t h p e n e t r a -

t i o n l i m i t a t i o n , c a u s e d b y p l u g g i n g , o f g r a v i t y c o r -

e r s . I t cons i s t s o f a se r i es o f conn ec t e d bar r e l s , a

l a r g e , h e a v y b e a d w e i g h t , a t r i g g e r a r m a n d a t r i g g e r

c o r e r . A p l a s t ic c o r e l i n e r is u s u a l l y e m p l o y e d , b u t

t h e c o r e m a y b e e x t r u d e d o n d e c k a f t e r r e c o v e r y .

T h e h e a d m a y b e d e s ig n e d w i t h r e m o v e a b l e w e i g h ts ,

w i t h f i n s , o r w i t h h o u s i n g s f o r i n s t r u m e n t s s u c h a s

c a m e r a s , f l a s h - g u n s o r p i n g e r s . M o s t t r i g g e r a r m s

a r e f i t te d w i t h s a f e t y p i n s t o p r e v e n t a c c i d e n t s d u r -

i n g d e p l o y m e n t . H y d r o s t a t i c p i n s u s u a l l y r e t r a c t i n

t h e u p p e r o r m i d w a t e r c o l u m n , b u t a c o u s t i c r e -

l e a s e s c a n b e u s e d , w h i c h c a n p r i m e t h e c o r e r b y

c o m m a n d f r o m t h e s h ip a t a n y d e p t h . T h e s e o ff e r

i n c r e a s e d s a f e t y i n r o u g h w e a t h e r o r o n l e s s s t a b l e

s h i p s , w h e r e t e m p o r a r y r e d u c t i o n s i n l o a d o n t h e

t r ig g e r a r m d u e t o s h ip s ' h e a v e c a n c a u s e p r e m a t u r e

r e l ea s e o f t h e c o r e r .

T h e a d v a n t a g e s o f th e p i s t o n c o r e r o v e r g r a v i ty

c o r e r s i s t h e i n c r e a s e d d e p t h o f p e n e t r a t i o n a s a

r e s u l t o f t h e p r e v e n t i o n o f p l u g g i n g . T h i s s h o u l d

e n s u r e h i g h e r q u a l i t y c o r e s . T h e d e p t h o f p e n e t r a t i o n

d e p e n d s o n s e d i m e n t t y p e , b u t i n r e l a t i v e l y s o f t

m u d s , o v e r 2 0 m c a n b e o b t a i n e d ( K u i j p e r s

e t a l .

1 98 4) . T h e a c t i o n o f th e p i s t o n , w h i c h r e d u c e s t h e

i n t e r n a l f r i c t i o n , e n a b l e s t h e s e c o r e r s t o g e n e r a l l y

r e c o v e r m o r e c o m p l e t e , a n d l e s s d i s t u r b e d s e d i m e n t

s e q u e n c e s t h a n o p e n - b a r r e l g r a v i t y c o r e r s . T h e a t -

t a c h m e n t o f t h e m a i n w a r p t o t h e p i s t o n , h o w e v e r ,

c a u s e s s h i p s ' h e a v e m o t i o n s a n d e l a s t i c r e b o u n d o f

t h e w a r p a f t e r c o r e r r e l e a s e t o b e t r a n s m i t t e d t o t h e

p i s t o n d u r i n g c o r i n g , w h i c h c a n r e s u l t i n c o r e s h o r t -

e n i n g a n d / o r o t h e r s e d i m e n t d i s t u rb a n c e s ( M c C o y ,

1980 , 1985) . Defo rmat i on i s o f t en par t i cu l a r l y severe

i n t h e v i c i n i t y o f s a n d y l a y e r s w h i c h m a y b e t i l t e d ,

b e n t d o w n w a r d s a t t h e s i d e s o f th e l i n e r o r t o t a l ly

m i x e d b y t h e c o r i n g a c t i o n . I g a r a s h i

e t a l .

(1970)

s h o w e d t h a t c o a r s e p a r t i c l e s c o u l d b e d r a g g e d f r o m


5/16

MARINE SEDIMENT CORE METHODS 9

cable to ship7

cable running

through barrel'

piston>

cutter>

free f ll c ble

i / /~ p t r igger arm

e head

piston stop

barrel /chain

< catcher

trigger corer >

o ed

~Xpiston stop

~c o i~

:


6/16

9 P P E WEAVER AND P J SCHULTHEISS

broke at the barrel couplings on 30 -35 m deploy-

ments. Further problems were encountered with sta-

bilizing the piston and tripping the core. Driscoll and

Hollister (1974) discussed several modifications to

the system, including step tapered barrels, a re-

designed head with provision for instrumentation

and an acoustic release. In 1977 a m ajor redesign was

proposed (Driscoll and Silva, 1977) to inco rporate

these features into a technically sophisticated system

for taking 50 m-long cores. Initially called the L ong

Coring Facility, i t was later renamed the Advanced

Piston Corer (APC). (This should not be confused

with the Ocean Drilling Program's hydraulic

APC . )

The ins trumentat ion of the APC head includes an

acoustic release, a ti l t alarm, a rotation recorder, a

piston contro l system, an acoustic telemetry package

and a power and control package. These instruments

were designed to enable close contro l and moni toring

of the system during the coring operation. A simpler

version of this system (Fig. 3) was used on th e Gian t

Piston Corer (Silva et al. 1977) and on the Marion

Dufresne ESOPE expedition (Schuttenhelm, in

press). In the simple version, a parachute is attached

to the coring warp just above the core head, the warp

continues down the barrels to a standard-type piston

and is held in position at the core head by the

release. The p arachu te takes some o f the core weight

during lowering, but on approach to the seabed the

winch payout is speeded up so that the parachute

, I [ o

1 m

5 m

Om

p a r a c h u t e

c o r e r h e a d

w i th e lec t ron i cs

r

q~

iston

Fig. 3. The giant piston corer with parachute a) during descent; b) during penetration redrawn after Driscoll, 1981).


7/16

M R I N E S E D I M E N T C O R E M E T H O D S

91

t a k e s t h e w h o l e w e i g h t o f t h e c o re r , t h u s p ro d u c i n g

a q u a n t i t y o f l o o s e w a rp a n d e l im i n a ti n g a n y r e -

bo und ef fec ts o f the warp whe n the load i s re leased .

The co re i s t r ipped wi th min imal f reefa l l , and s l ides

pas t the p i s ton which i s he ld a t the same pos i t ion by

the s lowly descend ing parachu te .

A v e ry c o m p l e x s y s t e m h a s b e e n c o n s i d e re d fo r

s tab i l i z ing the p i s ton by decoup l ing i t f rom the main

sh ip ' s warp (Dr i sco l l , 1981) . Al though th i s sys tem

has n ever been bu i l t , i t i llu s t ra tes the ex ten t to w h ich

techno logy can be u t i l i zed in co r ing equ ipmen t . I t i s

k n o w n a s t h e H y d ro s t a t i c A c c u m u l a t o r P i s t o n

(H A P ) , a n d u t il iz e s a p a r a c h u t e l o c a t e d j u s t a b o v e

t h e c o re h e a d . Th e H A P p i s t o n c o n c e p t c o n s i s t s o f

two long chambers , the upper one a i r - f i l l ed and

connected to the lower oi l -f i l led one by a valve.

There i s a l so an acce le rometer to as sess the p i s ton

m o t i o n a n d a s o n a r t r a n s d u c e r a t t h e l o w e r e n d t o

mo ni to r the po s i t ion o f the sed imen t su r face du r ing

cor ing . The p i s ton i s connec ted to the parachu te as

befo re , bu t a f t e r t r igger ing the va lve be tween the o i l

and air f i l led chambers al lows oi l to pass , thus

comp ress ing the l eng th o f the p i s ton . The ra te o f

con t ra c t ion o f the p i s ton is des igned to keep the base

of the p i s ton s tab le du r ing pene t ra t ion (wh ich t akes

on ly a few seconds ) . The con t rac t ion ra te nee ded can

b e c a l c u l a t e d f ro m t h e p a r a c h u t e s i n k r a t e , o r f r o m

the acce le rometer wh ich mon i to rs the ver t i ca l mot ion

o f t h e b o t t o m p a r t o f t h e p is t o n .

Th e d r a w b a c k s o f th e A P C s y s te m a r e i ts g r e a t

complex i ty and the very l imi ted numbers o f sh ips

capa b le o f dea l ing wi th the l a rge loads exper ienced

dur ing pu l l -ou t .

S T C O R

The on ly o ther co rer wh ich has t aken co res in excess

o f 2 5 m is t h e S T A C O R ( s t a t i o n a ry p i s t o n c o re r ) ,

d e v e l o p e d b y t h e In s t i t u t F r a n e a i s d u P & ro l e , E l f

A q u i t a i n e a n d To t a l i n t h e m i d -1 9 7 0 s (M o n t a rg e s e t

a l .

1983, 1987). This corer u t i l izes a mechanical

sys tem fo r main ta in ing a s ta t ionary p i s ton , an idea

f i rs t sugges ted by Kul lenberg (1955) and la te r deve l -

o p e d b y K e rm a b o n a n d C o r t i s (1 9 6 8) . Th e p r in c i p le

of the s tat io nar y o r recoiUess pis ton is that i t is

c o u p l e d t o a b a s e p l a t e o n t h e o u t s i d e o f t h e c o re r v ia

a se r ies o f pu l leys and w i res (F ig . 4 ). The main sh ip ' s

warp connec t s d i rec t ly wi th the co rer head and i s

t h u s d e c o u p l e d f ro m t h e p i s t o n . Th e b a s e p l a t e a n d

p i s t o n b e g in t h e c o r i n g o p e ra t i o n a t t h e c u t t e r e n d o f

the barrel , a nd s ince the basepla te is 1.5 m ac ross , i t

remains a t the sed imen t su r face and ho lds the p i s ton

a t the same pos i t ion wh i le the co rer s l ides pas t . Th is

co rer t akes co res wi th a 110 mm in te rna l d iam eter

and , be cause o f the s ta t ionary p i s ton , genera l ly

ach ieves recovery approa ch ing 100 wi th exce l len t

co re qua l i ty .

Th e S TA C O R s y s t e m d o e s n o t d e p e n d o n s o p h i s -

t i ca ted t echno logy and has a h igh success ra te . I t has

so fa r been launch ed in th ree d i f fe ren t ways : over the

s te rn o f a sh ip , over the s ide ra i l and v ia a mo on

p o o l ( M o n t a r g e s e t a l . 1987) . The long ba r re l - l eng th

and the l a rge loads , par t i cu la r ly those assoc ia ted

wi th pu l l -ou t (ab ou t 200 kN ) res t r i c t it s u se to the

larger oceanographic vessels , or to vessels such as the

N a d i r w h i c h h a v e a l a rg e f r e e d e c k fo r d e p l o y m e n t

over the s te rn . The loads can be d ras t i ca l ly reduced

by us ing Kev lar cab les wh ich a re a lmos t weigh t les s

in wate r (Sch i ll ing e t a l . 1 98 8) . O n e o t h e r d r a w b a c k

w i t h t h e c u r re n t S T A C O R s y s te m is th e l e n gt h o f

t ime requ i red to comple te a co r ing opera t ion , wh ich

c a n b e a s m u c h a s 1 6 h o u r s . A l a rg e p ro p o r t i o n o f

th i s t ime i s spen t removing the co re f rom the bar re l ,

wh ich de t rac t s f rom the co rer ' s u sefu lness fo r geo -

c h e m i c a l w o rk w h e re c o re s o f t e n n e e d t o b e p l a c e d

rap id ly under n i t rogen (Schu t tenhe lm e t a l . in

p ress ). H owe ver , some redes ign fea tu res o f the sys -

t e m c o u l d p ro b a b l y o v e rc o m e t h e s e d r a w b a c k s .

Vibrocoring and ockd ri ll ing

Th e v i b ro c o re r e x t e n ds t h e r a n g e o f s e d i m e n ts w h i c h

c a n b e s a m p l e d t o i n c l u d e s t i ff a n d s t o n y d a y s , s o f t

rock and sands , a l l o f wh ich a re d i f fi cu lt to pen e t ra te

us ing conven t iona l g rav i ty o r p i s ton co rers . The

Bri t i sh Geo log ica l Survey ' s v ib rocorer (F ig . 5 ) con-

s is t s o f a t r i p o d f r a m e w i t h a b a s e -m o u n t e d ro t a ry

dr ive t ab le w h ich d r ives a 6 m-long hexagona l d r i ll

bar re l . The tw in v ib ra to r m oto r g ives a fo rce o f 6

t o n n e s a t 5 0 H z a n d t h e c o re c a n b e w i t h d ra w n f ro m

t h e s e d i m e n t b y a b a s e -m o u n t e d w i n c h c a p a b l e o f

exer t ing 12 tonnes fo rce . Th is co re r t akes 83 mm

diameter cores in plas t ic l iners . The system is electri -

c a l l y p o w e re d f ro m t h e s h i p , f r o m w h e re t h e o p e ra -

t i o n c a n b e m o n i t o r e d a n d c o n t ro l l e d (A rd u s e t a l .

1982) . The p resen t sys tem has be en tes ted to 1800 m

water dep th , wi th the main l imi t ing fac to r be ing the

p o w e r s u p p l y c a b l e .

The Br i t i sh Geo log ica l Survey ' s rockdr i l l i s


8/16

9

P.P.E. WEAVERAND P. J. SCHULTHEISS

co r e r r e l e a se

b a s e p l o t e

t r i g g e r we i g h t

w i r e co n n e c t i n g

p i s to n to b o se p l o te

b o s e p l a t e

p i s to n

ulrey

Fig . 4 . The s ta t iona ry p is ton co re r , S tacor , showing the ope ra t ing p r inc ip le o f how the base -p la te and p is ton rem ain a t the seafloor dur ing

pene t ra t ion red rawn a fte r Monta rges

e t a L

1983).

m o u n t e d i n t h e s a m e t r i p o d s t ru c t u re a s t h e v i b ro -

c o re r P h e a s a n t , 1 98 4) . U p t o 6 m p e n e t r a t i o n is

poss ib le wi th th i s sys tem and , as befo re , wi thd rawal

o f the co re i s ach ieved by the 12 - tonne winch. B rook

and Pe l le t i e r 1970) repor t a deep-sea rock d ri l l

wh i c h wo rk s b y h y d ra u l i c p o we r , b u t wh i c h i s a l s o

l imi ted to 1800 m wate r dep th . Th is sys tem u t i l izes

t h r e e h e a v y -d u t y g a s c y l in d e r s wh i c h a l l o w wa t e r t o

e n t e r b y h y d ro s t a t i c p r e s s u re wh i c h t u rn s a h y d ro -

s ta t i c moto r fo r a 7 .5 -minu te cyc le . The f i rs t ha l f o f

the cyc le dri l ls the core in and flushes the dri ll b i t and

the second h a l f o f the cyc le wi thd ra ws the co re .

inches and arps

T h e m a x i m u m l o a d s e x p er i e n c ed d u r i n g c o r i n g o p e r -

a t ions a re those assoc ia ted wi th pu l l ing the co rer ou t

o f t h e s e d im e n t . F o r m o s t g r a v i ty , b o x a n d p i s t o n

cor ing work the loads wi l l be genera l ly wi th in the

c a p a b i l i ty o f m o s t o c e a n o g ra p h i c w i n c h es . T h e d e s ir e

t o p ro d u c e p i s t o n c o re r s o f la rg e r d i a m e t e r, a n d

g ian t p i s ton co rers has , however , increased the po -

t e n t i a l m a x i m u m l o a d s t o s u c h a n e x t e n t t h a t c o n -

ven t iona l s tee l warps a re no longer adequate . The

prob lem wi th s tee l warps i s the i r l a rge weigh t in

water , so tha t in deep water , beyond a cer ta in l imi t

the increased s t reng th o f a warp i s l a rge ly used to

o ffse t the increased weigh t o f the warp i t se l f .

T w o n e w m a t e r ia l s h a v e b e e n u s e d i n r e c e n t y e a r s

t o o v e rc o m e t h is p ro b l e m ; Ke v l a r a n d b ra i d e d

po lyes te r . Bo th o f these mater ia l s hav e neg l ig ib le

we i g h t i n wa t e r . T h e Ge o l o g i c a l S u rv e y o f t h e

Ne t h e r l a n d s h a s e x p e r i m e n t e d w i t h Ke v l a r a n d

found tha t i t g ives exce l len t resu l t s when used wi th

p i s t o n c o re r s o f l arg e a n d s m a l l d ia m e t e r . T h e

Kev lar i s encased in a po lye thy lene shea th to p reven t

chaff ing and i s s to red on a d rum l ike s tee l warp

Schil l ing

e t a l .

1988) . The po lyes te r cab le was


9/16

MARINESEDIMENTCOREMETHODS

9

tic Vibrator Head

tension lines

tical guide

embers

300cm

200

100

. Hydraulic rotary

~ N ~ f . / . , . . '~ dr* 11 tab 1

~

~ u s t i c penetration

Y 2 , 7 . e . . . . e . t

L Power Switching

PLAN ~ ~ distribution

pod

Fig. 5. The British Geological Survey's vibrocorer/rotary drill (fr om Ph easant, 1984).

chosen for the APC corer Dzwilewski and Driscoll,

1980), even though this cable cannot be stored on a

drum because it requires a long time for strain

recovery after it has been loaded. This strain recov-

ery would exert tremendous force on the drum and

so the cable is stored loose in a container on deck

Driscoll, 1981). Both types o f cable stretch signifi-

cantly under load, which must be allowed for when

calculating the length of wire required for the free-

fall distances of piston corers. Both of these cables

have considerable advantages over steel cables: they

are light in water; they have smaller bending radiuses

than steel; they do not corrode; they can be cut and

spliced, and yet Kevlar is stronger than a steel cable

of equivalent diameter and polyester has about half

the strength of steel. Polyester has about the same

cost as steel for equivalent strength cable, but it has

the disadvantage of needing a long recovery time

after it has been stressed, and it cannot, therefore be

wound on a drum. Kevlar, by contrast, costs about

three times as much as steel, but it can be stored on

a drum.

Many oceanographic cruises today use charter

ships which may not have the necessary combination


10/16

94 P p E WEAVER AND P J SCHULTHEISS

of winch, warp and A f rame required for p is ton

coring. Houbolt (1971) described a system

combining all these features which could be trans-

ferred from ship to ship. The Houbolt winch has

been cons iderably improved and is now marketed by

the Dutch company Seabed BV. The winch is com-

pletely indep end ent of the ship, h aving its own diesel

hydraulic pow er pack, 10 000 nm of Kevla r cable, a

telescopic boom, a traction unit with 20-ton lif ting

capacity, and two handling winches. The whole sys-

tem has the dimensions and fitt ings of a s tandard

container and can thus be f reighted around the world

and bolted on to any ship with suitable container

anchorage points .

Whole ore Logging

Having obtained a core in a plastic liner, there are a

number of useful measurements that can be made

prior to sectioning or splitting. These com e und er the

general heading of whole-core logging. Whole-core

logging is defined as non-destructive measurements

that can be autom atically made o n cores within plas-

tic core liners at frequen t intervals or on a contin uous

basis. The m ost frequ ently used is gamm a ray atten-

uation which essentially provides a log tha t is a func-

tion o f the sediment density. This technique has been

used on cores obtained from the Deep Sea Drilling

Project for many years. Other non-destructive whole

core logging techniques which are being more fre-

quently used are P-wave velocity logs and magnetic

susceptibility. It can be argued tha t m uch da ta is lost

from m any sediment cores (especially those from the

ocean drilling legs) because the time constraints on-

board ship preclude many measurements being made

at appropriately small sampling intervals . Perhaps

any useful parameter that can be measured cont inu-

ously and rapidly and does not require extra person-

nel should be measured on a routine basis .

The potential value of some whole-core logs is

already clear (Schultheiss and Mienert, 1988); other

non-destructive logging techniques may produce

data of only limited intrinsic value, or of a value that

is currently unclear. However, while any single

logged parame ter may at wors t be o f marginal value,

the combination of logs is almost certain to provide

a valuable diagnostic data set in the same way that

down-ho le logs are most useful when used as combi-

nation logs.

Continuous whole-core logging, with a range of

different sensors would be of considerable scientific

value for many reasons:

(1) Comp lete continuous records of P-wave velocity

and dens i ty ( f rom gamma ray at tenuat ion) are

necessary to construct synthetic seismograms for

comparison with seismic reflection profiling

records a nd borehole velocity logs.

(2) Con tinuous den sity logs are needed to evaluate

the s ta te of compact ion of sedimentary se-

quences.

(3) Mag netic susceptibility logs provide rapid iden-

tif ication of terrestrially derived sediments and

can provide excellent, high resolution, interhole

correlations.

(4) An y logged param eter that is sensitive to subtle

changes in sediment type or sediment structure

can be invaluable as a guide for later detailed

sampling. They may, for example, reveal cycli-

city which is caused by climatic changes and

could be u sed directly for climate spectral analy-

sis.

An example of the type of automated whole-core

logging system that is required is the P-Wave-L ogger

(PWL) developed at the Ins t i tu te of Oceangraphic

Sciences Deacon Laboratory (Schultheiss and

McPhail, in press) for measuring the compressional

wave velocity of sediments. The PWL (Fig. 6) ace-

urately measures and automatically records the

P-wave velocity of soft sediments within a cylindrical

plastic core liner and produces a very detailed velo-

city log of the whole core,

Veloci ty measurements are automatical ly taken at

regular intervals along each core section as it travels

between a pair of ultrasonic transducers. The PWL

provides fine-scale velocity profiles that (a) enable

accurate correlations of adjacent cores or holes to be

made, (b) provide high-quality data for synthetic

seismograms, (c) help in th e interp retation of seismic

records and (d) indicate the nature of sedimentary

features not easily detected by conventional means.

The m echanical ar rangement of the t ransducers used

on th e PWL is shown schematically as part of the

overall system diagram in Fig. 6a. An e xample of the

type of data obtained f rom very high resolut ion

P-Wave logging is shown in Fig ure 6b which is taken

from the ODP Leg 1 8 lnitial Report (Schultheiss

and McPhail, in press), clearly illustrating the cyclic

nature of the sedimentary sequence.


11/16

M A R I N E S E D I M E N T C O R E M E T H O D S 9 ~

>

w) Hs

NOI~ I I -dNS

~

l

O

O

O

~


12/16

9

P P E WEAVER AND P J SCHULTHEISS

C u r re n t l y t h e r e i s s c o p e fo r t h e d e v e l o p m e n t o f

a mul t i - senso r who le-co re logg ing appara tus tha t ,

in add i t ion to p rov id ing a too l fo r the cu rren t ly

e s t a b li s h e d t e c h n iq u e s g a m m a r a y a tt e n u a t i o n ,

P-W ave ve loc i ty and magn et ic suscep t ib il i ty ) ,

w o u l d i n c l u d e o t h e r n o n -d e s t ru c t i v e m e a s u re m e n t s

such as e lec t r i ca l res i s t iv i ty , spec t ra l na tu ra l gamma,

radar , u l t rason ic re f lec t ion and poss ib ly neu t ro n

ac t iva t ion .

Subcor ing

Box co res re t r i eve l a rge vo lumes o f sed imen t wh ich

a re o f t e n s u b c o re d a s s o o n a s t h e c o re a r r i v e s o n

deck . The lOS box co rer i s f i t t ed wi th a ver t i ca l row

of sample po r t s w h ich can be eas i ly opened , g iv ing

access to a se r ies o f sed imen t l ayers wh ich can be

sampled wi th sy r inges befo re the co re i s opened

P e t e r s e t a l . 1980) . I f the upper c los ing spade o f

the box co rer i s removed the sed imen t su r face wi l l

be revea led wh ich can be sampled by push ing in

leng ths o f co r e l iner. Bet te r co res wi l l be t aken i f a

p i s ton i s u sed which can be he ld a t the sed imen t

su rface wi th a smal l j ig wh ich i s a t t ached to the

c o re r f r a m e . Th e b o x c o re r d e s c r i b e d b y P a p u c c i e t

a l . 1986) has a cy l ind r ica l bar re l wi th a segmen ted

l iner thus a l lowing the co re to be ser ia l ly sec t ioned .

For geo log ica l inves t iga t ions i t i s o f ten impor tan t to

t r a n s p o r t t h e w h o l e c o re t o t h e l a b o ra t o ry , a n d t h i s

may be done by us ing a p las t i c l iner as descr ibed by

Kar l 1976) , o r by us ing in te rchangeab le b oxes as

d e s c r ib e d b y B o u m a 1 9 6 9) .

C o r e

Cut t ing

Core l iners can be cu t wi th e i ther a saw o r a b lade .

R o t a t i n g s a w s a r e n o i s y a n d p o t e n t i a l l y d a n g e ro u s

a n d p ro d u c e l a rge a m o u n t s o f p la s t ic s w a r f w h i c h

f r e q u e n t l y b e c o m e s e m b e d d e d i n t h e s e d i m e n t .

O t h e r m e t h o d s h a v e t h e r e fo re b e e n s o u g h t w h i c h

o v e rc o m e t h e s e p ro b l e m s . K a w o h l a n d K u d ra s s

1987) repor t the use o f a v ib ra t ing saw w hich i s

l es s dangerous than a ro ta t ing saw and p roduces a

cu r led th read o f p las t ic in s tead o f swarf . In the i r

s y s t e m t w o p a i r s o f s a w s a r e u s e d t o c u t a s l ic e o f

s e d i m e n t F i g . 7 ) w h i c h c a n t h en b e X - r a y e d a n d

sampled , l eav ing two in tac t s t r ip s o f co re fo r s to r -

age . Th is sys tem has the advan tage tha t the co re

d o e s n o t r e q u i r e s u c h h e a v y e q u i p m e n t t o h o l d

i t in p lace du r ing cu t t ing as i t does wi th a b lade

sys tem.

C o re c u t t e r s w h i c h u s e b l a d e s o p e ra t e b y p u l l i n g

t w o b l a d e s o n e o n e a c h s i d e o f th e c o re ) a l o n g t h e

leng th o f the co re . I t has been found tha t sharp

b l a d e s d o n o t w o rk b e c a u s e t h e y c a n n o t b e m a d e t o

cu t abso lu te ly s t ra igh t , and once the cu t has gone

off line i t i s imposs ib le to p reven t the b lade f rom

break ing . The b lades a re therefo re f i l ed f l a t o r b ro -

ken , and th i s b lun t edge i s u sed . The fo rce requ i red

t o p u l l t w o b l a d e s t h ro u g h P V C o r p o l y c a rb o n a t e

l iner i s cons iderab le , and read i ly bends the l iner i f i t

i s no t he ld very r ig id ly . Th is requ i res very s t rong

c o re h o l d i n g e q u i p m e n t a n d i t h a s b e e n fo u n d t h a t

1 m-long co res a re much eas ie r to hand le than

1 5

m-long ones .

C o r e Spli t t ing

The open ing o f s to red co res i s ex tens ive ly d i scussed

by Boum a 1969) . The t echn iques a re al l s imple ,

us ing b lades o r wi res to cu t the sed imen t , a l though

Bouma sugges t s app ly ing a DC e lec t r i c cu r ren t to

ach ieve an e lec t ro -osmot ic e f fec t i f poss ib le . The

p re s e n t a u t h o r s u s e a c o n s t a n t c u r r e n t D C p o w e r -

supp ly cap ab le o f p rov id ing 0 .5 am p a t 30 vo l t s ,

a n d a t t a c h t h e c a t h o d e t o t h e c u t t i n g b l a d e a n d t h e

a n o d e t o a p l a t i n u m w i r e w h i c h i s k e p t i n c o n t a c t

wi th the sed ime n t and in c lose p rox im i ty to the

b lade du r ing cu t t ing W eave r and Schu l the is s ,

1983b) . Sed imen t su r faces cu t in such a way are f ree

o f smear ing , sho w burrow s and l i tho log ica l

boun dar ies in minu te de ta il F ig . 8 ) and can revea l

smal l -sca le sed imen t d i s tu rbances due to co r ing . A

g o o d q u a l i t y c o l o u r p h o t o g ra p h o f a n o s m o t i c

kn i fe -cu t su r face combined wi th a descr ip t ion p ro -

d u c e s a m u c h b e t t e r r e c o rd o f a c o re t h a n a d e s c r i p -

t i o n a l o n e . W e a l s o u s e t h e o s m o t i c k n i f e t o c u t o f f

t h e a r c h i v e c o rn e r f ro m t h e K a s t e n l o t c o re s , a n d t o

t r im samples fo r geo techn ica l t es t ing .

One o ther m ethod o f co re sp l it t ing i s to f rac tu re

the co re in a con t ro l l ed way . Frac tu red su r faces can

revea l the max imum deta i l wi th in the sed imen ts be-

cause the su r faces have no t been d i s to r ted due to

loca l shear ing as wi th a cu t su r face . Th is method i s

u n p o p u l a r b e c a u s e o f t h e s e m i - r a n d o m s p li t w h i ch i s

a c h i ev e d a n d t h e p o o r v i su a l a p p e a ra n c e o f t h e

s u r fa c e . I t m a y b e n e c e s s a ry t o p h o t o g ra p h t h e c o re ,

thus p roduc ing a two-d imen s iona l image , befo re al l


13/16

MARINE SEDIMENT CORE METHODS 97

worm g ar

/

I

G

i

| '1 I |

.or,,g,,r I I

I ~- h.die

Fig. 7. Vibra t ing saw for spl i t t ing sediment cores into three sec t ions . The centre sec t ion is used for X-radiography (from Kawohl and

Kudrass, 1987).

t he fea tu res a re read i ly apparen t . An acc iden ta l lon-

g i tud ina l core f rac tu re was p roduced in a Kas ten lo t

core by We aver and S chul theiss 1983a) which re-

vea led numerous minu te open burrows each abou t

0 .5 mm across . S uch fea tu res would have been v i r tu-

al ly impossible to see in a cut surface even with only

smal l amounts o f smear ing .

ore Description and Photography

Cores are usual ly described immediately after spl i t -

t ing before any de ter io ra t ion o f the sed iment

sur face occurs . Trad i t iona l ly , the sed imento log i s t

records al l vis ible features of the core, such as

colour, texture, l i thological boundaries , burrows etc.


14/16

9 P. P. E. WEAVER AND P. J. SCHULTHEISS

Fig. 8. Examples o f heavi ly burrowe d sediment showing the clar i ty of detai l produced by a n electro osmotic knife cut surface.

The combination of a good quality colour pho-

tograph from an unsmeared osmotic knife-cut sur-

face with a multiple whole core log of the core can

provide much o f the information in a more objective

way than from a simple description. Ideally the

photographs and whole core logs should be backed-

up by comments from an experienced sedimentolo-

gist and the whole must be synthesized into

diagramatic form for publication. One advantage of

this method is that information can be interpreted

directly from the core even if the core is not avail-

able without having to also interpret the potentially

poor core description with its mis-interpretations or

omissions.

onclusions

The present range of available corers includes box

corers gravity corers piston corers giant piston

corers and vibrocorers. If:care is taken in selecting


15/16

MARINE SEDIMENTCORE METHODS

t he corer mos t appropr ia t e fo r the sed iment type and

mos t app ropr ia t e fo r the ana lyses to be conducted a

maximum of 20-30 m of co re shou ld be ob ta inab le .

Box corers provide good qual i ty cores with large

volumes from the upper 0 .5 m and often ret r ieve the

sed iment -water in te r face . S ing le-spade box corers

are more successful in obtaining cores in sandy sub-

st rates . Gra vi ty corers should be avoided or cores

f rom the m t rea ted wi th cau t ion fo r phys ica l p roper ty

s tud ies o r any work which requ i res p rec i se da ta on

the depths of boundaries within the sediment . This is

because they are subject to errors of mis-sampling

which cannot always be assessed. The one except ion

ma y be the Kas ten lo t co rer which because o f i ts

large cross-sect ional area gives relat ively undis tu rbed

cores in soft sediments . Pis ton corers produce more

representat ive resul ts than gravi ty corers but even

these are subject to mis-sampling errors due to move-

ment o f the p i s ton dur ing the cor ing opera t ion .

These corers ca n pen etrate up to 20 m in sui table

sed iments . Gia n t p i s ton corers a re capab le o f co r ing

to 30-50 m. At present the ST AC OR i s opera t iona l

and funct ion s wel l ret r ieving relat ively undis tu rbed

cores . The GPC has no t comple ted i t s des ign phase

and so i ts abi l i ty to recover long undis turbed cores is

cur ren t ly unknown.

New developments in corer hand l ing inc lude the

use of Kevlar cables which al low more precise moni-

to r ing o f opera t ions and the p roduct ion o f a con-

tainerized winch which al lows access to a much

larger number o f sh ips fo r runn ing cor ing opera-

t ions .

Whole-core logg ing t echn iques combined wi th

co lour pho tographs o f osmot ic kn i fe-cu t su r faces o f

spl i t cores provide excel lent and permanent records

of the core which are o f more va lue than descr ip t ions

on the i r own. The au thors s t rong ly com men d the

adop t ion o f these t echn iques by o ther workers in the

field.

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Documents

3b_Current Methods for Obtaining Logging and Spliting Marine Sediment Cores