Seismic and borehole analysis of Pantai Kundor, Melaka

Transcription

1 Gel. Sc. MalaYJia, Bulletin 35, July 1994; pp Seismic and brehle analysis f Pantai Kundr, Melaka ABD. RAHM SAMSUDN AND UMAR HAMzAH Jabatan Gelgi Universiti Kebangsaan Malaysia Bangi, Selangr D.E. Abstract: Twenty bring lgs were studied in cnjunctin with fur shallw seismic reflectin data. The brehles lcated clse t the seismic lines were drilled at depths ranging frm 12 t 40 metres. By cmparing the brehle data with the seismic recrd, empirical relatinships between the character f the seismic recrds and the crrespnding lithlgy was established. The seismic data were analysed using a velcity f 1,600 mlsec t cnvert the tw way travel times int depth. Fur distinct lithlgical units were recgnized and classified int units A, B, C, and D. The uppermst unit, D representing sft clay with sme carse sand is characterized by strng hrizntal stratificatin n seismic prfiles. Unit C which is made up f medium t carse sand with sme clay and shell fragments is recgnized by a chatic reflectin pattern in seismic recrds. Unit B is represented by dense silty sand with numerus mica flakes and it is interpreted as weathered granite. The lwer mst unit, A cmprises lwly weathered t fresh carse grained granites r bedrck and shws up as a dmed reflectin pattern n seismic sectins. Using these empirical relatinships, all the fur seismic prfiles were interpreted and analysed. NTRODUCTON This paper presents the results f seismic subbttm prfiling interpretatin and brehle analysis perfrmed t prvide subsurface infrmatin f a prpsed Petrnas marine facilities site at Pantai Kundr Melaka (UKM, 1990). The marine facilities cntain several majr cmpnents, ne f which is the exprt jetty capable f accmmdating up t 120,000 DWT tankers. The exprt jetty extending 1.0 t 1.5 km ffshre requires the cnstructin f a dredged apprach channel and a berthing basin with depth levels at 19 m belw chart datum (CD). Hwever, the ffshre depth in the prpsed area is rather shallw varying frm -10 m CD t -14 m CD. t is therefre necessary t dredge the apprach channel and the berthing area t -19 m CD t allw a passage way and berthing f 120,000 DWT tankers. Prir t the dredging activity, Sepakat Setia Perunding, acting as a cnsultant t Petrnas authrity had cnducted a high reslutin shallw seismic reflectin study as well as subsea sil investigatin in the prpsed area. The EG & G unibm sub-bttm prfiling system was used t prvide cntinuus shallw seismic prfiling alng the trestle area and acrss the prpsed jetty site. The equipment was perated with filter settings f 600 Hz fr lw-cut and 3,000 Hz fr high-cut, firing interval f375 ms (millisecnd) using a sweep speed f 125 ms (millisecnd). The seismic recrds and brehle data submitted t Petrnas were then used in this study t interpret the subsurface lithlgy and structure f the uncnslidated sediments as well as the nature f the bedrck in the study area. SURFACE AND SUBSURFACE GEOLOGY The gelgy f the cast adjacent t the prpsed site f the jetty, adapted frm the Preliminary Petrnas Melaka Refinery EA reprt (UKM, 1988) is shwn in Figure 1. n general, there are nly tw main rck types in the area i.e. granites and quartz mica schist. The granites cver the nrthern part f the area whilst the schists ccupy the suthern area. Tpgraphically, the granite frms a relatively flatter terrain than schists. t is the carse grained phrphyritic type f which the phrphyritic bitite granite is the dminant variety. Large phencrysts are a cmmn feature f this granite. The phencrysts are mainly micrcline feldspar measuring up t 5 cm in length. Bitite n the ther hand, has a much smaller size ranging frm 0.02 mm t 0.8 mm. The age f the granite is estimated t be either Upper Triassic r middle Lwer Jurassic. Generally, the granites are severely weathered. Hwever, relatively less weathered granite utcrps are fund alng parts f the shreline as well as ffshre. The seaward extensin f granitic bedrck is cvered by verburden which cnsists f uncnslidated, stratified yung sediments. A few tiny islands f granite expsures are fund ff the cast f Pantai -Tanah Merah. Other than Paper presented at GSM Annual Gelgical Cnference '92.

2 62 ABO. RAHM SAMSUDN AND U MA R HAMZAH 0.5 km GRANTES ::::J QUARTZ DKE EZa QUARTZ- MCA - SCHSTS Figure 1. Ge 10 gy f Pantal. Kundr, Melaka. Ge. L S c. Ma LayJ la,. BuLLetin 35

3 SESMC AND BOREHOLE ANALYSS OF PANTA KUNDOR, MELAKA 63 weathering features, the granites als shw several sets f jints and sme f the jints are filled by quartz. A big quartz dyke trending nrthwest is fund in the middle f the granite area and this is as a result f late filling f an pen jint r fissure. The granite is f Upper Triassic t Middle Jurassic age (Gbbett and Hutchisn, 1973). The schists have been described t display mre rlling and jagged landfrm as cmpared t that f the granites (UKM, 1988). t is interpreted t be f Lwer Palaezic age and as quartz-mica-schist type. Their clur ranges frm pinkish t greyish black. This clur reflects the irn cntent f the rcks. Mst f the rcks are fund t be weathered and changed t laterite at the tp. The laterites are fund nly in the schist area. The laterite is derived as a result f weathering f irn-rich schists and it is fund in the frm f huge blcks. These blcks f laterite are fund alng the shre near the site f the prpsed jetty. n the ffshre area, subsurface sil frmatins encuntered were mainly 5 m t 20 m thick sft alluvial clays, verlying firm t stiff clayey silts with significant mica cntents, prbably a result f decmpsitin fhrnfelses, which in turn verlies medium dense silty carse sand frm the weathering f rcks f granitic rigin. With depth, the decmpsitin f granite reduces in intensity resulting in prgressively denser and harder sill rck hrizns. The hrnfelses are likely a result f cntact metamrphic prcesses arund the fringes f granite intrusives. SESMC AND BOREHOLE ANALYSS A ttal f twenty brehle lgs were studied in cnjunctin with fur seismic sub-bttm prfiling data. These raw data were available frm the cnsultancy reprt (Sepakat Setia Perunding, 1988 and 1989) submitted t Petrnas Penapisan (Melaka) Sdn. Bhd. Mst f the brehles were lcated alng r very clse t the seismic reflectin prfiles. The exact lcatin f the brehles and the seismic lines are shwn in Figure 2. The brehles have depths varying frm t 38.8 m belw sea bed. The brehle lgs were studied and their results were superimpsed nt the relevant seismic recrds. n this way, the empirical relatinship between the character f the seismic recrds and the crrespnding lithlgy was established. The seismic data were analysed using a velcity f 1,600 mlsec t cnvert tw way time t depth. Frm the brehle lgs, fur distinct lithlgical units were recgnized and their descriptin are given belw: Unit D: The uppermst unit immediately underlying the sea bed is characterized by sediments JuLy 1994 f sft greenish grey clay (D2) and in part intermixed with light grey carse sand (Dl). On the seismic prfiles, these sediments are characterized by a fine texture and weak t strng stratificatin which is generally hrizntal. A carser texture with relatively weak stratificatin generally ccurs where the sediments are dminantly sandy. This type f material frequently have very lw mechanical strength and frm very pr fundatins fr ffshre structures. Unit C: This unit is characterized by yellwish grey sediments f medium t carse sand. t is lse t medium dense with clay seams and shell fragments. Carse grained sediments are nrmally characterized by a chatic reflectin pattern (line 4) in seismic data. Unit B: This unit is represented by samples f light grey and brwnish yellw silty fine t carse sand. The samples are very dense and in sme parts it is high and mderately dense with numerus mica flakes. t is interpreted either as decmpsed r weathered granite. This weathered layer is shwn as relatively reflectin free areas in seismic recrds. Unit A: The lwermst unit which cmprises lwly weathered t fresh strng carse grained granite (bedrck). This unit is fund in several brehles and is seen as a 'dmed' verlapping reflectin pattern n seismic sectin. Using the abve empirical relatinships, all the fur seismic prfiles (Fig. 3) were interpreted and their results are presented in Figure 4. The resulting gephysical prfiles are described individually belw: Line 1 (Figure 3a and 4a): Line 1 cmprises units D (1 and 2), C, B and A. The surface f the lwly weathered bedrck (Unit A) in this sectin lies at abut 35 t 40 meters. t is verlain by a thick layer f weathered bedrck (Unit B). A thin wedge f Unit C lies abve Unit B. The tp mst Unit D is characterized by parallel and relatively weak dipping layers. Line 2 (Figure 2b and 4b): Line 2 prfile is very similar t that as interpreted fr line 1. The nly difference t nte is that Unit C is relatively thicker than Unit Band that the tpgraphy f Unit A appears t be mre undulating. Line 3 (Figure 3c and 4c): Unit D is the uppermst unit in this sectin. This unit is relatively thick (> 10 meters) and is verlying Unit B. The unit underlying Unit B is designated Unit A. Unit C being absent at this particular lcatin.

4 64 ABO. RAHM SAMSUDN AND UMAR HAMZAH N t l > ". "' 8 c > ". "' <.. ". "' LNE 4 0"'.> N 't "' N B 107. Bre hle N Figure 2. Seismic lines and lcatin f the brehles. Gel. Sc. MalaYJia, Bulletin 35

5 SESMC AND BOREHOLE ANALYSS OF PANTA KUNDOR, MELAKA 65 a) L E b) LNE :,;::-.:-:...:.,,.' 1 : 1 Figure 3a. Seismic sectins fline l ea), and line 2(b). " p -

6 66 ABO. RAHM SAMSUON AND UMAR HAMZAH d) LiNE 4 Figure 3b. Seismic sectins f line 3(c) and line 4(d). Ce!. Sc. MaLaYJ ia, BuLLetin 55

7 a) LNE 0,5 0,0 1.5 km 100! 200. t 300 ii:::: C 400 B B B V A "-- A T B 116 B 14 BH2 B 118 \' ,5 -! 25,0 3' 37,0 3" c 50,0 b) LNE 2 0, km 10 20,... E 'li C ' r '--"' r\ C 'L - 02 C 0ty "-- ('U -... A A BH BHO BH2 BH8 BH5 12,5 -! 25,0 3' CD iii 37,5 3 50,0 62, C Sft greenish grey cly with sme time,... Light grey ne! brwnish yellw silty crae inter mixed with light grey crse sand B e+' sand l very dense) high t mderately e +0-+ ldecmpsedr weathered granite) Yellw greydmedium t carse sand lse t medium ense with cly seams + shell fragment l E ;; Lwly weathered t fresh granite c) 0,5 2.0km' 0 0 u > SEA BED 10 ::E ; E 30 " :;: 37.5,.. 3 " 50.0 A?? 40- A 0 t A A m DSTANCE d BHi 05 LNE 4.. 0,5 10 GO ,37:5 30.: 50,O 40-3 i 62, DSTANCE (kml Figure 4. Depth sectins and gelgical interpretatin fline l(a), line 2(b), line 3(c) and line 4(d).

8 68 ABO. RAHM SAMSUDN AND UMAR HAMZAH Line 4 (Figure 3d and 4d): All the fur units are present in this sectin. Unit D is the uppermst unit which appears t be thicker t the suth-west. Unit C appears as lenses. All ther units shw strngly erded surfaces and sme parts are truncated. A predminant dip twards the suth-west is shwn by the sediments sectin, an indicatin f sediment supply frm the nrth-east. The units in the suth-west sectin appear t exhibit evidence f rewrking. The highly chatic and internal structure and extreme variatin in unit thickness als tends t indicate that the sediments were depsited in a highly active fluvial envirnment. CONCLUSON The subsurface gelgical sectins alng the fur seismic lines were successfully btained by establishing the empirical relatinship between the character f the seismic recrds and the crrespnding lithlgy f the brehle data. A cmplete gelgical sectin f the prpsed site is well represented by the interpreted seismic prfile f line 4 which shws the presence f all the fur lithlgical units A, B, C and D. n average, the maximum depth t which granite bedrck (Unit A) culd be reliably mapped is abut 30 t 40 m belw sea level. The granite bedrck surface ffshre is characterized by steep slpes and high relief and an verall relatively gentle slpe twards the suthwest. The 40 m depth f bedrck belw sea level ccurs at abut 1.5 km frm the castline. The tpgraphy f the bedrck parallels the shreline is relatively undulating with n apparent change f slpe in that directin. Penetratin f seismic data in this area was limited, prbably in the mst part f high sand cntent in the upper sils but als by the quartz sands and micas f the weathered granite. Brehle recrds indicate that this material type is prevalent beneath the clays and silty sands depsited frm elsewhere. Due t the lack f internal structure exhibited by the weathered igneus rcks it has been difficult frm this survey, t indicate the depth t bedrck and the maximum depth f sft sediments. The seismic data in cnjunctin with brehle data des, hwever indicate the extreme variability in thickness f the sft sediments. REFERENCES GOBBElT, D.J. and HUTCHSON, C.S., Gelgy f the Malay Peninsula, Wiley-nterscience, New Yrk. SEPAKAT SETA PERUNDNG, Reprt hydrgraphic survey assciated wrks Petrnas Refinery Prject Melaka. SEPAKAT SETA PERUNDNG, Subsea sil investigatin secnd Petrnas Refinery Prject, Melaka. UNVERSm KEBANGSAAN MALAYSA, Preliminary EA Petrnas Secnd Refinery Prject Melaka, Final Reprt. UNVERSm KEBANGSAAN MALAYSA, Envirnmental mpact Assessment fr Petrnas Refinery Marine Facilities Melaka Manuscript received 1 April 1993 GeL. Sc. MaLay"ia, BuLLetin 35