--'SU DUNG PHUONG PHAP MINH GIAI KET HOP TAI UEU TRONG LUC, TU VA D\A CHAN OE XAC OINH SU P H A N B 6 PHUN T R A O BAZAN
NUI LUA TREN THEM LUC DIA NAM TRUNG B O V A L A N C A N
TS. Iran Tuan Dung Vlin Dia chdt vd Dia vgt If bien
Tom tat
Hogt dpng phun trdo bazan nui liia tren them luc dia vd bien Odng xdy ra khd mgnh, song rdt kho xdc dinh phgm vi, vj tri khong gian. Bdi bdo gidi thieu phUc/ng phdp chuyen titd vidp thdp vixich dgo nhdm tgo mdi tUang quan tdt han giCta di thudng vd nguon gdy di thUdng tii; phuang phdp Igc trUdng tdn sdcao diphdn tdch trudng trgng lUc, tCt vd cdc thdng tin vephun trdo bazan nui lira dphdn tren cua vo trdi ddt; phuang phdp gradient todn phdn 3D xdc dinh vj tri khdng gian cdc khdi mat dp, tdtinh cao. Tdc gid ddxdc dinh sUphdn bdphun trdo bazan nuiluTa bdng phep phdn tich tuang quan da chieu gida trudng trgng It/c vd tn/dng tdftdn sdcao vdi truimg trgng sd gradient todn phdn 3D.
Kit qud nghien cdu dUac kiem chdtig bdng mptsd tdi lieu dja chdn vd giing khoan hien cd trong khu vUc nghien nib.
Tit khda: Phuang phdp chuyen tii ve xfch dao. loc trudng theo tan so gradient todn phan 3D, phun trdo bazan nui ilia, them iuc dia Nam Trung Bd.
Bien Ddng hinh thanh sau sU va cham giQa cac mang An Dp va Au - A. Mpt sd bdn trung dugc md theo kieu keo tach (pull-apart) gay ra hien tuong tach gian vd thach quyen, chia c i t cac m i n h tieu lue dja... Qua trinh md bien Ddng dan den k i l u tach gian dai duong hinh thanh nen cic bon trung. Tich gian cham dot vao cuoi thdi k^ Miocene, sau sy va cham giQa cac t h i n h tao vung Bac Palawan vdi cung d i o Tly Philippines v l giUa d i o Dai Loan vdi lye dja Trung Hoa. Giai doan dau cua qua trinh taeh gian thach quyen thudng kem theo hoat ddng magma {phun trao lan x i m nhap), Tuy nhien, d bien Ddng hoat dpng nui IQa k h i r i m rp sau t i c h gian va phun t r i o x u l t hien khdng ehi trong khu vUc bien Ddng ma cdn nhieu noi khie d Ddng Nam A.
Hoat dong nui IQa xuyen e l t them dai duang v l cic t i l u luc dia nhu Hoang Sa, TrUdng Sa, Reed Bankva s i n sinh cac tang bazan npi mang tai mpt so vung tren bien Ddng.
107 108 109 110 111 112 113
107 108 109 -25 -16 -8 -3 5 14
1 1 0 1 1 1 1 1 2 1 1 3 79 118 147 167 1B8 262 325
EmGal Hinh 1. Di thudng trong luc Bughe
1 4 DAU KHl 3 0 8/2014
Nhieu y k i l n eho r i n g q u i trinh md bien Ddng phQc tap hon nhieu so vdi cac tai lieu hien cd. Theo ket q u i dja nhiet v l trpng lyc, mQc dp sut lun vd cung c h i m hoac ele ddng nhiet x i m n h i p tQ dUdi i^n r l t cao. Quan he thdi gian v l khdng gian cCia hoat dpng phun trao sau t i c h gian khdng ro rang, sd lieu dja vat ly khdng thdng nhat, do dd chua xac djnh ddng lye ehu dao cho hoat ddng phun trao ia sQ n i n g len cua manti hay t i c h gian thach q u y l n [13,15].
Cic boat dpng nui IQa xay ra trong nhilu pha va phun trao bazan nui IQa phat trien rpng rai tren b i l n Odng v l cac ria lyc dja. CIc phun trao bazan nui IQa tud1 Mesozoic mudn chu yeu I I d l granite tap trung d ria phia Bic bien Ddng, t h i m luc dja Ddng DUOng v l khu vUc quan d i o Hoang Sa. Cac loai phun t r i o nui IQa Cenozoic chCi y l u la bazan, pho bien d ria luc dja va vd dai duong. Nhin chung, d i e d i l m p h i n bd phun trao bazan nui iQa ed mdi quan he chat che vdi qua trinh hoat dpng k i l n tao - dja ddng lyc khu vyc b i l n Ddng [10,13].
113
Hinh 2. Di thudng trgng luc tU^ta
Phun trao bazan nui IQa la nguyen n h i n chfnh gay ra elc dj thudng trpng lyc, tQ dja phuong. Dya tren sU khic biptve mat dpva tQtinh giUa phun t r i o bazan nui IQa v l t r i m tfeh, nhieu nha khoa hoc da suf dung cac phuong p h i p dja vat ly nhu trpng lyc va tQ, dja chan de xac dinh nhan dang phun trao bazan nui ICfa. Tuy nhien, do hoat ddng kien tao phUc tap, sU khic biet Idn ve c l u true vd t r i i dat, sU da dang eua cic loai dat d i , sU bien ddi trUdng dja vat 1^ d vi dp thap, viec i p dung cac phuong p h i p dia vat ly thdng thudng hole don le se khdng dat hieu q u i tot.
Vl vly, can p h i i ap dung duoc mpt td hop phuong phap minh giai hieu q u i ddi vdi cic tai lipu trpng lyc va tQ cd ket hgp so sinh vdi tai iieu dia chan, gieng khoan [10,17].
Nham loai bd cle sai sd cQa dj thudng tQ d vi do thap do gdc lech va gdc nghieng tQ hoa, mdt so nha khoa hpc da dua ra cic phuong p h i p b i l n ddi trUdng tQve eye hoac v l xich dao trong mien khdng gian va m i l n tan sd, cd dp tin cly cao [7,17]. Viec ap dyng cic bp Igc t i n thap, tan cao cho phep xic djnh dj thudng trgng lyc, tQ khu vUc va dj thudng dia phuang. 6 day, cac dj thudng dia phuong - dj thudng tan cao dugc sQ dung de g i l i thfch, xic djnh phan bd phun trao bazan nQi IQa [10, 12]. Phuong phap gradient toan phan 3D thudng dUge sQ dung de xic djnh vj tri khdng gian cua cic khdi phun trao bazan nui IQa [3],
Ngudn tai lieu trong lye, tQ, dia chan tren vung b i l n Viet Nam chu y l u tU cac chuyen do true t i l p tren bien cCia cac cdng ty dja vat ly Lien bang Nga, Viet Nam, Phap,DUe, My, Nhat Bin.Trong cic de tai cap N h i nude thude chQOng trinh nghien cUu b i l n do Vien Dia chat va Dja vat ly bien chu tri (nhu KHCN-06-12, KC-09-02...) da thu thap, xQ ly va bd sung mdi k i t q u i nghien cQu ve trudng dja v i t ly rat cd gia trj tren khu vuc them luc dia Viet Nam va lan can [1,19].
BSn canh dd, t i c g i l cdn sQ dung ngudn sd lieu do dac chu dpng tQ ve tinh k i t hgp vdi do dac trQc tiep tren bien.Trong do, mang ludi sd lieu di thudng trong luc, til mdi nhat do cac n h i khoa hgc My xay dyng vdi dp chi t i l t dat d i n I'x
DIU KHl-sd 8/2014 15
l'{Sandwell and SmithV21.1) [16], dp s i u cac dai dUong tren the gidi 2,5 x 2,5km (Hinh 1, 2, 3). Tli lieu trUOng tQ (EMAG2) cua Trung t i m DQ lieu Qudc gia My (USNDC) la k i t q u i eua dU an hgp t i e toan eau v l dQ lieu dja vat ly, thudng xuyen duge cap nhat, bd sung mdi. D l y I I ngudn sd lieu dja vat iy cd do p h l n g i i i ddng deu nhat, dp bao phd rpng, dp chinh xac cao, duac sil dung hieu q u i eho cic nghien cQu cau true dia chat tren bien Ddng [16, 19], Khu vUc nghien cQu t i c g i l lUa chon trong ptiam vi tQ 107 -113° kinh Ddng va 9 -16° vT Bac.
Ngoai ra, t i c gia tham khio, sQ dung cle ngudn tai lieu dia chan tQ cac dy I n k h i o sat tham dd dau khi trong va ngoai nQde nhuVOR93,TC93, CSL07,PK08...[14].
2. PhUtfng phap ^p dung Nghien cQu x i c djnh p h l n bo cae loai phun trao bazan nui IQa ( k l e l phun trao hay x i m nhap) tren bien Odng can ed eli nhin tdng t h i nhat v l cau true dja chat va mdi tuang quan vdi phun t r i o bazan nui IQa d nhUng giai doan khic nhau. Die biet, can ap dung td hgp elc phuong phap d[a vat ly de hieu chinh trudng dj thudng tuang quan tdt vdi ddi tUgng gay dj thudng;
de p h l n t i e h cae dj thudng g i y ra bdi phun t r i o bazan nui IQa gan be mat;
de xle djnh pham vi, vj tri khdng gian cua cic ddi tuong gay di thudng.
2.1. Chuyin tdvixich dgo Viec p h l n tieh d|nh iugng tai lieu tQ d Viet Nam g i p khd k h i n do hien tQgng tQ hda nghieng, vj trf dj thudng v i ngudn g i y dj thUdng khdng trung nhau d vung vT do cao. Vi t h i , mudn xic dinh ehfnh xae elc vat t h i di tU, can thyc hien phep chuyen trudng v l eye. Tuy nhien, phep b i l n ddi dd kha phQc tap va khd hpi ty d vung vT dd thap, do t o l n tQ ehuyen trudng v l cue
Hinh 3. Do sdu ddy bien
khdng dn djnh, khulch dai c l nhieu cd trong sd lipu quan s i t v l thudng tao nen elc di thudng g i l dang keo dai ian at tfn hieu hufu ich [8],
Trong khi dd, vung nghien cQu nam d vT dp thap, gan xfch dao - nai dp tU hda nghieng va di thuOng tQ cd mdi tUOng quan k^m vdi ddi tUgng dia chat g i y dj thQdng. Vi vly, t i c g i l sQ dung phuong phap ehuyen tQ v l xfch dao, D i y l i phuong p h i p mOiap dung chovCingvTddtQ thap n h l m hieu chinh vj t r i d | thudng tQve trung tdt nhat vdi ngudn gay ra chung, k h i l n viee minh gill dj thudng tQde dang han ma khdng lam mat di y nghia dia vat ly [5,7,17].
Trong nghien eQu nay, t i c g i l I p dung phuong p h i p chuyin tQ v l xich dao trong cdng trinh [7,17]. Gdc lech tQva gdc nghieng tQ hda dUgc tinh cho diem trung t i m cua khu vUc nghien cQu [6].
Ham tinh chuyin trudng tQ ve cQc dugc b i l u dien nhu sau [5,7];
L(,h
n(l) -1 cos(l) X COS(D -fi)]H^sA
t%) + cos'(la)xcos'(D-^j\>{sin'(l) + cos'(l)xcos'(D-6'Jl Trong dd:
D: Gdc leeh tQ;
q: Hudng sdng;
I: Gde tQ khuynh;
ia: Gdc hieu chinh bien do.
16 DAUXHI so 8/2014
110 111 112 113
Hinh 4. Di thudng tUda tfnh chuyen vexi'ch dgo Hinh 4 I I bUc tranh dj thUdng tQ sau khi d l tfnh chuyin v l xich dao.
So sinh sd lieu tren Hinh 4 vdi sd lieu ban dau trong Hinh 2, cd t h i t h i y di thudng tQ sau khi tinh chuyen ve xich dao d l co sy thay ddi manh c l ve phuang p h i t t r i l n v l dp Idn (Hinh 4).
2.2. Lgc tn/dng theo tdn sd
Trong nghien cQu nay, phUOng p h i p Igc trUdng theo tan so phan taeh dj thudng trgng lye, tQ g i y bdi elc ddi tugng dia chat tang ndng ra khdi trudng tong de xle djnh ranh gidi v i vj trf khdng gian cua cae phun trao bazan nui IQa p h i t t r i l n d phan tren cua vd trai da't.
TrQdng trpng lUc Bughe dupe Ipc vdi cac buOc sdng X khac nhau bdi p h l n mem FIV[12].
Vdi cle budc sdng duoc lya chon, di thudng trgng luc tan thap duac tinh theo cdng thQc sau [12]:
Vdi bp loc Gauss:
^gu-Ff^-y)-—-2]j^(K^^M^u.Jexpli(;i,x+A^,)]dJi,dA^ (2)
Trong dd;
Ag: TrUdng trgng lye Bughe;
Ag^j,: Gil trj loc trudng tan sd thap;
\ntcfr ^^^^ sdng tach Ipc.
Trong cac budc tiep theo, ed tinh den trudng tan so cao, gia trj thu dUgc bang cleh lay trudng dj thudng tdng Bughe trQ di trudng tan so thap d cdng thQc (2):
Ag^P= Ag-Ag^pp (3) Sau khi da loc tan vdi cac bUde sdng X khae nhau, di thudng trpng lye tuong Qng vdi cac bUdc sdng ndi tren se duac sQ dung d l tfnh t o i n va minh g i i i cae eau true dja chat d cac chilu sau khac nhau.
6 day, dj thudng trong lUe, di thudng tQ tan eao dugc tinh eho toan bp khu vyc vdi bp ipc Gauss theo nghien cQu [12] vdi budc sdng Ige "k. Vdi mdi bUde sdng loc X se eho mpt bQc tranh p h l n bd trudng d elc dp s i u khac nhau. So s i n h trudng Igc tan cao d eae bude sdng X nhat dinh eho thay, dj thudng trong lyc, tQ tuong Ung vdi budc sdng A = 50km (Hinh 5, 6) ed mdi lien quan kha chat che vdi c i c ddi tugng dja chat d tang ndng. Vi t h i , chung dugc SLf dung de xay dyng md hinh 3D gradient toan phan tinh toan x i c djnh vj trf, pham vi cac phun t r i o bazan nui IQa.
2.3. Gradient todn phdn 3D Phuong phap gradient toan p h l n 3D dj thudng trgng lyc, tQtrUOng dugc sQ dyng de xae dinh pham vi, vi trf cung n h U l l chieu siu tOi ddi tugng g i y di thudng. Trong nghien cQu nay, t i c g i l chgn phUOng phap dUgc trinh bay trong nghien cQu [3] xac djnh gradient toan phan 3D dj thudng trpng lye, tQ cho khu vUc nghian cQu. Cdng thUc tfnh gradient toan phan 3D dugc bieu dien nhu sau;
A: Dj thudng trgng lyc hole tU.
DAU KHl-SO 6/2014
107 108 109 110 111 112 113
Hinh 5. Di thudng trong luc tdn cao bUdc sdng Igc A = SOkm
Hinh 6. Di thudng tUtdn cao budc sdng locX = SOkm
Phuong p h i p gradient toan phan 3D di thudng trong lyc va tU cho thay sU p h l n di kha ro net v l p h l n bd khdng gian cic phun trao bazan nQi IQa (d phan tr^n eua vd trii d i t ) . Tuy nhien, de g i l m bdt tinh da trj v l n i n g cao mQc dp tin cay eua phUOng phip, e i n p h i i cd sU danh gia p h l n tfeh tong hgp vdi cic tai lieu dja chat - dia vat iy khic. Kit q u i tinh toan gradient toan p h l n 3D the hien tren Hinh 7, 8.
3. Ket q u i va thao luSn
Tren khu vUc nghien cQu, phun trao bazan nui IQa ed m l t do cao va tU tinh manh hon ddi vdi mdi trudng dat da xung quanh. VOi cle phun trao bazan nui ICfa cd t h i n h p h l n khic nhau thi cd sUthay ddi tUOng ddi v l g i i trj mlt 66 v l tU tinh. Qua p h l n tieh so sinh vdi cic ngudn t l i lieu khic [2, 8, 13, 14], t i c g i l thiy r i n g phun trao bazan nui IQa trong khu vQc cd mat dp b i l n ddi trong khoing tQ 2,63 - 2,87g/
cm^ tQ tinh cd g i l trj bien doi trong pham vi tQ +50den+170nT.
TQ k i t q u i thu dUgc, t i e g i l thyc hien phep tich hgp thdng tin v l d i e diem phln bd dj thudng trpng lUc, tQ va gradient toan phln trong mdi tuong quan 3D xac djnh mQc dp tuong quan giUa chung. He sd tUong quan da chieu thu dMc ed g i l trj bien ddi trong khoing tQ-0,71 d i n +0,63 (Hinh 9a). K i t hgp cung vdi elc t l i lieu dia chan, g i l n g khoan [2,8,13,14]
{Hinh 9b), t i c gia da d i n h g i i , p h l n cap tuong quan, tQ dd dUa ra dMc bQc tranh ehi t i l t ve p h l n bd khdng gian cQa phun trao bazan nui IQa tren khu vye them luc dia Nam Trung Bp va l l n can {Hinh 10).
Qua nghien cQu, so s i n h vdi k i t q u i cCia mpt sd cdng trinh nghien cQu trude dd v l phun trao bazan nui IQa tren b i l n Ddng, t i c g i l thay ed sU trung hgp d i n g k l . Cae vung phan bo chCi yeu cQa phun trao bazan ndi IQa duge ehia so iugc t h i n h elc khu vyc: ddi n i n g ria lyc dja Trung Bp, Tu Chinh - Vung May, ddi p h i hQy vat mdng Tly Nam trung s i u bien Ddng.
Trong Hinh 10, vCing duoc khoanh dau m l u dd la vung cd tUOng quan tdt giQa phun trao bazan nui IQa va cac trUdng dja vat \'y - ed
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1 9 110 111 112 113
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2J^^S|M|i|Bj||
^C\'\i!>,^.. - "
^^S-'-' ^
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107 108 109 110 111 112 113 0.1 03 OS 06 OS 1.0 ^2 t.4 16 1.9 23 2.8 40
Hinh 7. Gradient tOdn phdn 3D trudng trgng lUc Hinh 8. Gradient todn phdn 3D trUdng td
Hinh 9. Hi s6 tuang quan khdng gian giUa trudng trgng luc tif tdn sd cao vd trucmg trong so gradient todn phdn 3D vdi phdn bo phun trdo bazan nui Ida (a); Phun trdo bazan nui Ida bieu hiin trin tdi liiu dia chan vd gieng khoan (b)
nia KHl - sg mou 1 g
107 109 110 [^11 Phun trao bazan nijr li>!
| C 3 I Ranh gi6l vi dai difonH - luc dja | : ^ ^ | Dittgay
Hinh 70. Phdn bdphun trdo bazan nui Ida trin them luc dia Nam Trung Bg vd Idn cdn
he so tuong quan Idn. Khu vye nay dugc danh g i i la vi trf khdng gian cua cac phun t r i o bazan nui IQa - cd mat dp cao va tQtinh manh.
Ngoai ra, qua phan tfch khdng gian bQc tranh dj thudng trpng lyc, tQva mdi tUOng quan vOi gradient toan phan, t i c g i l cho rang sy p h i n bd phun trao bazan nui iQa cd the edn chju i n h hudng bdi cic cau true sau ben dudi.
4. Ket luan
Nghien cQu da ap dung hieu q u i t d hgp cac phuang phap dia vat ly x l e djnh phan bo phun t r i o bazan nui IQa.
Phuong phap chuyin tQ ve xfch dao d vi dp thap tao nen mdi tuong quan tdt hon giQa dj thUdng va nguon g i y dj thudng tQ. Phuong phap lgc trudng tan sd cao da phan t i e h tdt dj thudng trong lye, tQ v l cac thdng tin v l phun t r i o bazan nui IQa d phan tren eua vd trai dat. Phuong p h i p gradient t o l n phan 3D xac djnh tin cly vj trf khdng gian cac khoi m l t dp, tQtinh cao.
Phuong p h i p p h l n tfch tUong quan da chieu giQa trUdng trpng lyc v l trudng tQ tan sd eao vdi trudng trong sd gradient toan phan 3D cho phep xic dinh kha chinh xAc pham vi cung nhu vj tri khdng gian cQa phun trao bazan nui IQa. De xic djnh chfnh xac sy phan bo phun trao bazan nui IQa, can ap dung to hgp phuong p h i p ^ a vat ly va cac tai lieu dja chan, dja hoa, t l i lieu gieng khoan...
Tai lieu tham khio
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312 trang.
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U r i Tnjc tech gian
20 DAU KHl ~ Sd B/2014
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19. Tran Tuan Dung, Nguyen Quang Minh, Vu Thu Anh. Anh hudng dja hinh day bien len di thudng trgng luc tren khu vi/c Bien Dong vd Idn can. Tuyen tap Blo cao Hpi nghi Qudc te Bien Ddng 2012: 90 nam cac hoat ddng hai duong hgc tren vung bien Viet Nam va lan can. 2012.
Determination of volcanic basalt distribution In Vietnam's south-central continental shelf and adjacent areas based
on integrated gravity, magnetic and seismic data
Tran Tuan Dung
Institute oflVlarine Geology and Geophysics Summary
Volcanic basalt eruption activities in Vietnam's continental shelf and the Bast Sea are very strong. However, it is difficult to define their ranges and spatial locations. This article presents the method of reduction to the equator of magnetic anomaly in low latitudes to bring out a better correlation between magnetic anomalies and their sources;
the high-frequency filtering method which is used to separate gravity and magnetic anomalies as well as information about the volcanic basalts in the upper part of the Earth's crust; and the 3D total gradient method to define the spatial location of high density and magnetic bodies. The distribution of eruptive volcanic basalts is determined by multi- dimensional correlation analysis between high frequency gravity and magnetic anomalies with weighted 3D total gradients. The achieved results are checked and confirmed by existing boreholes and seismic data in the study area.
Key words: Reduction to the equator of magnetic anomaly, frequency filtering, 3D total gradient. Volcanic basalt, Vietnam's continental shelf.
