Tap chi Cac Khoa hoc yh Trai Bit, 37(1), 48-56
(VAST)
\'ien Han lam Khoa hoc va Cong nghe Viet Nam Tap chi Cac Khoa hoc vS Trai OSt
Website: http:/ www,vjs.ac.vn index.phpjse
1
^ Kdt qua ap dung phuang phap tu telua nghien cuu he dia nhiet a khu vuc nguon nuac nong Bang - Quang Binh
Doan Van Tuy^n, Trdn Anh Vu, Lai Hop Phong, Le Van ST, Pham Ngoc Dat, Duong Thi Ninh, Dinh Van Toan, Nguyen TM Hong Quang
Vien Dia chdt, Vien Hdn lam Khoa hoc vd Cong nghe Viet Nam Ngay nhan bai; 11 -t
CMp nhan dang: 15 - -2014 2-2015
ABSTRACT
Resatts of magnetotelliuic si y for studying geolhermal system in the Bang a r e a , Q n a n g Binh province This p a p a piesQits the first results fiom die implication of magnetotelluric method (MT) using die new equipment NfTU 2000 (Canada) and analysis software to investigate Ihe sliuctine of geolhermal area around the Bang hot water source (Quang Bmh jHOvince). Results of d ^ a analysis by MT ID and 2D models to a dqith of 20 Icm show low resistivity zone in the southwest of Bang hot w ^ e r (100°C) and allow for inteiprebng the sbuctuial elements of alhdiydio- geothennal system. Hiis includes a very low resistivity layo- u depth of 2 km suggesting a clay cap (heat resistive shield), a letatively low resistivity zone at d^tfa > 2 km reflecting fiactuied locks containii^ geothennal fluid and hot steam. A lower resistivity body at dqUh of 12-14 km located about LS km fiom die hot water smnce indicates the existence of a heat source or a hot mass of intrusive magma., commonly diought to be sourees of typical bydio- geothemial systems potential fiir e n e i ^ eiqiloitaiioii. The obtained resuhs not only provide new information for better understanding geodiennal resouioe in die surveyed area, but also point out the methods and t e c h n o k ^ needed to improve the efTectiveness Ibr assessmg potratial of geothemial resources elsewhere in Vionam.
©2015 Vietnam Academy of Scioice and Technology
1. Mo^ dau*
Tren lanh tho Viet Nam da diong ke duoc hon 200 diem nuoc nong dia nMet xuAt Io hay phat M|n ducrc trong cac io khoan co nMet dp tu 30''C den 100°C; tren them luc dia cung 3a. phat M^n nMeu nguon rnroc nong tiong cac 16 khoan tham do dAu kM (TrSn Huyen va nnk, 1999; V6 Cong NgMep, 1998), mpt so k6t qua ngMen cuu khac ciing da phat Men mot so vung di tfauong dong nMet cao tren dat lien va them luc dia, hoat dpng magma Neogen-De tii xuat Men a nMeu noi, dac biet nui lira tre da tiing gM nhan vao nam 1923 tai
Tic gia lien h$. Email: [email protected]
Dao Tro thupc tinh Binh Thuan (Duchkov et al., 1992; He Lijuan, 1999; Kulinich, et al., 1989; V6 Cong NgMep, 1998),... chiing to ngudn dia nMet o nuoc ta phan bo rong rai, la doi tugng can quan tam ngMen cuu de danh gia tiem nang cac nguon dia nhiet nay cho khai thac phat trien nang lugng.
Trong nMeu nam qua cac nha ngMen ciiu trong nuoc cung nhu nuoc ngoai va ca cac nha diu tu ciing da quan tam Ska ngMen cihi kha nang khai thac nguon dia nMet cho phat trien nang lupng or Viet Nam (Gumming, 2009; Flynn et aL, 1997;
Hoang Huu Quy, 1998; Koemg et al., 1981;
Thomas Mathews et al., 2008; V6 Cong NgMq), 1998). Tuy nMen, kM khao sat thuc xk va xem xet
D. V. Tuyen va nnk/T^p chi Cac Khoa hpc ve Trai Dat, Tip 37 (2015) tai li?u deu nhan dinh rang Vi?t Nam co nguon dia
nhiet phan b6 rong rai, co thi khai thac cho phat trien nang luong ke ca phat di^n, nhung cac kk qua ngMen cmi c6n rat tan man, so sii va kem tin c§.y do mcri cM dupc dau tu cho cac difiu tra phat hien, tien hanh cdc phan tich mau nuoc xuat 10, ap dung cac phuong phap dia hoa, do dja v^t ly nong,... la nhirng phuong phap don gian ban dau cua giai doan nghien ciiu so b6, chua c6 dupc cac du li?u ve cau true, vi tri phan bo bSn dia nhiet can thiet cho giai doan tham do.
De dap ling yeu cau do can trien khai ap dung cac phuong phap dja vdt ly do sau, thong thuong cac bon dja nhiet phan bo 6 dp sau > 1km va cac yeu to cau true ciia he dia nM^t con phan bo is do sau l(in hon niia, nen doi hoi phai co phuong phap va thiet bj co kha nang nghien ciiu chi tilt toi dp sau tii l-2km din 10-20km va di^u nay chl co thi thirc hien dupc bang phuong phap tii telua va dta chin do sau (IGA report, 2013; He Lijuan, 1999;
Pellerin et al., 1996). Ket qui nghien ciiu thvtc nghi?m va tai li^u thvrc te da cho thSy, vat chSt trong cdc yeu to cau true ciia h€ dia nhi^t cilng nhu moi truong co moi quan he chat che hon ca voi tham so dan dien c6 the nhan dupc tu ket qui ap dung cdc phirong phdp tham d6 dien tii: chit long dia nhi?t trong b6n nhi$t luon hoa tan cdc muoi va khodng vat c6 dp din di?n cao hon dd binh thudng, nguon nhi$t co nhi^t dp rat cao cung 1dm tang dp dan di^n so vol cdc da trong v6 Trdi Dat,,.. Vi vay, dl xac djnh vj tri phan bo, d ^ dilm cdu tnic he dia nhiet thi phuong phap di?n tir, trong do phuong phdp tir telua c6 kha nang nghiSn ciiu dp sSu Ion toi hang ch\ic km co vai tro chii d^o; trong nhilu van li^u quoc tl gpi Id "phuong phdp chudn"
(standard method) hay chu dao trong tham d6 dia nMpt (Cumming, 2009; IGA report, 2013; Munoz Gerard, 2014).
Trong ndm 2013 trong khuon kh6 nhi?m vy va kinh phi cua dl tdi cap nhd nu6c KC08.16/ 11-15:
"Nghien ciiu danh gid mpt so nguon dia nMet trien vong va CO dilu ki?n khai thac cho phdt trien nang lupng o Vi?t Nam" vin sy ho trp tMit bi MTU2000 ciia D^i hpc qu6c gia C:hung Cheng - Dai Loan, lan ddu tien Vi|n Dja chdt da thuc M?n 35 dilm do tir telua phan bo tren dien tich 4 x 2,5 km^ dl khdo sdt ngu^n nuoc nong Bang 100°C (thupc huy^n Le Thiiy, tinh Qudng Binh). Bai bdo gioi thi^u cdc kit qua phan tich tdi li#u tu telua nhan dupc da the hipn mo hinh ve cau tnic mpt he
thiiy dia nhiet cho thong tin ro hon ve tiem nang nguon dia nMet is day vd dinh huong cho budc tham do nguon dia nhiet o giai doan tiep theo.
2. D^c di^m dia chdt - kiln tao \k bieu hi^n dfa nhiet khu vyc nghien cmi
Dja diem khdo sdt tir telua nam trong khu vyc, ve mat dja chat, la phan doan cua dai udn nep Truong Son, dupc gioi h^n boi hai diit gay Ion:
Khe Giiia - Vmh Linh a phia dong bac va Da Krong - Hue 6 phia tay nam. Ve dja ly, khu vyc ngMen cim bao gom phan ria nam tinh Quang Binh va toan bp dien tich cua tinh Quang Trj, phia tay gidp voi lanh tho Lao, phia dong la Bien Dong (Mnhl).
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Hinh 1. So do d{a chil khu vyc Quang Tr) va phia nam tinh Quang Binh (theo ban do d|a chat Viet Nam ty l | 1' 1 000 000 (Ban ih Dia ch^t va khodng sdn Vi$t Nam ty le 1:1.000.000 Ofc Dja chit vi Khoang san V i | l Nam xuat ban 2004) va cac tai lieu cong bo khac)
Chii giii. ky hi^u da vi luoi dja chit. I-Tram tich Ocdovic mupn • Silua som. 2-Da magma xiim iih$p Paleozoi; 3-Basalt phun trdo D^ Tii; 4-Basalt phun tiko Neogen- De Tii; S-6iit g3y kien t^o b | c 1-2; 6- Bin gay kien tao bSc 3-4; 7-Ngu6n nude nong va k^ hieu
Tren bdn do dia chat Vi^t Nam ty le 1: 1.000.000 (Ban dl Dia chit va khoang sdn Vi?t Nam ty le 1:1.000.000. Cyc Dia chdt va Khodng san Vi?t Nam xudt bdn 2004) trong vung nay xudt 10 dd ciia cac thanh tao dia chat c6 Moi tii Cambri den Dp Tii. Dien phan bo rpng rai hon ca chilm phdn Ion dien tich khu vuc Id da ciia he tdng Long Dai CO tuoi Ocdovic mupn - Silua som (O3-S1 Id) chieu ddy dat toi hon 2.000m. Cdc khSi dd magma xam nhap Paleozoi (yS^) phan bo tren di?n rpng 6 49
Tap chi Cdc Khoa hpc phia tay va tay nam gan bien gioi Viet Nam - Lao;
cdc khoi basalt phun trdo Neogen - De tii (PN2-Q1) vd D | tit (pQiv) xudt Men rdi rdc tren todn khu vuc, CO d i ^ lp rpng d phia dong bac thupc huypn Vmh Linh tinh Quang Tri chiing to o ddy da dien ra hau het cdc hoat dpng magma cho d£n giai doan h i ^ dai; cdc dd khdc thudng co dien Id nhd dang dai hep.
Cdc he thdng dut gay kiln t?o khdc nhau tir quy mo Idn khu vyc the Men ro tren bdn do vd quy mo nhd cd the quan sdt dupc tren th\rc dia: Cdc diit gay bac 1 va bac 2 cd phuong tay tay bac - ddng dong nam la diit gay Khe Giiia - Vuih Linh va DaKrong - Hul, he thing diit gay bac 3 - 4 c6 phuong tdy bac - dong nam xudt Men sdm hon he thdng diit gay theo phuong ddng bac - tay nam.
Cdc diem nuoc ndng xuat Id trong vung da tdn t^i va dupc phdt hien tgi 3 dia diem: (i) d khu vuc Bang gdm cdc dilm 1^ nude ndng cd nMet do 75 - 100°C (nude ndng xuat 16 cd nMet dp cao nhat tren lanh thd Vift Nam) phan bd dpc khe sudi chieu dai 200m theo phuong d kinh tuyIn, noi cd dii cdc he thdng dut gay kiln tiio cat qua; (ii) d khu vyc Da Krong cdc diem xuat Id nude ndng nhiet dp 47 - 75''C phan bd rai rac tren cMeu dai 3 km d ria pMa bac sdng Qudng Tri, tning vdi mdt diit gay bac 4 cd phuong d vT tuyen phdt triln tir diit gay khu vuc DaKrong - Hue; (iii) d khu vuc Tan Lam nude ndng CO nhi?t dp 47''C xudt Id tren nen da vdi va noi giao cat ciia hai diit gay kien tao bac 3 co phuong tay bac - ddng nam va dut gay bac 4 cd phuong ddng bac - tdy nam xudt phat tir dirt gay DaKrong - H u l .
Dilu kien dia chdt - kiln tao neu tren vd su xudt lp nude ndng nMet dd cao la nhung tien d l cho thay 0 day cd khd nang ton tai cac ngudn dja nhi|t ed nMet the cao, cd trien vpng cho khai thdc phdt trien nang lupng can dupc Iriln khai cdc phuong phdp nghien ciiu de xdc dinh vi tri phan bd vd cau tnic ngudn dja nMet Id mpt yeu cau khoa hpc vd thuc tien cdp tiiilt.
3. Ket qua ap dung phinmg phap tir telua nghien cihi cau true nguon dia nhiet d khu vuc Bang
3.1. Cong tdc thuc dja do tir telua
Trong nam 2013 cdc cdn bd khoa hoc ciia Vien Dia chat - Vipn Han lam Khoa hoc vd Cong nghe Vi?t Nam da thirc hien do tu telua tai khu vuc Bang vdi bp thilt bj MTU 2000 (Canada sdn xudt)
v l T t d i Ddt, 37(1), 48-56
do cdc nha khoa hpc ciia Dai hpc Qudc gia Chung Cheng (Ddi Loan) cho thue. Day la bd iMet bj tu telua thi he mdi, Men dai cd khd nang chdng nMlu cao, mdy gM lmi sd lieu cd bd nhd I Gb gpn nhe (3kg) dung ngudn acqui 30 Ah, cdc ddu do thao 1 ^ nhanh thuan tipn cho vipc van ehuyen timg bO phan d i n cdc d i l m do khdng cd dudng giao thdng, bp phan m i m xir ly cd tdc dp nhanh,... ddp ling dupc cdc yeu cdu cho khao sdt cM tiet vd dieu kien dia hinh phiic tap trong ngMen curu dja nMet d Viet Nam va khu vuc Bang noi rieng.
Do tir telua bdng tMit bi MTU 2000 thuc hi^n theo cac yeu cau vd nguyen tac chung cua phuang phdp tu telua: d i l m do phdi cd be mat tuong ddi phang de b l tri dupc 2 cap dien c\ic chieu ddi mdi cap > 50ni, va hai Shu do tir thdnh phdn nam ngang dat tren mpt h§ true vudng gdc vdi quy udc tryc x theo phuong nam - bac, tryc y theo phirong tay - ddng; dam bdo cdc yeu cdu ve Men dien thfi nho giiia cdc dien cue, dien trd tiep ddt ciia cdc di^n c\rc,.., Doi vdi tMet bi MTU 2000 diem do tir telua phai cd dieu kien de tMet bi dinh vi GPS dat troBg mdy thu dupc tin hipu cua it ohkt 4 ve tinh, thuc hien viec chuan may va cdc dau thu tin hi^u (calibration) bang mpt phan mem d l lira chpn tham sd gM sd lieu phii hpp vdi dac diem trudng tir d dja diem do, thdi gian thirc hien 1,5 - 2 gid. Quy trinh do thuc dia thuc hien theo sach hudng ddn sii dung kem theo tMgt bi (MTU2000).
Tai moi d i l m do tren thuc dia thilt bj MTU 2000 ghi lien tuc vd lmi vao bd nhd tin Meu dao dpng theo thdi gian ciia 5 thdnh phdn dien tuf trong ddi tan sd 5000 - 0,003 Hz, gdm: thdnh phdn dien Ex cua c£tp dien cue vd thanh phdn tut nam ngang Hx dat theo tryc x, Ey eiia cap dipn cyc thii hai vd Hy ddt theo true y, thdnh phan tii thdng diing Hz.
Tai khu vyc Bang, budc ddu tien thuc hien 2 diem do tM ngMem vd xii ly so bd tdi lipu d l lya chpn thdi gian ghi thich hpp: vdi thdi gian ghi 20 gid (1 ngay dem) cho phep nhdn sd lieu cd chdt lupng tdt d ddi tdn sd 5000 - 0,01 Hz vd thdng tin v l mpt Idp ddn diSn d dp sdu khoang 2 km theo dudng COD§
Bostik; vdi thdi gian gM 10 gid (1 ngdy) cho sd lifu bao dam chdt lupn^ d dai tin sd 5000 - 0,1 Hz cung cd du thdng tin ve lop ddn d i | n ndy. Dya t r ^ ket qua do thi ngMem da thyc hien cdc diem do tiep theo vdi thtri gian ghi 10 gid (ban ngay) de dam bdo an todn va cM pM han chl. Kit qud cong tac thyc dia da nhan dupc 35 di£m do co chat lupng phdn bd tren khu vuc vdi dien tich 4 X 2,5km bao quanh diem 16 nude ndng Bang; do
D. V. TuyIn vd nnk/T^p cM Cac Khoa hpc vl Trai Dat, Tap 37 (2015) 3.2. Ket qud xu ly tdi lieu dieu kifn dja Mnh phiie t ^ (sdng, suli, nui cd
vdch vd sudn ddc cao) gdy khd khan cho viec tim mat bang de ddt tMet bi nen khdng bd tri dupc m ^ g ludi diem do cd khoang cdch deu theo yfiu cdu (hMh 2).
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Hinh 2. So d^ phSn b6 di^m do v^ cAc tuyen phin tfch t ^ li^u tit (elua khu vvc Bang
Chu giii: ' Ba8. vj t i va ky higu dilm do; A-A' - Toyin phSn tich ii\ li|u m6 hinh ID; B-B", C-C- TuyIn phSn tich tii lieu mfl hinh 2D; 'Bang - Diem xuit lp nuiic n6ng Bang (100°C)
Sau khi kit thiic mdi ngdy do t^i thyc dja, tin hieu ghi t^i moi dilm do dupc xir ly so bp bang phdn mIm SSMT 2000 va MTPlot nhan dupc hon 10 tham s6 khdc nhau dupc luu trii trong may tinh va cho phep ddnh gid so bp chdt lupng tdi li^u, thdng tin vl mdi trudng tr&i cdc tham so co ban is dang tdi lieu thd la cdc dudng cong dipn trd suat, cdc dudng cong pha, dudng cong ve tinh lien kit (Coherency),... ddc biet dudng cong Bostik cho thdng tin so bp ve sy the hipn vd dp sdu phdn bd tdng din dien la doi tupng can quan tdm. Todn bp cdc %h lipu thyc hi§n do tir telua d khu vyc Bang dupc luu trii tai Vipn Dja chat.
Tin hi^u dien tir telua thudng bi nhieu m ^ do cdc nguon dien trong ho^t d$ng cdng nghi^, giao thdng, dan sinh vd he thdng vien ttidng M|n nay cd d khap mpi noi. De cd so li§u dii chat lupng cho phdn tich dinh lupng cdn tiln hanh yi?c xir ly sd lipu ddi hdi nMlu thdi gian bdng phdn mIm MT- Editor, chi cd the thyc Men du9fc d van phdng sau dot thyc dja, budc ddu bang che dp tu dpng, sau dd su dung chiic nang xu ly thu cdng (Manual processing) de lo^i bd dupc nhilu bifin do Idn (Data Processing User guide, 2005). Sau kM thuc Mpn budc xii ly ndy_da nhdn dupc tai lifu cd chdt lupng tit d ddi tdn sd 5000 Hz d^n 0,1 Hz tren cac diem do vdi thdi gian gM 10 gid, d ddi tin s6 5000 Hz den 0,005 - 0,003 Hz cho cdc dilm do vdi thdi gian ghi 20 gid.
3.2. Kit qudphdn tich tdi li^u
S6 li§u chinh ddm bdo ch^t lupng nhdn dirpc tii ket qud xii ly sd lifu sir dyng de phdn tich djnh lupng t^i mdi dilm do la cap dudng cong dien trd suat cd tinh phdn cyc di^n (TE) la dudng cong pEity, cd tinh phdn cyc tu (TM) - pEyx vd c§p duimg cong pha tuong ling. Sd lipu cua todn bd cac dilm do dupc sir dung de phan tich theo md hinh mpt cMeu (ID), chpn sl Ii§u cdc dilm do phan bd dpc theo cdc tuyen cd phuong khdc nhau dl phan tich theo md Mnh 2D bang cdc phan mIm WinGlink, SondMT (WmGLink User guide). Trong bdi bdo ndy chi trinh bay kit qua phdn tich theo mo hinh ID cho tuyen A-A' vd phdn bl dien trd suit Idp din di^n tren dipn tich khdo sdt; kit qud phdn tich theo md Mnh 2D cho 2 tuydn B-B'^ vd C-C (hinh 2} the Mpn dupc nhfing thdng tm cdn quan tdm ve cdu tnic h? dia nhi^t. Tdi li^u iil4n dupc ciing du dieu ki?n eho phdn tich theo md hinh ba chidu (3p), nhung M§n nay chung tdi chua cd phin mem ndy.
Ket qud phan tich tai li^u theo md hinh ID dpc tuyen do A-A' cat qua diem Bai Id nai xuit lp nude ndng Bang, cho thay sy tdn t^\ Idp ddn dipn (dipn trd suat thip tii don vj din hdng chyc Ohm.m) phdn bd r^ng rai d dp sdu l,5-3km (hinh 3). Ldp phii tren cd difn trd suat cao (>500 Ohm.m) vd cac Idp ben dudi cd dien trd sudt rat cao (>1500 Ohm.m) phdn dnh tinh chat dd rdn chac khdng chiia nude, cho ph6p dy dodn Idp ddn dien cd kha nang la tdng chiia nude ndng (bon nMet) - ngudn cimg cdp nude eho dilm xudt lp Bang, di?n phdn bd vd gia tri dipn trd suit cua ting ddn di?n ndy thi hien tren hmh 4, cho thdy no cd xu the phat triln ve pMa ddng va tdy nam dilm nude ndng xuit lp.
Tap chi Cdc Khoa hpc v l Trai Ddt, 37(1), 48-56
Hinh 3, K ^ qui phin tich tii h$u tii telua md hinh 1D theo tuy^n A-A' (A-MJi cjl di?n Era suit bi&i kien; B-Mat
Hinh 4. Dien phfln b6 va d i ^ tro suit a khu vyc Bang
Chii glii • T B a l - Vj tri v i ky hieu
•Bang - Diem xuit lp nuoc n6ng 100°C;
dien tr6 suit thap, < 20 Ohm-m.
a ting din di^n diem do
— Ranh tu telua, gioi viuig
Thyc hi?n phdn tich tai lieu theo md hmh 2D bang phan mem thuong mai WmGlink (ban quyin sii dung - khda ciing do cdc nhd khoa hpc Dai Loan cho mupn) dya tren thuat toan phd bien Id phuong phdp phdn tir hihi han, tdi uu hda bang phuong p h ^ lap - lya chpn, dupc hudng dan chi tilt trong tdi lieu (WmGLink User guide). Phin mdm nay cd chl dp giao dien tren mdn hinh rit thuan ti^n cho thyc Men nhanh cdc phuong dn phan tich vdi cac tham sl khdc nhau: cdu true ludi md hinh, cMeu sdu khdo sat, dien trd suat ban ddu, sd lieu dien trd sudt vd pha theo phdn cyc dien (TE) va phdn cyc tii (TM) rieng re hodc ddng thdi ca hai, phuang phdp giai phuong trinh,... Tir dd se lya chpn dupe phuang an phdn tich vd md hinh hpp ly nhdt cho tap tdi heu do ihyc te.
Vdi cdc sd lipu thyc t l d Bang, phtrong an phdn tich vdi ludi md hinh dudi tuyIn do gdm 40 hang theo tryc z chifiu sau 20km, sd cpt giua cdc diem do la 4, di?n trd sudt ban ddu 300 Ohm.m, phuang phdp lam tron Occam, sd lipu dipn trd s u ^ va pha dang phdn cue TE hay TM, sd Idn lap din 10 dat dupc sai sd trung binh giira sd lipu do va tinh trfin md huih thay ddi trong kbodng 5 - 11%
phu thupc s l lipu tren tung tuyIn, Id k i t qua phii hop vdi thdng tm ve dia chdt va bieu Mpi dia nhiet.
D.V. TuyIn va nnk/T^ chi Cdc Khoa hpc vl Trdi Dit, T|p 37 (2015) Kfit qud phdn tich tdi li§u theo md hmh 2D den
dp sdu 18km theo hai tuyIn B-B' vd C-C' thi Men ro nhat thiet dien ddi dipn trd sudt thdp trong pham vi khdo sat vdi nhiing thdng tin phong phu hon (hinh 5) so vdi ket qud phdn tich tdi lieu theo mo hinh ID neu tren. Theo dd, ddi di£n trd suit thip (hdng chyc Ohm.m din 500 Ohm.m) phdn bd d nira dau cdc tuyen cd khd nang phdt trien sdu hon cd dp sdu khdo sdt dupc (18km) vd tilp xfic vdi vdng di?n trd suat tang cao (>1000 Ohm.m) d cudi Myen bdi ranh gidi cd gdc cdm gin nhu thdng diing. Trong ddi di?n trd suat thdp ndy thi hipn rd tinh phdn di (phdn Idp) di§n trd suit theo cMdu sau, bao gdm: lop phii tren cd dipn trd sudt tuong ddi cao (500-600 Ohm.m), Idp dan dien (hang chyc din 100 Ohm.m) phdn bd d dp sau tir 2 din 3kin, viing dien trd suat trung binh (100 - 500 Ohm.m) phdt trien sdu han 18km, trong dd d ria ben dudi diu tuyIn xuit hi§n diu nhd cua mpt ciu true difn trd suat tuong ddi thap (200 Ohm.m) d dp sau 12 - 14km. Theo kit qud phdn tich vdi sd li^u hifn cd, thilt di^n ciia ddi difn trd suit thip cd kich thudc keo ddi khoang 1,5km theo tuyen B-B' phuong d kinh tuyen (hinh 5B), diem xudt lp nude ndng Bang nam d ria bac ciia ddi, trong khi theo tuyen C-C phuong d vi Myen kich thudc tMet dien ciia nd nhd hon nhieu chi khodng 0,5 km.
Hluh S. M$t cit c i u true di@n trd suit theo ket q u i p h i n tich tai li^u mo hinh 2D theo tuySn B-B' v i C-C' Wiu vvtc Bang
Luu y rang, theo ket qud phan tich md iilnh 2D (Mnh 5) lop dien trd suat t h ^ (lap ddn dipn) cdn cd diu ht|u tdn t^i keo ddi ve phia cudi tuyen, nhimg do chi€u ddy nhd so vdi dd sdu phdn bd lai nam trong dd gdc dien trd sudt rat cao nen khdng the Mpn dupc. Ddi di$n trd suat thap cd diu hieu phat trien keo ddi ve pMa dau tuyen cdn bo sung md rpng dipn do tu telua ve phia nam se cho thdng tin day dii ve cdu tnic cua ddi nay. Ngodi ra, mot sd tham sd tu telua khdc nhu Skew chi thj tinh chat bdt ddng nhdt; vector dp ddn (nhdn dupc tir ket qua do 3 thdnh phin tii Hx, Hy, Hz) cM thj vl bidn dp vd hudng bien d6i dp dan dipn, neu dupc khai thdc se cho th£m thdng tin vl tinh chdt cua mdi trudng cung nhu ddi tupng can quan tam trong phfun vi khdo sdt.
4. Luan giai md hinh ciu true h^ dja nhi^t d khu vyc Bang theo tdi lif u tu* telua
4,1, Mo hinh nguySn tdc cau trdc hf thiiy dja nhift vd tham so di^n trd'suat
Phan tich ket qud dp dung phuong phdp dja vdt ly trong nMlu nam qua de khdo sdt xdc djnh cdu tnic, so sdnh vdi tdi li^u khoan, do nMet dp vd phdn tich mau d nMeu bdn dia nhi|t tren the gidi, cdc nhd ngMen ciiu da xdc lap dupc md hinh nguyen tdc (conceptual model) vl ciu tnic ciia mot he thiiy dja nhiet vd tham so dien trd suat the h i ^ trfin kit qud phdn tich tdi li^u tii telua (Cumming 2009; Munoz Gerard, 2014; Pellerin et al., 1996) (hinh 6). Md Mnh ndy cung cip nhimg thdng tin co bdn vl cdc ylu tl cdu tnic cua mpt he dia nhiet, dong thdi cdn Id nhirng chi tieu can thiet de luan gidi tdi li^u tii telua trong khdo sat vung dia nhift cy the, kM chua cd cdc tai li$u khoan sau. Theo dd, mpt he thiiy dja nhipl bao gom cdc ylu tl ciu true chinh vdi ddc tnmg dien trd sudt nhu sau:
(i) Lap mii s6t (clay cap) d ben tren dupc t^o thanh do qud trinh ddi lmi, dan dung dich vd hai nhiet tii dudi sau l£n gan be mat, cd ddc trung dif n trd suat rat thap (<10 Ohm.m). Thanh phin khodng v^t ciia mu set phy thudc nhi^t dp vd thdnh phan khoang vat ciia da chiia trong bin dia nhipt:
Khoang vdt smectite ddc tnmg cho bin nhi?t dp thip din trung bmh (70 - 150°C), hSn hpp illite- smectite cM ttii nhipt dp b6n nMet trung binh (150- 180°C), clorite vd epidote cM thj bdn nhipt cd
T?.p chi Cac Khoa hpc nhiet dp rdt cao (> 220''C). Mu set ndy ddng vai trd ehdn nMet, cdc diem xudt lp nude ndng len be mat chi tdn tai d nhihig nai cd khe niit cat qua mu set.
(ii) Bdn nMet Id vung da niii ne, cd hoc rong cua mpt thdnh hp dia chdt hay dupe tao ra bdi cac phd huy kien tao la noi cd dieu kipn tich luy dung djch dja nMet va dudng dan hoi nhipt tu ngudn nhipt di len, cd dac tnmg d i ^ trd suit tuang ddi thdp phu thudc vdo dipn trd suit ciia da chiia, thdnh phan dung djch, nMet dp,... nhung thudng Id cao hon dien trd sudt cua mu set ben tren.
(iii) Ngudn nMet hay ngudn cung cip nMet Id cdc the magma xdm nhdp cd nhiet dp rdt cao (>300°C) thudng phdn b l d dp sdu 8 - 12km, cd ddc trung dien trd sudt thip hon dd vdy quanh va di^n trd suat ciia bdn nhif t, cd thi nhdn biet dupc tren tai Ii?u tir telua. Nhieu van lieu thg gidi cdn cho biet, trong nhieu tnrcmg hpp ben canh cdc ciu true dipn trd sudt lien quan din cac ylu to hp dia nhipt cdn xuat hien nhfing cdu tnic dipn Ird suit thap gdy ra bdi cdc thdnh h? dja chit hay khdi qudng cd chua cdc khodng vat dan dien nhu cdc kim loai, sulpMte, graphite,... (hinh 6B) ddn din nhihig ludn giai sai Ipch vl ciu tnic he dia nhiet dya tren kit qua phdn tich tdi lipu tu telua (Munoz Gerard, 2014).
Hp dia nMet cd dii cdc yeu td ciu tnic neu tren dupc gpi Id ngudn thuy dia nMpt ngudn gdc magma va theo phan loai trong dja nMet ling dyng Id he dja nMpt km cho phep khai thdc nMet dn dinh bdi bon nhipt ludn dupe cdp nhiet tir ngudn nMet nhi?t dp cao theo co chl ddi luu nhipt vd khdng bj xd nhipt len bl mat (Data Processing User guide, 2005).
Hinb 6. Mo hinh nguyen tic ciu tnic v i tham dt?n iro suit mgt b6n thiiy dia n h i ^ A) Ly hrdng (Mum Geiard, 2014); B) Voi tai lieu thvc te (Pellerin et a l , 1996)
54
ve Trdi Ddt, 37(1), 48-56
4.2. Mpt so luan gidi ve mo hinh cau true he ^ nhift & khu vuc Bang
Ket qud phdn tich cac tai lifu: tii telua, dac tinh dia hda nude ndng xudt lp, thdng tin ve dieu kien dja chdt - kien tao d khu vyc Bang vd Idn can, md hinh nguyen tac cua mdt hp thiiy dja nhiet tren van lipu,. . cho phep dua ra mpt sd luan gidi ve md hinh cdu true ciia he dia nhiet d khu vyc nay.
Tdi lieu tii telua theo md hinh ID (Mnh 3, 4) mdi chi cung c ^ nhung ttidng tin ban ddu ve kha ndng tdn tai vd dien phdn bd mpt Idp ddn dien d khu vyc lan cdn diem xudt 10 nude ndng Bang (100°C) la cM thi cua mpt tang chiia nude dja nhi|t - ngudn cung cap nude ndng cho dilm xuit lp; kit qud phdn tich tdi lipu theo md hmh 2D (hinh 5) the hipn rd hon cdc chi thi ve yeu to ciu tnic vd vj tri phdn bd cua he dja nhipt. Theo dd, ddi dien trd suat thap phat trien sdu cd phuong d kinh tuyIn keo ddi I,5km tii diem nuoc ndng Bang vl pMa tdy nam vdi chieu rpng 0,5km, thi hipn cac cM ttii di?n trd sudt cua he dja nhift, tuong tu vdi cau true vd dipn trd suat cua md hinh hp thiiy dja nMet cdng bd trong van lieu qudc te neu tren (hinh 6):
(i) Ldp din dien (dipn trd suit thip nhit tii hdng chyc den 100 Ohm.m) phdn bd d dp sdu khoang 2 km phdn dnh tinh chit cua ldp (mH) set.
Ve dja chdt cd the ldp ndy phdn bo trong dd ciia cdc phy hp tang dudi vd giira ciia he ting Long Dai (Os-Si Idi.i) cd thdnh phan dd hat thd cMlm mi thi Id dieu kipn thuan lpi cho vipc ddn, chiia dung djch vd tich ty set.
Viing dien trd sudt trung binh (100 - 500 Ohm.m) phdt trien sau hon 18km phdn dnh tinh chdt cua da nut ne chiia nude vd hoi dia nMet cM thi ve mdt bdn dia nMpt. V l dja chit ddi ndy cd thi dupc tao thdnh bdi mpt phd hiiy kiln tao tre phuang a kinh tuyIn cat qua cdc thanh hp dja chit chua dupc xdc dinh, cd thi Id dd cd cua nin kit tinh tien Cambri vd da granit cd tinh chat dipn trd sudt rit cao nhu d ben ngoai ddi (han 1000 Ohm.m phdn dnh dd khdi rit rdn chic). Kit qud phdn tich bang cdc phuang phap dja hda tir cdc mau nude d ngudn xudt lp Bang chi thj nude d ria bdn nMet vd theo phuong phdp dia nMpt kl hda hpc nMet dp bdn nhiet dat tdi 160-180°C (Doan Van Tuyen et al., 2014; Hoang Huu Quy, 1998).
D.V. Tuyen vd nnk/Tap cM Cac Khoa hpc ve Trdi Ddt, Tap 37 (2015) (ii) Cd the Id tinh chit vd tham sd ciia chinh
bon dia nhiet ndy.
(iii) d ben dudi ria phia nam vd tdy nam ciia ddi d dp sdu 12 - 14 km xudt Men diu nhd cua m0t ciu true di|n trd sudt tuang ddi thip (200 Ohm.m) cho phep dy dodn vl su tdn tai mpt nguIn nhiet cd nhipt dp rat cao, cd the la mpt khdi magma xdm nhdp ? Tdi Ueu hipn cd chua the hien du dipn tich, dp sdu phdn bd ciing nhu thdnh phdn vat chit cua cdu true ndy, nhung xem xet tren bdn dd dia chdt (hinh 1) cho thiy, vl phia bdc dilm Id Bang khodng 8km va xa hon ve phia ddng - ddng nam (khu vyc Vmh Linh) lp ra cdc khdi phun trdo basalt tuli De Tii (PQiv) chimg td hojit dpng magma tre trong viing ndy da dien ra rdt manh me, do Id dieu kien d€ tdn t^i cdc ngudn magma xdm nhap, trong do cd nguIn nhiet dy bao trong ddi dipn trd sudt thip d khu vuc Bang. Vdi nMet dp nude xuat lp IOO''C, du bdo nhipt dp bon nhiet 160 - I80°C thi nhi^t dp nguIn nhipt (magma xdm nhdp) hodn toan cd kha ndng dat > 300°C nhu md hinh ddn trong van lieu the gidi.
Md hinh hp dja nhipt d Bang theo tdi lieu tii telua c6 chi thj tuong ty ve edu tnic so vdi md hinh trong van li^u quic tl, tuy nhifin vl tham so dipn trd suit cd cao ban, cd thi giai thich dupc bdi sy khdc bipt ve dieu kien dia chat, tinh chit din di|n ciia da chira va dung djch dja nhiet d mdi nai: hp dja nhidt d Bang tdn t?i trong mdi trudng da cd dipn trd suit rit cao (hdng nghln Ohm.m), cdn di^n trd sudt mdi trudng da trong md hinb dan chung cd thi cd dipn trd suat thip ban nhieu. Mu set d phdn gifla va culi tuyIn do trong he dia nhipt d Bang cd chieu ddy khdng Idn so vdi dp sau phdn bd chimg nen khdng thi hipn dii tren ban ve; tham sd dipn trd suat xdc dinh dupc bdng tai li^u tir telua cung thudng cao hon gid tri thyc Id nhihig van dl binh thudng trong phdn tich tdi lieu dja v|tt ly.
S. Ket lu|n
(i) De cd dupc thdng tm vl v) tri phdn bo vd can true cua mpt hp dja nhipt la yfiu cdu quan trpng c ^ giai do^n nghien ciiu so bp trong ddnh gid triln vpng khai thac nguon dja nhipt, cdn thiet phdi triln khai dp dyng cdc phuong phdp dja vat l^, trong dd phuang phdp tii telua cd vai trd chii dao, ddp ling dupc yeu cau chi tilt va dp sdu nghien cuu din 10 km vd sdu hon.
(ii) Dya tren ket qua dp dyng phuong phap tir telua Idn ddu tien d Viet Nam tren mdt dipn tich 2,5 >: 4 km^ bdng hp ttiilt bi hien d^i MTU 2000 dl khao sat hS dia nhipt d khu vuc ngudn nude ndng Bang (tinh Qudng Bmh) da xdc djnh dupc vj tri phdn bo vd ciu tnic ciia mpt ddi di^n trd sudt thdp d pMa nam - tdy nam diem nude ndng xudt lp, cd cdc cM thj ve cac ylu tl cau tnic cua mdt hp dia nhiet.
(iii) Md Mnh dien trd sudt ciia he dja nhi^t theo tdi lipu tii telua nhdn dupc d khu vuc Bang cd cac yeu tl cdu tnic vd tham sd dipn trd sudt tuong ty vdi md hinh nguySn tdc rapt hp thuy dja nMpt ngudn gdc magma trong vdn lieu qudc tS, bao gdm mu set (ldp chdn nhiet), bdn nMet vd ngudn nhi^t cd the Id khdi magma xdm nhap ndng hinh thanh trong giai doan phun trdo basalt Dp tii (PQrv), Id d^g ngudn nMpt cd dieu kien thuan lpi va Meu qud cho khai thdc ndng lupng. Md hinh cdu true hf dja nhi$t nhdn dupc d khu vyc Ban^ phii hpp vdi cdc thdng tin vl dieu kipn dja chdt, boat ddng magma, ket qud phdn tich dia hda, nhiet dp nude xuit lp,... Id CO sd dfi; lifu mdi rat can thiet cho vipc lua chpn vi tri thich hpp dfi tien hdnh khoan tham dd cho nghifn ciru chi tilt, xdc djnh trir luong bdn nhiet d giai do^n tiep theo.
(iv) Ket qud vd kinh nghiem dp dung phuong phap tit telua d khu vyc Bang cd the sii dyng cho edng tdc nghien ciru, khdo sdt danh gia tiem nang cdc ngudn dja nhipt khdc d Viet Nam.
Ldi cam on
Kit qud cdng bl trong bdi bao ndy Id mpt phin ndi dung ciia De tdi cap nha nude "NghiSn ciiu ddnh gia mdt sd ngudn dja nhiet trien vpng va cd dilu kien khai thdc cho phat trien nang lupng d Vipt Nam", ma sl KC08.16/ 11-15, cdc tdc gid chdn thdnh cdm on GS. Chau-Huei Chen, D^ii hpc qudc gia Chung Cheng - Dai Loan da hd trp thiet bj vd phin ralm de thyc hipn phuang phdp tit telua.
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