utility of magnetotelluric data in unravelling the stratigraphic ......utility of magnetotelluric...

Utility of Magnetotelluric Data in Unravelling the Stratigraphic-StructuralFramework of the Nechako Basin (NTS 092N; 093C, B, G, H), South-Central

British Columbia, from a Re-Analysis of 20-Year-Old Data1

by J.E. Spratt2, J. Craven3, A.G. Jones2, F. Ferri and J. Riddell

KEYWORDS: magnetotelluric data, elec tro mag netic data,Nechako Ba sin

INTRODUCTION

The Ca na dian Cor dil lera, where the Juan de Fuca oce -anic plate is cur rently be ing subducted be neath the NorthAmer i can con ti nent, is com prised of oce anic and is land arcter ranes accreted to the west ern edge of an ces tral NorthAmer ica since the Neoproterozoic (Gabrielse and Yorath,1991; Mon ger et al., 1972; Mon ger and Price, 1979; Mon -ger et al., 1982). The Me so zoic Nechako Ba sin, lo catedwithin the Intermontane Belt of the Ca na dian Cor dil lera,in cludes over lap ping sed i men tary se quences de pos ited inre sponse to this terrane amal gam ation. Re gionaltranscurrent fault ing and as so ci ated east-west ex ten sion,be gin ning in the Late Cre ta ceous, was ac com pa nied by theex tru sion of ba saltic lava in Eocene and Mio cene times thatforms a vari ably thick sheet, av er ag ing 100 m in thick nessand pos si bly ex tend ing up to 1 km thick in iso lated places,cov er ing much of the ba sin. The main geo log i cal el e mentswithin the south ern Nechako area in clude Mio cene floodba salt, Ter tiary vol ca nic and sed i men tary rocks, Cre ta -ceous sed i men tary rocks and Ju ras sic sed i men tary rocks(Fig 1). Un der stand ing the dis tri bu tion and struc ture ofthese sed i men tary rocks at depth is vi tal for as sess ing pos si -ble re sources. An as sess ment of the hy dro car bon po ten tialbased on ex ist ing geo log i cal in for ma tion and well data,per formed by the Geo log i cal Sur vey of Can ada in 1994,sug gested that the Nechako Ba sin may con tain as much as atril lion cu bic metres of gas and a bil lion cu bic metres of oil(Hannigan et al., 1994). The thick vol ca nic cover lim its thetransmission of reflection seismic waves and has made itdifficult to determine the physical boundaries of the basin,impeding exploration.

Magnetotelluric (MT) data can dis tin guish be tweenlithological units, as flood ba salt and ig ne ous base mentrocks typ i cally have elec tri cal re sis tiv ity val ues of>1000 ohm-m, whereas sed i men tary rocks are more con -duc tive with val ues of 1 to 1000 ohm-m. In the 1980s, the

Uni ver sity of Al berta re corded MT data across theNechako Ba sin be tween 52° and 53° lat i tude us ing shortpe riod au to matic MT sys tem (SPAM) in stru ments that re -corded data in the fre quency range of 0.016 to 130 Hz(Fig 1; Majorowicz and Gough, 1991). These data, alongwith other geo phys i cal in for ma tion, have been re-ex am -ined us ing mod ern anal y sis tech niques to as sess the use ful -ness of un der tak ing MT in the Nechako re gion and to de ter -mine spe cific data ac qui si tion tech niques for fu ture MTsurveys that will provide higher-resolution crustalimaging.

Ini tial anal y sis of the data col lected in the 1980s (Fig 1) re vealed struc ture that was ob tained from the phasepseudosections along a pro file con sist ing of 26 sites. Depthes ti mates were based on a one-di men sional in ver sion of sixof these sites (Majorowicz and Gough, 1991). The data re -vealed an an oma lously con duc tive up per crust (10–300 ohm-m) in the east ern half of the pro file and was at trib -uted to the pres ence of sa line wa ter in pore spaces and frac -tures. The west ern half of the pro file showed the pres enceof an east ward-dip ping re sis tive fea ture. The re sis tive body has been in ter preted to rep re sent granodiorite or other crys -tal line rocks of the Coast Belt that ex tends be neath a thinlayer of ba salt (Gough and Majorowicz, 1992; Majorowiczand Gough, 1994; Jones and Gough, 1995; Ledo and Jones,2001).

Since the early 1990s, con sid er able ad vance ments inpro cess ing soft ware and tech niques, as well as mod el lingand in ter pre ta tions pack ages, have sig nif i cantly im provedour abil i ties to an a lyze and in ter pret MT data. Many ofthese new tech niques, in clud ing strike anal y sis, dis tor tionde com po si tions and mod ern 2-D mod el ling in ver sions,have been ap plied to the MT data col lected in the 1980s.The re sults of these anal y ses and an ex am i na tion of the re -solv ing power of MT based on bore hole logs have im -proved our un der stand ing of the data lim i ta tions and en -hanced our knowl edge of the res o lu tion of the 2-D mod elsin the area, par tic u larly at crustal depths within theNechako sed i men tary ba sin. Our stud ies in di cate that fu -ture MT stud ies can reveal substantially more shallowinformation related to the basin itself.

MAGNETOTELLURIC DATA ANALYSIS

The magnetotelluric (MT) method in volves mea sur ing nat u ral vari a tions in the Earth’s elec tric and mag netic fields in or der to pro vide in for ma tion on the elec tri cal con duc tiv -ity struc ture of the subsurface (e.g., Jones, 1992). The mea -sure ment of mu tu ally per pen dic u lar com po nents of theelec tric and mag netic fields at the sur face of the Earth al -lows us to cal cu late MT re sponse curves, phase lags and ap -

Geo log i cal Field work 2006, Pa per 2007-1 395

1Geoscience BC con tri bu tion GBC032; Geo log i cal Sur vey ofCan ada con tri bu tion 20060441

2Dublin Institute for Advanced Studies, Dublin, Ireland3Geological Survey of Canada, Ottawa, ON

This publication is also available, free of charge, as colourdigital files in Adobe Acrobat® PDF format from the BCMinistry of Energy, Mines and Petroleum Resources website athttp://www.em.gov.bc.ca/Mining/Geolsurv/Publications/catalog/cat_fldwk.htm

par ent resistivities at var i ous fre quen cies for each site re -corded. Since the depth of in ves ti ga tion of these fields isdi rectly re lated to the fre quency of the mea sure ment andthe re sis tiv ity of the ma te rial, es ti mates of the phase and ap -par ent resistivities pro vide a rough guide to the true spa tialvari a tion of re sis tiv ity be neath each site. For ex am ple, ifap par ent resistivities rise as the fre quency drops, that is anin di ca tion that the true re sis tiv ity is in creas ing with depth.To ar rive at a more re al is tic depth im age of the subsurfacere sis tiv ity dis tri bu tion, a model must be gen er ated withsyn thetic or cal cu lated re sponses that match the mea sureddata within mea sure ment er ror. The choice of an al go rithmto cal cu late the syn thetic data is strongly de pend ent on thecom plex ity of the subsurface re sis tiv ity dis tri bu tion. Inturn, the com plex ity of the re sis tiv ity dis tri bu tion is a func -tion of a num ber of fac tors, in clud ing lithological vari a -tions, struc tural fab rics, rock po ros ity and sa lin ity ofground wa ter within pore spaces. It is com mon in geo log i -cal sit u a tions to have pre dom i nantly two-di men sional dis -tri bu tions (i.e., the elec tri cal con duc tiv ity is invariant inone direction, the strike direction) and therefore most of the modelling algorithms in use today assume a two-dimensional structure.

Be cause or thogo nal com po nents of the elec tric andmag netic fields are mea sured, four re sponse curves can befor mu lated to de fine the four el e ments of the 2 by 2 MT ten -sor. The ten sor con tains the in for ma tion to dis cern the com -plex ity of the Earth’s elec tri cal dis tri bu tion. In a two-di -

men sional Earth, two of the el e ments are zero and two arenot. The two non-zero el e ments are re ferred to as the Trans -verse Elec tric (TE) and Trans verse Mag netic (TM) modesof elec tro mag netic (EM) field prop a ga tion. The four com -po nents of the ten sor are non-zero in three-di men sional en -vi ron ments. The two modes of elec tro mag netic field prop a -ga tion dem on strate equiv a lent phase lags when thestruc ture is one-di men sional or lay ered. Math e mat i cal de -com po si tion anal y sis on the ten sor data is typ i cally un der -taken to de ter mine if it is ap pro pri ate to use two-di men -sional mod el ling al go rithms and to as sign the pre ferredgeo-elec tric strike di rec tion for the sites along a pro file(Groom and Bailey, 1989). The re sults of de com po si tionsalso pro vide the TE and TM-mode re sponse curves thatmost ac cu rately re flect the two-di men sional struc ture.Two-di men sional mod el ling of these data will then de ter -mine the most rea son able re sis tiv ity struc ture of thesubsurface be neath a pro file. The anal y sis be gins by re-examining the data, using modern tensor decompositions,to determine if two-dimensional algori thms areappropriate.

Decomposition Analysis

Sin gle-site and multisite Groom-Bailey (1989) de -com po si tions, us ing the method of McNeice and Jones(2001), were ap plied to the MT sites along the pro file andshowed much of the data to be one-di men sional as max i -

396 Geoscience BC, Re port 2007-1

Fig ure 1. Ge ol ogy of the Nechako re gion. The blue dots show the lo ca tion of magnetotelluric sites re corded in the 1980s. The red and bluelines in di cate the ori en ta tion of two-di men sional MT mod els gen er ated for these data. The black dots in di cate the lo ca tion of bore hole wells. Mod i fied from Ferri and Rid dell (2006).

Quaternary

Anahim Volcanlca; Mlocene

.,,,,,",,.,,,,~,,",. ,~,~ basales

• Gambler Gp; Early Crel volc.onh;.

" •

SlIkl,.. Te mln,: late Trtualc to midJurass lc volu nlc . and sedlme nts; Includ es HUI "on Gp

mum phase dif fer ences be tween the two modes were be low10°, par tic u larly at fre quen cies above 1 s (Fig 2). For thefew sites with a strong two-dimensionality, and that ex hib -ited larger phase dif fer ences, the pre ferred geo-elec tricstrike an gle was de ter mined to be –35° de grees, i.e., north-north west, ap prox i mately par al lel to the geo log i callymapped strike of the belts. The mod els gen er ated in theMcNeice-Jones de com po si tion anal y sis show that the datafit well within 3.5%, equiv a lent to a 2° er ror floor in phase.This geo-elec tric strike an gle is con sis tent through out thedataset and ap pears to cor re late well with struc tures re -vealed in the grav ity data (Fig 3) in di cat ing that the strike islikely to be dominated by regional large-scale two-dimensional structures.

Preliminary Borehole Comparisons

Sev eral bore holes have been drilled within the south -ern Nechako Ba sin re gion since the 1960s and re sis tiv itywell log data were ac quired for some of them (Fig 4). Most

MT sites are too far away from the wells to be use ful incom par ing mea sured and mod elled re sponses; how ever,well log a-4-L is lo cated close to MT site ten020 and can beused for this com par i son. Syn thetic ap par ent re sis tiv ity and phase curves were cal cu lated us ing the Geotools MT in ter -pre ta tion pack age from long nor mal and deep in duc tionlogs mea sured at well a-4-L (Fig 5a). From these syn theticcurves, as well as mea sured curves from the data re cordedat site ten020, one-di men sional mod els were gen er ated(Fig 5b). The 1-D mod els for both sets of curves, syn thetic(from the bore hole data) and mea sured (at the MT site), aregen er ally quite sim i lar with just two ma jor dif fer ences: 1)the start ing re sis tiv ity value is nearly one or der of mag ni -tude higher in the MT data com pared to the syn thetic welldata; and 2) the dif fer ent re sis tiv ity lay ers ap pear to bedown shifted, that is, that the bound aries be tween the lay ersoc cur at deeper depths in the 1-D model for the MT site.Con trary to the as sump tions made by Majorowicz andGough (1991) — that there was lit tle static-shift ef fect onthe data — these dif fer ences in di cate that there is most


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Phase Difference

Fig ure 2. Maps show ing the geo-elec tric strike di rec tions de ter mined from Groom and Bailey (1989) gal vanic de com po si tion mod els foreach site at four dif fer ent pe riod bands be tween 0.01 and 100 s. The colours rep re sent the max i mum phase dif fer ence be tween the Trans -verse Elec tric (TE) and Trans verse Mag netic (TM) modes. Each band sam ples pro gres sively deeper into the crust and rep re sents the best-fit two-di men sional strike of the elec tri cal units in the subsurface. Small phase dif fer ences are in dic a tive of one-di men sional or lay ered ge -om e tries in the subsurface.


Fig ure 3. Bouguer grav ity for the Nechako re gion and strike di rec tions ob tained us ing multifrequency McNeice-Jones de com po si tions forpe riod bands 0.1 to 1 s and 1 to 10 s. The black lines mark the pro file traces of the MT mod els. Mod i fied from Ferri and Rid dell (2006).

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likely a sig nif i cant ef fect of static shift on themea sured MT data and these ef fects need to beac counted for in fur ther mod els. The sim i lar i tiessug gest that the MT data is ca pa ble of im ag ingthe shal low con duc tiv ity lay ers ob served in thewell. The dif fer ences in di cate that there is a needfor ad di tional mea sures to be taken in fu ture MTdata ac qui si tion in this re gion in or der to get anac cu rate depth es ti mate for these lay ers. For ex -am ple, the model ob tained from the data (Fig 5b)is en tirely fea ture less at depths shal lower than100 m, sug gest ing that the fre quency band widthuti lized by the SPAM sys tem (up to 130 Hz) is in -suf fi cient to ex plore at shal low depths. High-fre -q u e n c y a u d i o - ma g n e to t e l l u r i c ( A MT )soundings up to 10 000 Hz would help to con -strain the re sis tiv ity struc ture at shal low depthsand other meth ods, such as time-do main elec tro -mag netic, borehole resistivity logs or electricalrock property measurements performed in alaboratory, could be used to correct for thisstatic-shift effect.

DATA MODELLING

Two-di men sional mod els along Pro file 1(Fig 1) were de ter mined for the de com po si tion-cor rected MT re sponses at a geo-elec tric strikean gle of –35° of both the Trans verse Mag netic(TM) and Trans verse Elec tric (TE) modes, aswell as the TE mode only. Since the na ture of MTmod els is non-unique, care ful steps need to beun der taken to en sure that the model gen er ated isthe best rep re sen ta tion of the struc ture of theEarth. A trade-off is re quired be tween thesmooth ness of the model be tween cells, the tauvalue (τ), and the fit of the model to the data, theRMS (root mean squared) mis fit value. In versemod el ling us ing Rodi and Mackie’s (2001) code, as im ple mented in Geosystem’s WinGlink soft -ware pack age, was run for 100 it er a tions formod els with vary ing smooth ness pa ram e ters (τ).Higher val ues of τ in crease the smooth ness andthere fore the fi del ity of the model, but de gradethe fit of the model as rep re sented by the RMS.From the L-curve trad ing-off fit (RMS) againstτ, the best trade-off was de ter mined to be at τ = 3(Fig 6). In or der to ac count for static-shift ef -fects, the data were in verted to pref er en tially fitthe phase data, with high er ror floors (25%) setfor the ap par ent resistivities. The data were alsoin verted with er ror floors for both the phase andap par ent resistivities equiv a lent to 2° (as de ter -mined ac cept able from the dis tor tion anal y sis)and af ter 100 it er a tions, a static-shift cor rec tionwas ap plied to the model pa ram e ters. The struc -tures of the dif fer ent mod els and the as so ci atedcon duc tiv ity val ues were very sim i lar and the fi -nal model gen er ated achieves an RMS mis fit of2.4. Pseudosections al low a vi sual com par i sonbe tween the cal cu lated for ward re sponse of theap par ent resistivities and phases, and the orig i nal data (Fig 7). These show that there is areasonable fit at most sites along the profile,particularly at shallow depths.


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Fig ure 4. Re sis tiv ity data mea sured at four of the wells in the south ern Nechakore gion.

Fig ure 5. a) Re sis tiv ity log data re corded at well a-4-L re-plot ted in log do main;b) one-di men sional mod els of syn thetic data from bore hole log a-4-L andmagnetotelluric (MT) site ten020 at the same lo ca tion.

Resistivity Log Data

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Resistivity (ohm·m)

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Main Model Features

The model shows dis tinct vari a tions in the con duc tiv -ity struc ture along the pro file and re veals a mod er ately con -duc tive layer (40 ohm-m) that var ies in depth up to ~5 km(Fig 8). There is a good cor re la tion be tween the grav itylows, which are as sumed to rep re sent sed i men tary se -quences, and the high con duc tiv ity layer (<100 ohm-m)where it thick ens be tween sites 922 and 908 in the 2-Dmodel along Pro file 1 (Fig 8). Where the MT model showsre sis tive struc ture near the sur face at both ends of the pro -file, there are grav ity highs that have been in ter preted torep re sent vol ca nic base ment. This in di cates con sis tencybe tween the two datasets and sug gests that the MT data areca pa ble of dis tin guish ing the dif fer ent lithological units atdepth and defining the boundary of the Nechako Basin.

Model Focused on Shallow Basin Structure

A new model was gen er ated for the east ern edge of theNechako Ba sin us ing the high-fre quency data only (1–130 Hz) in or der to de ter mine how sen si tive the data are tothe shal low struc tures in this re gion. This model shows sig -nif i cant con duc tiv ity changes at shal low depths along thepro file (Fig 9). A com par i son of cal cu lated mod elled re -sponses to the ob served data is shown in Fig ure 10. To the


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Fig ure 7. Plots of the mea sured data and the model for ward re sponses of the ap par ent resistivities and phases for both Trans verse Mag -netic (TM) and Trans verse Elec tric (TE) modes of the two-di men sional model shown in Fig ure 8.

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south west, there is a con duc tive (~100 ohm-m) re gionover ly ing re sis tive (>1000 ohm-m) ma te rial; Majorowiczand Gough (1991) in ter preted this to rep re sent sed i men tary rocks in Nechako Ba sin over ly ing gra nitic in tru sions. Tothe north east, the con duc tive re gion be comes re sis tive

across the bound ary be tween the Nechako Ba sin and theCache Creek Terrane (Fig 1, 9). The model also shows twovery con duc tive fea tures (<10 ohm-m) that cor re late withthe surface-mapped Tertiary volcanic and sedimentaryrocks (Fig 1).


Fig ure 8. Two-di men sional re sis tiv ity model ob tained from MT data across the Nechako Ba sin. The model was gen er ated us ing both Trans -verse Elec tric (TE) and Trans verse Mag netic (TM) mode dis tor tion-de com posed ap par ent re sis tiv ity and phase data, and a root meansquared (RMS) mis fit of 2.4 was achieved. The red box in di cates the re gion of lo cal ized mod el ling shown in Fig ure 9. The black bars abovethe model il lus trate the re gions of high and low grav ity data, as seen in Fig ure 3.

Fig ure 9. A two-di men sional model fo cus ing on the shal low struc tures cross ing the east ern bound ary of the Nechako Ba sin. The model wasgen er ated us ing high-fre quency data (1–130 Hz) for both the Trans verse Elec tric (TE) and Trans verse Mag netic (TM) modes and a rootmean squared (RMS) mis fit of 1.4 was achieved.

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Hy poth e sis test ing of these shal low con duc tors wasun der taken by re plac ing the high con duc tiv ity val ues witha higher re sis tiv ity value (~200 ohm-m, sim i lar to the sur -round ing val ues) and run ning a for ward re sponse. TheRMS val ues in creased dra mat i cally (from 1.4 to 8.2) and acom par i son of the ca l cu l a ted ver su s mea suredpseudosections of both the TM and TE modes, in both theap par ent resistivities and phases, showed dis tinct dif fer -ences. This in di cates that the shal low con duc tive struc turesare re quired by the mea sured data and are not ar ti facts of the in ver sion al go rithm and that, al though cur rently sparse, theMT data is ca pa ble of dis cern ing shal low struc ture withinthe Nechako ba sin. The SPAM MT sys tems used for ac qui -si tion of the mea sured data re corded a max i mum fre quencyof 130 Hz (0.007 s). Mod ern au dio-magnetotelluric (AMT) in stru ments can re li ably ac quire data to 10 000 Hz(0.0001 s). With a dense site spac ing, ad di tional AMT andbroad band MT data ac qui si tion would be use ful in de ter -min ing the shal low struc ture within the Nechako Ba sin andwould al low the ba sin bound aries to be clearly iden ti fied.

CONCLUSIONS

Magnetotelluric data col lected two de cades ago are ca -pa ble of pen e trat ing the Ce no zoic vol ca nic rocks and im ag -ing the shal low fea tures of the Nechako Ba sin. The two-di -men sional mod els show a good cor re la tion be tween along-strike con duc tiv ity vari a tions and mapped geo log i cal unitsas well as ob served grav ity anom a lies. A com par i son with

mea sured bore hole resistivities al lows us to quan tify static-shift ef fects and in di cates the need to ac count for these ef -fects in the in ter pre ta tion of the ex ist ing data. Ad di tionalMT data ac qui si tion us ing mod ern high fre quency (AMT)and broad band in stru men ta tion would be a highly cost-ef -fec tive method to de ter mine the thick nesses and bound -aries of the Nechako ba sin. This in for ma tion is key foradvancing our understanding of the region’s resourcepotential.

ACKNOWLEDGMENTS

Help ful re views by R. Stevens and C. Lowe are grate -fully ac knowl edged. Bore hole data was pro vided byJonathon Mwenifumbo of the Geo log i cal Sur vey of Can -ada. The pro ject has re ceived fi nan cial sup port fromGeoscience BC and the BC Min is try of En ergy, Mines andPe tro leum Resources.

REFERENCES

Ferri, F. and Rid dell, J. (2006): The Nechako Ba sin Pro ject: newin sights from the south ern Nechako Ba sin; in Sum mary ofAc tiv i ties 2006, BC Min is try of En ergy, Mines and Pe tro -leum Re sources, pages 89–124.

Gabrielse, J. and Yorath, C.J., Ed i tors (1991): Ge ol ogy of theCordilleran Orogen in Can ada; in Ge ol ogy of Can ada, num -ber 4, Geolog i cal Sur vey of Can ada and in Ge ol ogy of North Amer ica, vol ume G-2, Geo log i cal So ci ety of Amer ica.


Fig ure 10. Plots of the ob served data and the data cal cu lated from the model for ward re sponses of the ap par ent resistivities and phases forboth Trans verse Mag netic (TM) and Trans verse Elec tric (TE) modes of the two-di men sional model shown in Fig ure 9.

_ 10-)

!E-"0 10·' 0 .!ii 10° o.

'"' _'0·2 !E-'B 10. 1

'!ii '0° o.

'"' TE-mode

'iiJ10·:

~IO" .!ii 100 ' I ... .. o. Ot!served phase '"' -,0.2

• i 'o·' ·!ii,oo o.

'"' " '" O".ancc (Km)

'0': .. - '0·'

~ '"" • o. '"'

_ '02 • ~ 'O· ' .~ '0° o.

'"'

10.2

~'O· ' :g '0° • 0.. 10' .. '0·:

- '0·' '8 .!ii ,o0

o. '"'

'00

TM-mode

" '" O"lance (Km)

512

'50 128 64 32

Gough, D.I. and Majorowicz, J.A. (1992): Magnetotelluricsoundings, struc ture and flu ids in the south ern Ca na dianCor dil lera; Ca na dian Jour nal of Earth Sci ences, vol ume 29, pages 609–620.

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utility of magnetotelluric data in unravelling the stratigraphic ......utility of magnetotelluric...

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