Magnetotelluric Method. Stephen Park IGPP UC Riverside

Transcription

1 Magnetotelluric Method Stephen Park IGPP UC Riverside

2 So, what is the magnetotelluric method? The magnetotelluric (MT) method determines the tensor electrical impedance of the earth through measurement of naturall varing M fields, and then uses computer modeling to find cross sections of electrical resistivit that ield theoretical responses similar the observed ones. And wh is it abbreviated MT? 1. It is the empt method because of the long waiting times in the field needed for data collection (MIT field camp students, 1981). 2. It describes the look on the faces in the audience when the above description is given. 3. The initials stand for MagnetoTelluric (Cagniard, 1953). But seriousl.. What can it tell us about the arth?

3 MT is one of the few techniques capable of sensing through the arth s crust to upper mantle.

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5 IN T CRUST Silicate minerals comprise 95% of the crust and silicate minerals are ver resistive* (< 10-6 S/m). lectrical currents do not like resistors! The observed finite conductivit ( S/m) of the crust is due to small fractions (ppm-10%) of interconnected conductive material. MT cannot be used to determine mineralog but can be used to identif small fractions of: aqueous fluids ( S/m) partial melt (2-10 S/m) graphite (10 6 S/m) metallic oides and sulfides (10 4 S/m) MT has been used successfull to locate: Underthrust sediments Regions of metamorphism and partial melting Fault ones (fractured, fluid-filled rock) *At crustal temperatures!

6 IN T MANTL Temperatures are sufficientl high (> 800C) that mobilities of crstal defects and impurities are enhanced. Ionic mobilit lectrical conductivit! nhanced mantle conductivit is caused b higher temperatures partial melt (> 0.01 S/m) hdrogen (and carbon?) diffusion MT has been used successfull to identif: partial melt variations in lithospheric temperature asthenosphere

7 What IS MT?.

8 ionosphere

9 Not all MT signals are from interactions with the solar wind: Micropulsations Global lightning Range of frequencies used to probe lower crust Murph s law is hard at work!!

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11 Let s look at the governing equations j ωµ J s + σ These break down into components: j ωµ j ωµ σ σ j ωµ σ Consider a halfspace and a verticall incident plane wave: Is there an difference between one point and another 1 km awa? NO! So, what terms vanish above?

12 jωµ σ jωµ σ σ 0 jωµ 0 Note lack of vertical fields and similarit of equations for (,) and (-,) jωµ Assume solutions of form ep(jk), and get k+/- (jωµσ) ½ and final result of: ωµσ ωµσ j jωt 2 2 Ae e e ωµσ ωµσ σ j jωt 2 2 (1 + j) Ae e e 2ωµ 0

13 Note that both of these contain an undetermined constant, A, that is set b the strength of the source field. in order to get rid of this constant, we eamine the impedance of the arth: Z/ Z 1 (1 + j) 2ωµ σ Note that phase is constant at -45 and amplitude is proportional to frequenc and resistivit (1/σ). This leads to the concept of apparent resistivit: 2 1 a Z ωµ ρ MT responses are represented b phase and amplitude (apparent resistivit)

14 Assignment: Derive equations for, and Z. What similarities or differences do ou see with Z? jωµ σ jωµ σ σ 0 jωµ 0 ωµσ ωµσ j jωt 2 2 Ae e e ωµσ ωµσ σ j jωt 2 2 (1 + j) Ae e e 2ωµ Z 1 (1 + j) 2ωµ σ 1 ρ a Z ωµ 2

15 ωµσ ωµσ j jωt 2 2 Ae e e ωµσ ωµσ σ j jωt 2 2 (1 + j) Ae e e 2ωµ Z 1 (1 + j) 2ωµ σ SAM apparent resistivit and phase is 135 (-1 is 180 ) different from Z. Summar Laered halfspace characteristics: apparent resistivit is independent of frequenc phase is either 45 or 135 apparent resistivities for two modes (, and (,) are equal NO vertical fields.

16 In a 1-D earth (laered geolog) and a verticall incident plane wave source, what terms can be eliminated? Asssignment: j ωµ j ωµ σ σ j ωµ σ In a 2-D earth (variations in conductivit in and onl) and a verticall incident plane wave source, what terms can be eliminated?

17 0 Z1 -Z1 0 0 Z1 Z2 0 Z1 Z2 Z3 Z4

18 T

19 When we have multiple sites, we plot a pseudosection:

20 Interpretation: 1. 1-D modeling, inversion fast, eas, readil available, almost alwas WRONG! 2. 2-D modeling, inversion slower, more difficult, programs usuall available, ma have 3-D effects in data D modeling used to verif 2-D results, programs available but onl simple models possible. Inversion not et available. 2-D inversion is standard tool for interpretation.

21 A sstem of equations for,, and (called the TM mode): σ σ jωµ and a sstem of equations for,, and (called the T mode): jωµ jωµ σ Note similarities in equations if, switched and σ, -jωµ switched. This leads to some simplifications in programming the forward solution! ach mode is simpl ecited b an equivalent current sheet in the appropriate direction at the surface (J for the TM mode

22 These sources lead to problems in solving both sets of equations with one forward solution! In M, basic boundar conditions at Interfaces are: 1)continuit of tangential fields 2)continuit of normal current densit Consider the TM case (with J source): J Because J at the surface must be continuous both across the air-arth interface and between the adjacent prisms, J is constant everwhere on the surface and therefore is a equivalent to an MT source with a uniform plane wave. Thus, the current sheet is placed at 0.

23 Consider the T case (with J source): J 1 2 Continuit of tangential at the surface requires that be continuous across the air-arth interface AND at the edges of the prisms. Because J σ, J must be DIScontinuous at the edges of the prism. This means that J varies in the direction across the model and does NOT represent a uniform source! SOLUTION: Add air laers to top of model to a sufficient height that J is once again uniform (tpicall 8-10 laers to a height of ~100 km or more).

24 Tpical steps for interpretation: 1. Identif T, TM modes based on a. comparison to geologic strike b. decomposition of impedance tensor c. similarit of with horiontal T mode: Induction arrows horiontal I 2. Design starting model based on a. geologic structure b. other geophsical data c. guesses 3. Run inversion and tr to fit data 4. Perform sensitivit analsis to determine which bounds on modeled structure.

25 MT can provide resistivit sections at man scales from the uppermost crust igh resolution MT profile in Krgstan to determine neotectonic structure to the entire crust.

26 MT profile across Sierra Nevada and eastern California: KVF 37N GV Isabella anomal K SN 22 S OV DP DV W T100 s 118 Scale MT modeling and inversion are regional problems! Data in the Sierra Nevada are affected b the highl conductive Pacific Ocean (and all of the structure in between). Mackie et al. (1996) showed with a 3-D model of California that the Transverse Ranges resistivit affected electric field levels in Death Valle.

27 Depth, km W SL GV S DP PV DV >100 <100 <30 >100 >100 >300 < Some sensitivit to depths up to 400 km Distance, km Log10(ρ) owever, what ou reall need not electrical resistivit..