3D Anisotropic velocity model of the Los Humeros

3D Anisotropic velocity model of the Los Humeros geothermal field, Mexico, using seismic ambient noise

tomography

Ivan Granados1, Marco Calò1, Ángel Figueroa Soto 2, Philippe Jousset 3

1. Institute of Geophysics, UNAM.2. Universidad Michoacana de San Nicolás de Hidalgo.3. GFZ, Potsdam, Germany.

www.gemex-h2020.eu

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 727550 and the Mexican Energy Sustainability Fund CONACYT-SENER, project 2015-04-68074.

3D Anisotropic velocity model of the Los Humeros geothermal field, Mexico, using seismic ambient noise tomographyIván, Marco, Ángel, Philippe

Seismic imaging of the deep structures for the Los Humeros caldera, a super-hot geothermal system in Mexico, as part of the framework of the consortium GeMex (Mexico – European Union).

3D anisotropic model for the geothermal system, using the seismic ambient noise tomography.

Los Humeros Seismic Network

SEISMOMETERS

DATABASE AVAILABILITY

Since July 2017, until October 2018 (almost 440 days).

HEAT FLOW IN MEXICOProl-Ledesma y Morán-Zenteno (2019)

mW/m2

720 MW

10 MW

93.4 MW

BROAD-BAND

SHORT-PERIOD


Trillium compact120 s – 100 Hz

Digitizer: Data-Cube3

Mark L-4V-3D1 – 100 Hz

Digitizer: EarthData PR6-24

DATABASE QUALITY CONTROL


TIME SYNCHRONIZATIONWe checked the time continuity of the database by analyzing the daily cross-correlation behavior, in order to identify if time deviations are present.

Z CROSS-CORRELOGRAM DS09-SS41

300

350

400

450

500

Day

s

0 10 20 30-20 -10Time [s]

-30

0 10 20 30-20 -10Time [s]

-30

REFERENCE GF

300

350

400

450

500

Day

s

0 10 20 30-20 -10Time [s]

-30

CORRELOGRAM OF CORRELOGRAMSa b c

Z DS03-SS41 ORIGINAL

Da

ys

40

70

100

130

160

10

335

40

70

100

130

160

10

335

Da

ys

0 25 50-25Time [s]

-50 0 25 50-25Time [s]

-50

Z DS03-SS41 CORRECTED STACKED GREEN’S FUNCTIONSd e f

0 25 50-25Time [s]

-50

0 25 50-25Time [s]

-50

original

corrected

The time correction helps to avoid wrong wave packets.



CORRECTION OF SENSOR’S ORIENTATIONWe must assure the correct orientation if we want to analyze the anisotropy patterns. By taking several regional and tele-seismic earthquake records, we used the polarization of the surface waves in order to identify the correct orientation, based on the theoretical back-azimuth for each earthquake with respect the network position.

PERIOD [s]

U [

km

/s]

10 15 25 40 602

3

4

5

θ=265o

Z

RT

TT

Z

RR

TR

245o 255o 265o

275o 285o

FTAN [R]θ (265 ±45o)

PERIOD [s]10 15 25 40 60

200 250 300 350100 150

Time [s]

400 450

Z

E

N

Time [s]

200 250 300 350100 150

Time [s]

400 450

FILTERED AT 10 – 60 s ROTATED TO BACK-AZIMUTH ROTATED TO BEST SOLUTION

θ=246o

FTAN [Z] 5

4

3

2

5

4

3

2

5

4

3

2

5

4

3

2

5

4

3

2

PERIOD [s]10 15 25 40 60

PERIOD [s]10 15 25 40 60

10 15 25 40 60 10 15 25 40 60

200 250 300 350100 400150 450

Comparing the dispersion pattern (FTAN diagram) for the horizontal components rotated at different angles, we were searching the angle where radial component are so much similar to the vertical component one, due the P-SV polarization of the Rayleigh waves.



VARIATION OF THE GREEN’S FUNCTION RETRIEVALBy using same processing scheme, variation of noise level during the day impacts on the retrieval of GF. Taking only “night hours” get a better recognition of the surface wave dispersion, even more than the sum of causal and anticausal part.

FULL DAY CC FULL DAY CC (CAUSAL+ANTICAUSAL) ONLY ‘NIGHT HOURS’ CCONLY ‘DAY HOURS’ CC

The best dispersion pattern continuity.

At short periods, dispersion pattern is not clear.

Dispersion pattern not focused.

Dispersion pattern not focused.

GREEN’S FUNCTIONS


Retrieved GF for the Rayleigh (a) and Love waves (b). Velocities of the wave packets for the longer periods rounds 3 km/s.

PACIFIC OCEAN350 - 405 km

GULF OF MÉXICO70 - 130 km

LOS HUMEROS

The gather of the Rayleigh waves, aligned in the NE-SW direction (c) shown a high directivity of the noise sources coming from the NE to the SW (causal part), related to the swell of the Gulf of Mexico, located at 100 km NE of the LHVC.

Because of that, we used the NDCP code (Granados et al., 2019) for the picking of the dispersion curves in the causal and anticausal parts of the GF.

GREEN’S FUNCTIONS


Mean dispersion curves obtained for group and phase velocity, for Rayleigh and Love waves, from 0.3 to 10 s, and a velocity range from 0.5 to 3 km/s.

After the picking of the dispersion curves, we tested different cell-size grids, in order to obtain the better image at each period. We used cells from 1 km to 2.5 km, along the whole bandwidth.

VM [km/s]-10% +10%

Seismometer

a

0.20 1.00.4 0.6 0.8

VM=0.9 km/s

1 km 0.75 s

Lat

itu

de

La

titu

de

Lat

itu

de

Lat

itu

de

2 km 4 s

VM=2.6 km/s

CHECKERBOARD TEST RESOLUTION

The best resolution is located inside the Los Potreros caldera, where the inner array of seismometers were placed.

2D VELOCITY MAPS


At short periods, we observe a low velocity ring-shape (0.55 – 0.7 km/s) pattern along the Los Potreros rim, surrounding a high velocity area (1.1 – 1.3 km/s) in its central part; along the Los Humeros fault for Rayleigh waves, and to the western part of the fault, for Love waves.

Group velocity (Love) 4.0 s

VM=0.93 km/s

VM=2.23 km/s VM=1.98 km/s

2.3

2.2

2.1

2.0

1.9

1.8

1.7

1.6

2.5

2.3

2.2

2.1

2.0

1.9

0.9

0.8

0.7

0.6

U [km/s] U [km/s]

X [km]

X [km] X [km]

Y [

km

]

Y [

km]

Y [

km]

Group velocity (Rayleigh) 4.0 s

2.4

VM=0.86 km/s

1.1

1.0

0.9

0.8

0.7

0.6

X [km]

Y [

km]

Group velocity (Rayleigh) 0.75 s

1.1

1.0

U [km/s]

1.2

Group velocity (Love) 0.75 s U [km/s]

1.2

At 4 s, a large low velocity structure can be observed at the center of the Los Humeros caldera, but it differs in its extension and velocity magnitude from one component to the other, being higher for the Rayleigh waves (≈ 1.9 km/s) than the Love waves, (1.6 – 1.7 km/s).

LOS POTREROS

LOS HUMEROS

LOS H

UM

ERO

S

FAU

LT

LOS POTREROS

LOS HUMEROS

LOS H

UM

ERO

S

FAU

LT

LOS POTREROS

LOS HUMEROS

LOS H

UM

ERO

S

FAU

LT

LOS POTREROS

LOS HUMEROS

LOS H

UM

ERO

S

FAU

LT

3D ANISOTROPIC MODEL


Joint inversion of group and phase velocities, for Rayleigh and Love waves (Herrmann, 2013).

0.7 1.4 2.1 2.8 3.5

VS [km/s]

4

8

12

DEP

TH

[km

]

Mean velocity model (VS).

VARIATION WITH RESPECT THE MEAN MODEL AT DEPTH (dVS)

3D anisotropic VS model

A’

A

B

C

B’

C’

In A-A’, we observed a low VS body at

from 1500 – 3500 mbsl, just below the seismicity stops.

In B-B’, a thick layer of low velocity is observed at 2000 masl (-10% V

S),

where the seismicity pass through it at the north.

In C-C’, this low VS body becomes thicker and deeper to the south.


3000

2000

1000

ma

sl sl

-1000

-2000

-3000

-4000

-5000

3000

2000

1000

ma

s l

sl

-1000

-2000

-3000

-4000

-5000

3000

2000

1000

ma

s l

sl

-1000

-2000

-3000

-4000

-5000

A’

dVS [%]

-15% +15%-5% 5%

ALos Humerossouth border

Los Potrerossouth border

B’BLos Humerosfault

Los Potreroseast border

Los Humeroseast border

Los HumerosEast border

C C’Los Humerosfault






A3000

2000

1000

ma

sl sl

-1000

-2000

-3000

-4000

-5000

3000

2000

1000

ma

s l

sl

-1000

-2000

-3000

-4000

-5000

A’

B’

C C’

ξ = (VSH

/VSV

)2

1.40.8 1.210.6

A’

A

B

C

B’

C’

RADIAL ANISOTROPY ξ = (VSH

/VSV

)2

3000

2000

1000

ma

s l

sl

-1000

-2000

-3000

-4000

-5000

BIn A-A’, a dominant VSV (ξ<1) upward pattern connect the low V

S with the

base of the seismicity clusters.

In B-B’, the seismicity pass through two ξ>1 areas, as a lateral discontinuity at 2000 masl, where the seismicity is located.

In C-C’, this ξ>1 pattern where located above several ξ<1 bodies, surrounding a ξ>1 body at 0 masl.

Los Humerossouth border

Los Potrerossouth border

Los Humerosfault




Los Humerosfault






X3000

2000

1000

mas

l sl

-1000

-2000

-3000

-4000

-5000

X’ A deep anomaly…

We were able to identify a low VS big area located in the NE

part of the Los Humeros caldera, where other geophysical methodologies have been observed a body of low density and high magnetization.

A electrical resistivity image obtained with magnetotelluric soundigns, revealed a low resistivity body (≈3Ωm) in this same area.

dVS [%]

-10% +10%-5% 5%



X X’ X X’

DENSITY FROM GRAVITY1500 mbsl

MAGNETIC SUSCEPTIBILITY1500 mbsl

[g/cm3] [A/m]

WP 5.3 GEMEX WP 5.3 GEMEX

LOS POTREROS

LOS HUMEROS

LOS H

UM

ERO

S

FAU

LT

LOS POTREROS

LOS HUMEROS

LOS H

UM

ERO

S

FAU

LT

Conclusions

- Good agreement with shallower geology of the caldera (i. e. Los Potreros rim, Los Humeros fault, etc).

- A low VS layer at 1 km depth (its top), with variable radial anisotropy pattern, is

located at the center of the Los Potreros caldera.

- A low VS body at 1500 mbsl, with an upward ξ<1 pattern (VSV

>VSH

), could be

related with the up-flowing paths of the geothermal reservoir.

- At the NE part of the Los Humeros caldera, we observed a big low VS body, from sea

level down to 4000 mbsl, that is located where a low density and a high magnetization anomaly was identified for other work package of the GeMex project.


Acknowledgements

The content of this presentation reflects only the authors’ view. The Innovation and Networks Executive Agency (INEA) is not responsible for any use that may be made of the information it contains.

We acknowledge the Comisión Federal de Electricidad (CFE) for kindly providing support and advice and for granting access to their geothermal fields. Data has been kindly provided by CFE.

We also acknowledge our European colleagues for their help an collaboration.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 727550 and the Mexican Energy Sustainability

Fund CONACYT-SENER, project 2015-04-68074


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3D Anisotropic velocity model of the Los Humeros