1. Insights from Audio-MagnetoTelluric data1
Imaging hydrothermal systems at Furnas caldera,
Insights from Audio-MagnetoTelluric data
Colin Hogg, Duygu Kiyan, Volker Rath,
Sveta Byrdina, Jean Vandemeulebrouck,
Caterina Silva, Fatima M. Viveiros, and Teresa Ferreira
22 April 2016, Vienna

2. Azores, Furnas: Geological Setting2
Azores, Furnas: Geological Setting
Azores Archipelago:
Triple junction (African, Eurasian & North Atlantic Plate) meet!
Sao Miguel Island (largest)
3 volcanic centres: Sete Cidades, Fogo and Furnas
Furnas Lake

3. Furnas Caldera - Sao Miguel Island
Fault
Furnas Lake
Furnas Lake
Across WNW-ESE trending dip-slip faulting crosses the volcanic edifice.
For fumarolic field aquifers 100 – 200 m depth were estimated and proposed to be supplied with steam/gas from volcanic bodies cooling at greater depths (Ferreira & Oskarsson, 1999; Viveiros et al., 2015). Intensive CO2 outgassing.
Geometry and character of the hydrothermal system unknown

4. Audio-Magnetotelluric Method (I)4
Audio-Magnetotelluric Method (I)
Sources: lightning activities, magnetic storms
Induced electric (“telluric”) currents in the subsurface
Impedance Tensor calculated from orthogonal E and H measurements
Impedance Tensor yields conductivity structure in terms of direction and extent
Frequency content determines depth of penetration

5. Audio-Magnetotelluric Method (II)
Processed MT station response Ex Ey Hx Hy
Apparent resistivity
a,xy() = Ex()
0 Hy()
Phase
xy() = tan-1(Ex()/Hy()) 2
Time
E = electric field, H = magnetic field
 = angular frequency (2f)
0 = magnetic permeability

6. AMT Site Locations/ Depth of Penetration6
AMT Site Locations/ Depth of Penetration
13 AMT sites in total, on average 10 hours recording time
Site Spacing: m
High quality data 10-4 s - 2 s
Average depth of penetration ~1 km
Conductor in the fumarole area reduces the penetration of the MT signal

7. 7
Data Quality

8. 1-D Models (invariants) 8 Profile1 S N Profile 2 S N
Background resistivity – Occam 1-D Inverse Modelling
Vertical bars – Sharp-boundary 1-D Inverse Modelling
Profile 2
Profile1
Profile2 S N

9. (based on 1-D AMT invariant inversions)
Profile 1 ERT /AMT
(based on 1-D AMT invariant inversions)

10. Data Analysis - Induction arrows
1010
Data Analysis - Induction arrows
Fault

11. Data Analysis – Phase tensor
The colour of the ellipses represents the skew angle – indicator of 3-D regional conductivity structures (Caldwell et al., 2014)
Most of the stations have high skew angle values (>3o) at longer periods

12. Preliminary 3-D Inverse Modelling Results
AMT data inverted using ModEM (Egbert & Kelbert, 2012; Kelbert et al. 2014)
12 +1 sites, full impedance tensor data and tipper data, period range: – 100 s
initial model resistivity of 100 ohm-m, with the lake resistivity of 70 ohm-m (measurements by Cruz et al., 2015) kept fixed during the inversion
3-D mesh of 75 (N-S) x 73 (E-W) and 147 (vertical) (plus 10 air layers) cells, with a horizontal cell size of 40 m x 40 m in the area of interest

13. Conclusions Excellent data quality, no indication for static shift
Difficult to model the data, analysis indicate 1D/2D/3D structures -> complicated geological structures, topographic and ocean effects important
Mainly well-conducting (<100 ohm-m). Data analysis (induction arrows and phase tensor) and data inversions indicate regions of high conductivity (3 - 5 ohm-m) :
1-D and 3-D results show a small highly conductive region beneath the main fumarole field, bounded by resistive strata.
A deeper, slightly south-westward dipping very low resistivity (<5 ohm-m shown in (c)) structure
Acquisition of BBMT data is required to image the base of the conductive structure and deeper features

14. On-going & Future Research
1414
On-going & Future Research
3-D inverse modelling of MT responses with prescribed lake bathymetry and optimized parameters
New MT and ERT field measurements in June 2016, close data gap
Joint interpretation with ERT and possibly other data (“joint inversion” vs non-invasive “image fusion”)
Other geophysical methods (seismic monitoring, gravimetry, magnetics, active seismics, new denser gravimetry, lake EM measurements)
What are we imaging - petrophysical interpretation of conductivity structures?
New MT stations:
Red and Green Balloons
TEM, Mollidor et al. 2013, Cologne University, DE

15. THANKS!

16. Bathymetry Effect on the AMT Data
Forward 3-D Modelling
Stations FUR013 and FUR011 are approx. 4 km and 5 km away from the Atlantic Ocean
The influence of the ocean can not be neglected beyond periods > 1 s.
Plan view of the regional model used for investigating the ocean effect on the AMT data. Squares mark the survey area.

17. Topographic Effects on the AMT Data

18. RMT, Bastani et al. 2015, Uppsala SE
TEM, Mollidor et al. 2013, Cologne University, DE

19. 1-D Models (invariant) SW NE
Background resistivity: Occam 1-D Inverse Modelling
Vertical bars: Sharp-boundary 1-D Inverse Modelling
Profile 1
Profile 2 SE NW

20. 3-D inversion
  3 m

21. (based on 1-D AMT invariant inversions)
Profile 2 ERT/AMT
(based on 1-D AMT invariant inversions)

22. Dimensionality Analysis
Phase Tensor Analysis
product of the inverse real matrix and imaginary matrix of the MT impedance tensor
graphically represented by an ellipse
axes indicate the max' and min' phase, proxy for electrical strike directions
skew angle, beta, is a measure of the “dimensionality” of the geology
(Caldwell et al, 2004)
Circle indicates 1D whereas and ellipse indicated 2D/3D environment, however skew angle outside +/- 3 degrees imply 3D...

23. Data Analysis – Phase tensor
The colour of the ellipses represents phase difference between the magnetic and electric field.
The black ellipses show where the minimum phase value is >50o corresponds to the region of high conductivity

24. Sensitivity map of Profile1
Preliminary 2-D inversion results: Profile 1
Profile 1 average strike angle of 72o East of N.
inverted using smooth model inversion code (WinGLink software package, Rodi & Mackie, 2001).
10kHz to 0.1Hz, app resistivity and phase. Error floors for phase and apparent resistivity data: 5%, 10% for both TM and TE modes.
Regularization parameter: τ = 3.0
Starting model: 100 ohm-m
Sensitivity map of Profile1
Log (sensitivity)