Low-Frequency Electromagnetic Methods for Multi-Scale ...
Transcript of Low-Frequency Electromagnetic Methods for Multi-Scale ...
Low-Frequency Electromagnetic Methods for
Multi-Scale Subsurface Planetary Exploration
Robert E. Grimm Southwest Research Institute, Boulder
International Workshop on Instrumentation for Planetary Exploration
October 2012
D. Durda
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Geophysical Methods
• Seismology – Gold standard for internal
structure, esp. global.
– Not very sensitive to water.
• Ground-Penetrating Radar – Good structural resolution
– Limited investigation depth.
– More sensitive to water.
• Inductive EM – Very sensitive to water.
– Very high sensitivity to selected properties for specialized investigations.
– Large exploration depth.
– Poorer resolution.
Source
Grant and West, 1968
Skin Depth (km) = 0.5 r/f = 0.5 T/s
f = frequency, Hz; T = period, sec
r = resistivity, W-m; s = conductivity, S/m
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DEEP SOUNDING (kms to >100 km)
Natural-Source Electromagnetics Magnetotellurics, Geomagnetic Depth Sounding
Target
Frequency
Water Table (kms) Habitability, geothermal gradient (joint with heat flow)
mHz – kHz solar wind, lightning
Crust (tens of km) Differentiation history (joint with seismology)
mHz – Hz solar wind
Lithosphere (> 100 km) Thermal state (joint with seismology, heat flow)
mHz (diurnal) to mHz ionosphere, solar wind
Mars Groundwater is a Near-Ideal EM Target
Grim
m, 2
00
2
-5 -4 -3 -2 -1 0 1 2 3 4 5 3.5
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4.5
5
5.5
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6.5
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7.5
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Fig. 1: Mars MT Response
Log Frequency, Hz
Rock
Rock+Ice
Rock+Ice+Water
Log A
ppare
nt R
esis
tivity,
Ohm
-m
Resistor (7 km) – Conductor (1 km) - Resistor
• Dry, very resistive cryosphere over saline, conductive groundwater
• EM discerns groundwater over wide frequency band
Sensors
• Sensor package is
essentially a classical
space-physics
experiment operated on
the ground.
• Magnetometers
– Fluxgates < 1 Hz,
Induction (Search Coils)
> 1 Hz
• Electrometers
– Response to near DC
under galvanic coupling;
capacitive coupling
requires high-impedance
preamplifiers, circuit
shielding and guarding.
THEMIS SCM Roux et al.
Balloon Electrode (top) and Ballistically Deployed
Ground Electrode (bottom); G. Delory
UCLA FGM
(R. Strangeway)
10 cm
EM Sounding: Implementation E1 (ballistic)
E2 (ballistic)
E3 (Basal)
FGM
SCM
Electronics
Generalized Lander
Lunette Lander (JPL)
PI. C. Neal, Notre Dame
INTERMEDIATE SOUNDING (10s m to few km)
Controlled-Source Electromagnetics Time-Domain EM, Surface Nuclear Magnetic Resonance
CURRENT IN TRANSMITTER LOOP
RAMP TIME
TIME-ON
RAMP TIME
TIME-OFFTIME-ON
ONE PERIOD INDUCED ELECTROMOTIVEFORCE CAUSED BY CURRENT
MEASUREMENTDURING
TIME-OFFCAUSED BY EDDY CURRENTSSECONDARY MAGNETIC FIELD
CURRENT IN TRANSMITTER LOOP
RAMP TIME
TIME-ON
RAMP TIME
TIME-OFFTIME-ON
ONE PERIOD INDUCED ELECTROMOTIVEFORCE CAUSED BY CURRENT
MEASUREMENTDURING
TIME-OFFCAUSED BY EDDY CURRENTSSECONDARY MAGNETIC FIELD
TDEM
Transmitter On DelayTime
Recording Time
SignalObserved
Transmitter Off
Instantaneous Response
SNMR Grim
m, 2003
NI-DAQ 6052E
333ksps, 16-bit,
16-channel
NI-DAQ Battery
Receiver Electronics
FPGA Controller
Battery
&
Power Supplies
PC / Laptop
Running Labview
RS232
IEEE 1394
Control
B,
dB
/dt
Dith
er
Preamp
Diff dB/dt Rcv
Coil
Battery
Transmitter
Electronics
Phase
Xmt
Coil
Enable
Grimm et al., 2009
Grim
m e
t a
l., 2
00
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Launcher Assembly:
Prototype
Launcher Assembly:
Flight Design
Grimm et al., 2009
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12 SHALLOW SOUNDING (m to 10s m)
Controlled-Source Geoelectrics Dielectric Spectroscopy
• Surface electrode array
• Inject I, measure V amplitude + phase,
convert to conductivity + permittivity.
• Investigation depth proportional to
electrode spacing: use multielectrode
array to generate 2D cross-section at
fixed location.
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Dielectric Spectroscopy Distinguishes
Ice and Adsorbed Water
• Ice relaxation + DC conductivity dominate in sands
• Broadband dispersion dominates in clays
• Martian regolith likely between sand and JSC-Mars-1.
– Assess H2O content and state to few % accuracy, ~1%
threshold.
– Complements neutron spectroscopy and GPR
Fine-Grained Sand JSC Mars - 1 Smectites, 3 ML
Coarse Fine
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Biogeophysics
• Living cells display several polarization mechanisms under
applied electric fields.
• Electrical properties through life cycle of microbial colonies
have been extensively studied in laboratory columns.
• Substantial investment by DOE, hydrocarbon industry in
using biogeophysics for cleanup monitoring.
• Use on Mars to assess microbial activity beneath few meters
of irradiated and oxidized regolith.
1. Bulk cell interior & external
counterions
2. Cell
membrane
3. Biofilm &
counterions
4. Nanowires
E. Atekwana
Conclusions
• Low-frequency EM better than GPR for investigation to
km depths and beyond.
• Natural-source methods require minimal resources and
have greatest investigation depth (kms – 100 km)
– Groundwater, crust, lithosphere.
• Large controlled source (200-m loop) optimal to assess
shallower groundwater (10s m to few km).
– Time-domain EM, Surface NMR
• Small controlled source (several-m electrode array)
measures both conductivity and permittivity to depths of
meters.
– Distinguish ice from adsorbed water at percent levels.
– Detect microbial activity.