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

2

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

3

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

4

4.5

5

5.5

6

6.5

7

7.5

8

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

9

Launcher Assembly:

Prototype

Launcher Assembly:

Flight Design

Grimm et al., 2009

11

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.

13

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

14

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.