Geophysics is expected to have a major role in lunar resource assessment when manned systems return to the Moon. Geophysical measurements made from a lunar rover will contribute to a number of key studies: estimating regolith thickness, detection of possible large-diameter lava tubes within maria basalts, detection of possible subsurface ice in polar regions, detection of conductive minerals that formed directly from a melt (orthomagmatic sulfides of Cu, Ni, Co), and mapping lunar geology beneath the regolith. The techniques that can be used are dictated both by objectives and by our abilities to adapt current technology to lunar conditions. Instrument size, weight, power requirements, and freedom from orientation errors are factors we have considered. Among the geophysical methods we believe to be appropriate for a lunar resource assessment are magnetics, including gradiometry, time-domain magnetic induction, ground-penetrating radar, seismic reflection, and gravimetry

Becker, A.

Goldstein, N. E.

Lee, K. H.

Majer, E. L.

Morrison, H. F.

Myer, L.

English

NASA Technical Reports Server

10 New Technologies (or Lunar Resource Assessment
Conclusions: We thus see that the adaptation of this well
logging technique is entirely consistent with most of the needs
for a robotic lunar mission (e.g., low power, small size, rugged
instrumentation) and could provide a broad range of elemental
data for the shallow lunar subsurface. With the neutron generator
turned off, the detector would provide natural and solar-flare-
produced activity data, which are likely to be proposed by other
investigators; by also using the neutron generator one can obtain
much more derailed composition data. One can envision dragging
the generator and detector package behind a rover for mobile
measurements, i.e..getting the source of radiation away from other
experiments. One could also design the whole package so that
it could be inserted into a shallow borehole for near-surface
measurements.
References: [1] Schweitzer J. S. (1991)Nucl. Geopnys., 5, 65-
90. (2) Ellis D. V. (1990)'Science, 250. |3] Hertzog R. er al. (1989)
SPE Formation Evaluation, 4, 153-162. [4] Schweitzer J. S. and
Peterson C. A. (1990) KTB Report 90-6a, 441-449. (51 Peterson
D C. A. e ta l . (1991) Tram. Am. N.«:/. Soc., 64. 212-213.
V93-17235
GEOPHYSICAL METHODS-AN OVERVIEW. A. Becker,
N. E. Goldstein, K. H. Lee, E. L. Majer, H. F. Morrison, and L.
Myer, Earth Sciences Division, Lawrence Berkeley Laboratory,
Berkeley CA 947 20, USA.
Geophysics is expecred to have a major role in lunar resource
assessment when manned systems return to the Moon (ca. 1995).
Geophysical measurements made from a lunar rover will contribute
to a number of key studies: estimating regolith thickness, detection
of possible large-diameter lava tubes within maria basalts, detection
of possible subsurface ice in polar regions, detection of conductive
minerals that formed directly from a melt (orthomagmatic sulfides
of Cu, Ni, Co), and mapping lunar geology beneath the regolith.
The techniques that can be used are dictated both by objec-
tives and by our abilities to adapt current technology to lunar
conditions. Instrument size, weight, power requirements, and free-
dom from orientation errors are factors we have considered in
this paper. Among the geophysical methods we believe to be
appropriate for a lunar resource assessment are magnetics, in-
cluding gradiometry, time-domain magnetic induction, ground-
penetrating radar, seismic reflection, and gravimetry.
Depending on weight limitations, all or some of these instru-
ments, together with their power supplies and data acquisition/
telemetry, would probably be mounted on a small rover and on
a towed nonmagnetic, nonconductive trailer. Rover and trailer
might be controlled remotely. To reduce power and data storage
requirements, the data would probably be taken at discrete loca-
tions, rather than continuously. We envision that the rover would
also be equipped with a lunar version of a GPS receiver to display
and record position and elevation.
Magnetics: Two- or even three-component flux-gate magne-
tometers could be attached via short booms to the trailer to
measure the total (scalar) magnetic field plus its horizontal and
vertical gradients. These small, low-power devices are commonly
used in many applications, including some of the past Apollo
missions. They have auseful sensitivityof 0.1 nT,which is perfectly-
adequate for resource evaluation and for the mapping of lunar
rock types.
Because the Moon presently has negligible, if any, internal dipole
field due to a metallic core, the small magnetic fields at the surface
are due to permanent magnetization, possibly TRM, of the lunar
rocks. There is also a small time-varying field of external origin
due to fields carried in the solar wind and the fields associated
with the Earth's magnetotail. Magnetic field variations over the
Moon's surface due to changes in rock magnetization may amount
to ±2000 nT. A major unresolved question concerns the nature
and origin of localized strong magnetic anomalies detected from
orbit.
Time-Domain Magnetic Induction: The absence of water
and conductive minerals gives the lunar surface environment an
extremely low, and presumably uniform, electrical conductivity
(less than 10H S/m) to considerable depths. Solar heating of the
surface will raise the conductivity several orders of magnitude
within a thin layer, but this does not alter the very resistive nature
of the Moon. Thus, conductivity measurements, such as the
shallow-probing time-domain EM measurements done on Earth
to search for conductive features like groundwater, might not be
a candidate for lunar geophysics were it'not for the fact that the
lunar regolith and crust contain micrometer-sized particles of
elemental iron and FeS. These single-domain magnetic particles
are superparamagnetic, which results in a frequency-dependent
magnetic relaxation loss phenomenon when rocks and soils are
subjected to an alternating magnetic field |1). This property can
be exploited to determine the vertical distribution of superpara-
magnetic particles, information that could be used as a guide to
regolith thickness and age.
The magnetic induction system might consist of a pair of
coplanar, colinear coils on the trailer (Fig. I). A transmitter coil,
about 1 m in diameter, surrounds a small-diameter, mult i turn
detector coil. The transmitter is energized by a series of square-
wave current pulses with sharp rise and decay times. During the
brief time current flows through the transmitter loop, a primary
magnetic field is created. At the instant the loop current goes
to zero, the only remaining magnetic field is a rapidly decaying
secondary magnetic field caused by a magnetic polarization current
Current in transmitter loop
Voltage Induced in receiver
coil - no eddy or polarization
currents in lunar regolith
Sample Decay
Voltage
Voltage induced in receiver
coil by superparamagnetic iron
minerals in lunar regolith
Fig. 1. Principle of a time-domain magnetic induction system.
https://ntrs.nasa.gov/search.jsp?R=19930008046 2020-03-24T07:24:57+00:00Z
LPI Technical Report 92-06 11
in the lunar material. The decay current is measured in terms
of a voltage induced into the detector coil. The voltage decay
curve is recorded within microseconds after current in the loop
goes to zero, and one can, in principle, determine from its shape
and magnitude the vertical distribution of polarizable material.
If one were to measure the change in magnetic field in the center
of the during the current on-time, relative to its free space value,
one could also obtain an estimate of the magnetic susceptibility
of the lunar soil.
Ground-Penetrating Radar (GPR): During the last decade,
GPR has become a staple method in shallow investigations. GPR
systems working in the 100-MHz to 1-GHz range are routinely
used to probe the Earth to depths of up to tens of meters. The
depth of investigation is limited by a number of system and
subsurface parameters, e.g., radiated power, frequency, and atten-
uation by wave spreading, conductive losses, and scattering, to
name a few. Because lunar surface rocks have very low conductivity
and low dielectric constant, radar waves penetrate more deeply
into the Moon than into the Earth; radar frequency penetration
may be hundreds of meters. Spatially coherent reflections, if they
are detected at all from shallow sources, would provide exciting
new information on the Moon's internal structure and possible
presence of either conductive mineral concentrations or an ice
horizon, which some scientists believe exists beneath the polar
regions.
No new technology would have to be developed to add GPR
to a lunar rover. Antenna design would have to be optimized for
a lunar mission, antenna size dictating the lowest frequencies.
Small, highly directional horn antennas could be used at frequen-
cies over 1 GHz for high-resolution shallow soundings; 1-m electric
dipoles could be used at frequencies of around 300 MHz. Currently,
Sensors and Software Corp., Mississauga, Ontario, is planning to
space harden and lighten a commercial GPR system for a possible
French-led Mars mission in the 1998-2000 time period.
Seismic Reflection: Seismic reflection may be the most dif-
ficult geophysical technique to adapt to a lunar rover because
the system must be lightweight and remotely controlled, and the
technique requires physical coupling of sources and detectors to
the Moon's surface.
Several source types used on Earth might be adaptable to a
rover, e.g., a hammer or an accelerated weight drop onto a plate,
a shotgun slug fired into the ground, or a mechanical vibrator
[2]. Of these, a mechanical vibrator with sufficient moment to
be useful is probably the least attractive candidate source because
of its size, complexity, and power requirements. However, it should
not be excluded from consideration at this time because new
technology could make piezoelectric devices, such as bimetallic
"benders," a candidate source if a good way were to be found
to couple energy into the Moon. The accelerated weight drop
produces the highest energy of all the small commercial sources,
but significant redesign and weight reduction of the Bison Elastic
Wave Generator is needed for a lunar source.
The seismic system would probably have only a few detectors
and corresponding data channels. Detectors might be embedded
into the wheels of the rover-trailer vehicle; a pressure-activated
switch in the wheel would relay a signal to the driver that the
detector(s) is (are) positioned for a measurement.
Gravimetry: Gravity measurements will provide the basic
information for determining the Moon's internal density distri-
bution, and thus will be an important method for discerning
subsurface structures and rock-type variations. Gravity measure-
ments are among the most tedious and time-consuming geophysical
measurements to make because the meter must be very precisely
leveled and elevation, terrain, and tidal correction factors must
be calculated and applied before the meter readings become useful
data. For example, we would need to know station elevation
relative to a datum surface to much better than 1-m accuracy,
the local topography to 1-m accuracy out to a radial distance of
at least 10 km, and the lunar tide effect. The latter requires a'
nearby monitoring station.
At present there is no gravity meter that meets the specifications
for a lunar resource assessment. The meter must be self-leveling
to 10 arcsec or better, provide digital readings under the direction
of system control, work in a low absolute gravity environment,
have a precision of 0.1 mGal or better, and have an operating
range of perhaps 2000 mGal. Scintrex Corp., Concord, Ontario,
has redesigned its CG-3 Automated Gravity Meter for the Can-
adian Space Agency for possible use in the French-led Mars
mission. The design calls for a smaller and lighter meter than the
10-kg CG-3, one that would be autoleveling to better than 200
arcsec, more precise leveling to be achieved by means of a numer-
ical correction.
References: [1] Olhoeft G. R. and Strangway D. W. (1974)
Geophysics, 39, 302-311. |2] Miller R. D. (1992) Geophysics, 57.
693-709.. _ . / -
N 9 3 - I 7 2 3 6 -
TOPLEX: TELEOPERATED LUNAR EXPLORER-
INSTRUMENTS AND OPERATIONAL CONCEPTS FOR
AN UNMANNED LUNAR ROVER. James D. Blacic, Los
Alamos National Laboratory, D462, Los Alamos NM 87545, USA.
We propose a Teleogerated Lunar Explorer, or TOPLEX, con-
sisting of a lunar lander payload in which a small, instrument-
carrying lunar surface rover is robotically landed and teleoperated
from Earth to perform extended lunar geoscience and resource
evaluation traverses. The rover vehicle would mass about 100 kg
and carry —100 kg of analytic instruments. Four instruments are
envisioned: ( l ) a Laser-Induced Breakdown Spectrometer (LIBS)
for geochemical analysis at ranges up to 100 m, capable of operating
in three different modes (1) ; (2) a combined X-ray fluorescence
and X-ray diffraction (XRF/XRD) instrument for elemental and
mineralogic analysis of acquired samples [2]; (3) a mass spectrome-
ter system for stepwise heating analysis of gases released from
acquired samples (3]; and (4) a geophysical instrument package
for subsurface mapping of structures such as lava tubes (4).
The LIBS (30 kg, 7 W) uses plasma atomic emission and (op-
tionally) mass spectrometry for elemental analysis of unreachable
locations such as cliff faces. Mineralogic information is obtained
by using the optical portion of the LIBS as a UV/VIS/near-IR
reflectance spectrometer. The XRF/XRD instrument (10 kg, 10 W)
requires powdering of scoop-and-screened or other acquired sam-
ples. Mineral structures are determined by Rietvelt analysis of
powder diffraction data combined with elemental analysis by XRF.
The mass spectrometer system (20 kg, 35 W) will be used to
measure isotope ratios of light-element gases released from step-
heated samples, and will include an evaluation of lunar H and
He resources. The geophysical instrument package (20 kg, 20 W)
uses a combination of high-frequency seismic and electromagnetic
sensors to measure subsurface physical properties and map struc-


Geophysical Methods: an Overview

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server