Sahara


Titles of Investigations:

I.  Geologic and Hydrologic Studies of Saudi Arabia Under the 
Shuttle Imaging Radar (SIR-C) Science Plan

II.  SIR-C Polarimetric Radar Image Simulation and 
Interpretation Based on Random Medium Model

III.  The Exploration and Reconstruction of the Middle to 
Late Cenozoic Drainages of the Sahara by Means of the SIR-C Mapping

IV.  SIR-C Surface and Subsurface Responses from 
Documented Test Site Localities in the Sahara, Namib, 
and Kalahari Deserts, Africa and the Jornada del Muerto, New Mexico

V.  Multifrequency Imaging Radar Polarimetry:  
Geophysical Factors from Penetration Phenomena

Principal Investigators

I.  Dr. Abdallah Dabbagh
  King Fahd University

II.  Dr. Jin Kong
  Massachusetts Institute of Technology

III.  Dr. John McCauley
  Northern Arizona University

IV.  Dr. Gerald Schaber
  U. S. Geological Survey

V.  Dr. Howard Zebker
  Jet Propulsion Laboratory

Site Description:

The Sahara, an area the size of the USA, became desert about 2 Ma, 
but prior to that was apparently well-laced with major drainage 
networks.  With increasing dominance of the wind, the sands in 
the channels and floodplains of the rivers were reworked into 
thin sheets and scattered dunefields.  These widespread aeolian 
deposits masked the Tertiary continental history of the region 
and hid many of the relict river systems from view until the 
advent of SIR-A.

Several targets in the Sahara have been selected as sites 
for the SIR-C/X-SAR mission.

Eastern Sahara:  Wadi Howar to Nile Valley and Gilf Kebir

Location:  
This area is adjacent to the Safsaf Calibration Supersite.  
It covers areas crucial for mapping paleodrainages between 
the Nile and Chad Basins.  It is an irregularly shaped area 
whose corner coordinates are:

 N 19°30', E 21°     N 20°, E 28°30'
 N 25°, E 25°20'     N 22°, E 30'
 N 21°, E 27°        N 14°30', E 24°30'
 N 15°15', E 23°10'  N 17°15', E 24°40'

Science:  
The radar rivers discovered on the SIR-A image swath across 
Egypt and Sudan are thought to extend into the Chad Basin 
and then into the Atlantic Ocean (McCauley et al., 1986 a, b).  
This Trans-African Drainage hypothesis will be tested by examination 
of SIR-C/X-SAR imagery for evidence of continuation of the radar 
rivers' distinctive patterns in the shallow subsurface of Libya, 
Chad, and Sudan.  This application of SIR-C/X-SAR for geological 
exploration is extremely important because of its potential 
for locating shallow groundwater in the sand-buried alluvial 
valleys of this impoverished region.  Such previously unknown 
water resources (and other mineral resources) have been found 
in the radar river valleys of southern Egypt and also in 
buried river valleys discovered by random and 
(and expensive) wildcat drilling efforts in Libya 
(McCauley et al., 1986 a).

This area covers part of the probable course, now mostly 
obscured by windblown sand, of Wadi Howar, a relict river 
that in wetter times flowed across northern Sudan and is 
thought by some to be a tributary to the Nile.  
The paleo-flow direction is uncertain and controversial; 
Wadi Howar may be an independent, pre-Nile, consequent 
stream.  SIR-C/X-SAR coverage may provide a means to 
discriminate among sizes of river gravels in buried 
fluvial deposits, and thus determine the run and the 
relation of Wadi Howar to the Nile River, if any.  
This region is of critical interest to all Paleodrainage 
Project members; areas around the Gilf Kebir Plateau 
and in the Nile Valley southwest of Aswan are well 
known to McCauley, Breed, and Issawi and the Wadi Howar 
area is a specialty of Prof. Pachur and his group 
in Berlin, and Maxwell.

Central Sahara:  Wadi Tafassasset

Location:  This area along the border of northern Niger with 
southeastern Algeria includes the course of Wadi Tafassasset, 
one of several relict tributary drainages that formerly flowed 
to the Niger River, but are now mostly defunct and obscured 
by sand sheets and dunes.

Science:  The Tafassasset area is bounded (approximately) by 
latitudes 20° and 24°30'N, and by longitudes 7°30' and 11°30'E 
(see map).  This area has been studied by French geologists, 
including H. Faure, using conventional geologic field methods.  
Faure's group is presently using SPOT imagery to map the 
drainage patterns that are visible in certain places; 
other, key areas are obscured by sand and will only be 
mappable from the SIR-C/X-SAR coverage.  SIR-C/X-SAR has 
the potential to greatly increase our knowledge of the 
evolutionary history of this part of the Central Sahara in 
relation to paleoclimatic conditions, regional tectonics, 
and the capture and reversal of the Niger River system.

Objectives:

I.  a) Major objectives are the utilization of multiple-polarization, 
dual frequency, radar remote sensing imagery, and synthetic 
aperture radar to detect lithological boundaries, distinguish 
tectonic features, detail fluvial geomorphology, and 
elucidate hydrologic systems in large areas having a thin sand cover.

II.  a) Demonstrate the applicability of the random medium 
model in simulating SIR-C Supersite imagery.

  b) Analyze and interpret SIR-C imagery for remote sensing applications.

  c) Investigate seasonal variations and atmospheric effects.

III.  a) Integrate and map relic Cenozoic drainage systems across 
the Sahara using SIR-C data in a synoptic mode with other 
remotely sensed data, field, and cartographic data.

  b) Demonstrate applicability of SIR data, used synergistically 
with Landsat, SPOT and high-altitude photographic data, 
for exploration geology.

  c) Produce a major report on the distribution of 
paleodrainages in the Sahara, their relations to the basic 
tectonic elements of North Africa (basins and swells), 
and their economic potential.

IV.  a) Determine the optimum SIR sensor configuration for 
detection of desert duricrust and to use this understanding 
to reconstruct the paleoclimatic history of two large desert 
regions in Africa.

V.  a) Model, experimentally characterize, and verify 
penetration phenomena in hyperarid and vegetated regions 
using the SIR-C multiparameter radar system and ground-based receivers.

  b) Invert measured radar backscatter as a function of 
frequency and polarization in terms of geophysical 
parameters of the surface and subsurface such as surface 
roughness and subsurface geomorphology.

  c) Display subsurface features in an image format, thus 
easing the interpretability of the results.

Field Measurements:

I.   a) Field observations will be carried out to collect 
ground truth data on joint systems, fold structures, 
fault systems, rock types, thickness of sand layer over 
paleodrainages, dune dimensions and orientation, and vegetation density.

II.  Kong please provide if applicable

III.  a)  Testpits will be excavated and documented to 
determine the distribution and composition of buried 
fluvial deposits located by their radar signatures 
on SIR-C/X-SAR images, for reconstruction of paleodrainage 
systems and potential aquifers.

  b) Samples will be collected in testpits to record the 
stratigraphic sequences of buried fluvial deposits for 
radiometric dating and interpretation of regional geologic 
and paleoclimatic history.

  c) Field measurements by Schaber will be used to support 
this research effort.

IV.  a) Deploy series of six to eight JPL-provided corner 
reflectors (8-foot diameter) required for minimum calibration 
in support of the Sahara EM-Theory supersite investigation 
during SIR-C/X-SAR.

  b) Excavate a series of backhoe trenches in order to document 
the physical and geologic (stratigraphic) controls on signal 
penetration and backscatter down several meters.  
Full photo-documentation and sketching of 
surface and trench walls.

  c) Collect soil, sand, and duricrust samples for 
determination of petrology, moisture content, and 
electrical properties measurements.

  d) If feasible, in situ electrical properties measurements.

  e) Location of trenches and corner reflectors using 
Magellan NAV-5000 GPS Navigator.

V.  a) Deploy ground receivers during the experiment to 
measure field strengths at both vertical and 
horizontal polarizations.  These in situ measurements 
will constrain and confirm our theoretical models. 

Crew Observations:

1) Crew Journal: Document dust storms and vegetation extent.  
Sketch any drainage patterns at the site.

2) Cameras: Hercules and Hasselblad will obtain low sun angle 
photographs of the desert.

Coverage Requirements:

1) Coverage of the Eastern Sahara area requires eight short swaths, 
with 10% side overlaps (total length approximately 5,000 km).  
All coverage for the Paleodrainage Experiment should be in 
Mode 11, with the widest possible swath.

2) Two boxes are shown on the map, representing both orientations 
of the potential SIR-C/X-SAR coverage.  The NW-SE, narrower 
but longer rectangle would be best for tracking the course 
of the Tafassasset paleodrainage system from the Djanet 
(Ft. Charlet) area into the sand-covered desert 
(Tenere du Tafassasset and Erg of Bilma).  The corner 
coordinates of that box are:

 N 21°45', E 7°50'   N 20°, E 12°
 N 22°45', E 9°20'   N 19°, E 10°40'

 Coverage of that box would require two short swaths, each 500 km 
long, with 10% overlap (again in Mode 11).

Anticipated Results:

I.  a) Significant new data on joint systems and fold structures, 
as well as some of the major fault systems.

  b) Exploration of Pleistocene paleodrainage system with 
major implications for regional hydrology.

  c) Contributions to the archaeological geology by way of 
ancient irrigation channels and now buried settlements.

II.  a) Predicted multi-frequency, multi-incidence angle, 
fully polarimetric supersite imagery prior to the 
actual SIR-C/X-SAR mission.

  b) Interpreted SIR-C/X-SAR imagery with applications to 
seasonal and diurnal change studies.

  c) Radar image simulation algorithm based on the random 
medium model for future radar sensor development.

  d) Improved radar image processing algorithms 
for terrain classification.

  e) Multi-layer random medium model for general earth terrain scattering.

III.  a) Major contribution to the knowledge of the nature and 
scope of the erosional events from middle to later Tertiary 
time, which gave rise to the present (Quaternary) 
geomorphology of North Africa.

  b) A major report will be prepared with accompanying maps 
that we hope will have broad scientific and economic 
applications (for both the public and private sectors).

IV.  a) An improved understanding of radar backscatter and 
penetration in hyperarid-to-semiarid terrains that was initiated 
during our SIR-A/B investigations (Elachi, Roth, and Schaber, 
1984; Schaber et al., 1986).

  b) Refinement of synergistic remote methods to identify 
various types and stages of datable, authigenic CaCO3 deposits 
related to successive changes in climate and surface geologic 
processes during the Quaternary.

  c) Improved models of geometric scattering effects on SIR signal 
penetration.

  d) Significant new data on the spatial and chronological 
distribution of semiarid paleoclimatic zones in Africa.

V.  a) An increased understanding of penetration phenomena in scattering;

  b) Identification of sources for backscatter in hyperarid 
subsurface imaging.

  c) Solutions for descriptive geophysical parameters in the 
supersites studied.  These results would be of great use to 
any investigators interpreting images acquired over similar 
targets by SIR-C/X-SAR or any other radar system.