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Contrasting
Architecture and Dynamics of the Transantarctic Mountains |
| TAM Home :: Pensacola-Pole Transect :: Migration |
Migrating radar records |
| Radar
sounding records are typically interpreted without substantial data processing.
In this study, seismic migration processing methods were applied to differentiated
radar sounding records experimentally to correct for angular relationships
of the bed surface (Fig. 11). Because radar records image only the air/ice,
ice/bed and air/bed interfaces without substantial bed penetration, only
morphological data may be extracted about the bed surface. Therefore, for
detailed structural analysis, it is imperative that ‘true’ angular
relationships are preserved. Seismic migration provided a method of processing
that more accurately displays planar dips. A 2-layer, 2-D finite difference migration was applied to all radar records to account for latitudinally varying ice and air thickness along the length of a transect. All migration was done using Paradigm Geophysical’s Focus version 4.3 seismic processing software. Median trace spacings for each transect were used. A 2-layer velocity model was derived using ice surface picks and nominal air/ice velocities. 80% Vr.m.s. velocities were calculated at each point as per the Dix (1959) equation: Vr.m.s.
= [((t1v12) + (t2v22))/(t1+t2)]
1/2 Where
v1 is the velocity of radar waves in air (300 m/µs) and v2
is the velocity of radar waves in ice (169 m/µs), t1 is the
distance to the air/ice surface interface in two-way time and t2
corresponds to the two-way time between the ice surface and the end of
the radar record and includes the ice/bed surface interface. More points
were picked when there was more topographic variation. |
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Migrated files were then imported into Schlumberger Geoquest’s IESX version 3.7 software for interpretation. The ice and bed surfaces were picked using the following criterion: 1. The strongest echo was picked with the assumption that it was the echo received from directly below the aircraft. 2. When a mountainous looking bed surface was superimposed upon a horizontal bed surface echo, the horizontal surface was picked due to the likelihood that the more mountainous echo came from off to the side of the aircraft. 3. When two equivalent (in strength) bed echoes were superimposed, the higher of the two was chosen because the lower echo probably came from the side. 4. The ice surface echo was chosen as the initial strongest ‘horizontal’ record. 5. Geologic maps were used where available to supplement and cross check bed surface picking. Migration improved the radar record by correcting for angular relationships that became cluttered from diffraction hyperbolae that result from imaging one point reflector at several aircraft locations. Point reflectors are displayed in the up dip direction and migration replaces point reflectors to their correct position below the aircraft as well as correcting for vertical reflection time.
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