Radar Interpretation

The co rrect geological in terp retatio n o f rad a r im ages depends c ritically on a know ledge o f how the rad ar energy in teracts w ith the surface. The brightness variations seen in M agellan im ages are c o n tro lle d p rim a rily by th ree d iffe re n t su rfa c e v a ria b le s: (1) topography, (2) surface roughness, and (3) electrical p ro p erties o f th e su rfa ce m ateria ls. T h ese v a ria b le s are in flu e n c e d to som e e x te n t by v a riatio n s in the rad ar in cid en ce angle and the look d irec tio n {Farr, 1993). Figure 3.5 and T able 3.1 show how the

30

I

1 -30 Cycle 1 — Cycle 2

Cycle 3 Maxwell Montes — — Cycle 3 stereo -60

-90

10 15 20 25 30 35 40 45 50 Incidence Angle, deg

Figure 3.5 Magellan SAR incidence angle as a function of latitude for cycles 1, 2 and 3. Maxwell Mons data were collected during orbits 4031 to 4131. After Plant (1993).

Latitude, deg

Incidence angle, deg Cycle 1 Cycle 2 Cycle 3, Maxwell

Montes Cycle 3, stereo 90 16.5 85 18.5 80 20.2 75 22.0 24.4 27.1 13.4 70 23.9 24.9 30.8 13.5 65 26.0 25.1 33.4 14.1 60 28.3 25.1 35.1 15.2 55 30.8 25.1 35.9 16.6 50 33.3 25.1 36.1 18.2 45 35.8 25.1 35.8 19.8 40 38.1 25.1 35.1 21.4 35 40.3 25.0 34.2 22.7 30 42.1 25.0 33.1 23.9 25 43.6 25.0 31.9 24.8 20 44.8 24.9 30.6 25.3 15 45.5 24.9 25.6 10 45.7 24.9 25.5 5 45.6 24.9 25.2 0 44.9 24.9 24.5 -5 43.8 24.9 23.6 -10 42.3 24.9 22.6 -15 40.4 25.0 21.4 -20 38.1 25.1 20.1 -25 35.5 25.1 18.7 -30 32.8 25.2 17.4 -35 30.1 25.3 16.2 -40 27.5 25.3 15.2 -45 25.1 25.3 14.3 -50 23.1 25.1 -55 21.6 24.7 -60 20.5 24.1 -65 19.7 23.1 -70 18.5 21.6 -75 16.3 19.7 -80 17.4 -85 14.8 -90 12.7

Table 3.1 Variation of incidence angle with latitude. Angles are representative for each cycle and are accurate to within 0.5®. After Ford (1993).

ra d a r in c id e n c e a n g le v a rie d w ith la titu d e . A s th e ra d a r in stru m e n t has a sid e -lo o k in g illu m in a tio n g eo m etry , all ra d a r images are distorted to some degree {Farr, 1993, Figure 3.6).

Topography

T o p o g ra p h y m ay h a v e a p ro n o u n c e d e f f e c t on r a d a r b ack scatter. A slope oriented tow ards the incom ing rad iatio n w ill re fle c t sig n ifican tly m ore energy than a fla t su rface or su rface w h ich slopes aw ay from the in co m in g ra d ia tio n . S tro n g ra d a r retu rn s received at the spacecraft from the surface o f V enus are tran sla te d in to p ix els w ith high D N v alu es, rec o rd ed as b rig h t areas in rad ar im ages. C onversely, areas w hich retu rn little rad ar en erg y have low DN pixel values and hence are d ark er in the radar images.

W ith in r a d a r im a g e s , b r ig h t a re a s m ay be s p a tia lly c o m p re sse d (fo re s h o rte n e d ) w h ile d a rk a re a s are e x p a n d e d . F o resh o rten in g (Figure 3.7a) resu lts in a given terrain or featu re appearing to have a steeper slope on its near range side, and an exaggerated shallow far range slope {Pettengill et al., 1991). W ith extrem e slopes, radar layover m ay occur w here the top o f a slope is im aged before the bottom (Figure 3.7b). A nother m ore com m on e ffe c t resu ltin g from p ro nounced to p o g rap h y is rad a r shadow ing, w here the b ack slo p e o f a m ountain or graben is n o t illu m in ated by the radar and no data are acquired for this region (Figure 3.7c).

Surface roughness

S u rfa c e ro u g h n e ss at the scale o f th e ra d a r w a v e le n g th d o m in a te s ra d a r b a c k s c a tte r in M a g e lla n im a g e s w h e re th e in cid en ce angle is greater than 20^ and less than 60^ {Farr, 1993, Figure 3.8). Incidence angles for the V19 and V31 quadrangles lie b e tw e e n 32^ and 460, hence surface ro u g h n ess is an im p o rtan t c o n sid e ra tio n w hen m apping surface m aterials. T he d e te ctio n o f s u rfa c e ro u g h n e s s is w a v e le n g th d e p e n d e n t, th e lo n g e r th e w avelength the coarser the surface m aterial m ust be for dom inant b a c k s c a tte r to o ccu r. T he w a v ele n g th o f the M a g e lla n SA R in stru m en t was fixed at 12.6 cm, so surface roughness w ill control

SAR Look^ A ngle S lant R ange S lan t R an g e Im age

k

G round R an g e Im age Near R ange

X

Far R ange S lan t R ange G ro u n d R ange Im age Im age

Figure 3.6 Slant-range and ground-angle geometries. By knowing the height of the spacecraft; and incidence angle a correction can he applied to slant-range geometry which causes compression of targets in the near range compared with those in the far range. After Farr (1993).

SAR Look A ngle SAR R adar B eam L ook A ngle I— R a d a r B eam R ad ar-lm ag e P lan e R ad ar-lm ag e P lan e R ad ar-lm ag e F o rm at R ad ar-lm ag e F o rm at F ar R a n g e

N ear R an g e N ear R ange F ar R an g e

SAR _{Look A ngle} ^ R a d a r B eam R adar-lm age P lane R a d a r S h a d o w F ar ^ R a n g e N ear R ange C D A

Figure 3.7 Illustration o f geometry distortions in radar images: (a) radar image foreshortening, where the slope facing the radar is compressed to segment A ’-B’ while the backslope is extended to segment B ’-C’; (b) radar layover, an extreme case of foreshortening, where the top of the mountain, B ’, is imaged

before the bottom. A’; (c) radar shadowing, where the backslope of the mountain, B ’-D’, is not illuminated by the radar, and no data is acquired. After Farr,

(1993).

+10 S m o o th S u rfa c e € M oderately R ough

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-10

I

_{R o u g h} -20 -30 40 60 20 0 In cid en ce A ngle 6, d e g

Figure 3.8 Radar backscatter as a function of incidence angle for representative surfaces. For angles less than approximately 25^, smoother surfaces have greater backscatter than rougher surfaces (Farr, 1993). Incidence angles for Venus quadrangle V31 and V19 range between 33.3® and 44.9® with cycle one data (see Table 3.1). After Farr (1993).

the d e g re e o f ra d a r b a c k s c a tte r (F ig u re 3 .9 ). In th e u n it d escrip tio n s below w av elen g th ro u g h n ess refers to m u ltip le s o f 12.6 cm (see Chapter 5 (5.2) for specific examples).

Dielectric properties

In addition to topography and surface roughness, variations in the dielectric constant o f surface m aterials can affect the intrinsic re fle c tiv ity p ro p e rtie s o f su rface m a te ria ls , in flu e n c in g im ag e b rig h tn ess. M aterials o f high d ielectric co efficien ts are g en erally good reflecto rs and hence poor em itters. Two p rom inent exam ples o f low em issiv ity read in g s and rela te d high d ielec tric co n stan ts are (1) near the sum m its o f the p la n e t’s h ig h est m ountains w hich are rad ar b rig h t, corresponding to areas ap p roxim ately above the 6053.5 km rad iu s co n to u r, and (2) p ara b o lic d eb ris a sso ciated w ith som e o f the venusian craters e.g. N eva crater (0.9® N, 338.7® E, V31) {Campbell et al., 1992; P la n t, 1993). An explanation for the e n h a n c e d d ie le c tric c o n sta n ts n e ar m o u n ta in su m m its is the c o n c e n tra tio n o f iro n su lp h id e such as p y rite and p y rrh o tite

{Pettengill e t a l , 1988; Klose etal., 1992).

Spatial comparison o f SAR data

Q u a n tita tiv e c o m p a riso n s b etw een m ap u n its can on ly be ach iev ed once the b ack sc atter co efficien t, gq (sigm a-0) is in its original lin ear form at. The backscatter coefficients on M agellan CD d ata are scaled in a logarithm ic form (value in dB) in ord er to co m p ress the very w ide dynam ic ran g e o f the SA R data. A ny av erag in g o f pix els w ith in M ag ellan im ages m u st be c alcu lated w ith b a ck sc atter co efficien t values in the o rig in al lin e a r fo rm at (see A ppendix I). S ig n ifican t errors m ay occur if D N values are averaged p rio r to lin ear rescalin g {C a m p b ell, 1995). A M acintosh program w hich calculates Gq from the D N value and averages the m e a su re m e n ts w ith in th e sam p le a re a has b e e n c re a te d by

C a m p b e l l (1995). A ll num eric radar data in this thesis have been red u ced to obtain the true rad ar b ack scatter co efficien ts. F u rth er c o n strain ts o f rad ar data w hen used fo r g eo lo g ical m apping are discussed in Chapter 5 (5.2).

Isolated flows

S m o o th : no re tu rn

i

T

« X

Smooth plains

Slightly R ough: slig h tly d iffu se

i

j

<X

Mottled plains f

YYYYYYYY\

M oderately R ough: m o d erately d iffu se

Tessera

Very R ough: v e ry d iffu se

i

T

» X

Figure 3.9 Surface roughness effects on radar backscatter. Surfaces whose roughness is much less than the radar wavelength scatter in the specular direction. Rougher surfaces scatter more energy in all directions, including hack to Magellan’s high gain antenna, and hence appear brighter in radar images. Typical units identified in this study which have different backscatter properties are noted. After Farr (1993).

In document Observations of stratigraphy and volcanism from guinevere and sedna planitiae, venus (Page 88-97)