The Virunga Volcanic Chain

ELSEVIER

The Effect of Varying Acquisition Parameters on the Interpretation of SIR-C Radar Data: The Virunga Volcanic Chain

Mary E. MacKay* and Peter J. Mouginis-Mark*

In 1994, the Spaceborne Imaging Radar experiment C INTRODUCTION (SIR-C) acquired five data takes with different acquisi- Tile Spaceborne Imaging Radar C (SIR-C) was flown tion paranwters over the Vimnga volcanic chain, Zaire. Changes in incidence angle (20.0-48.1°), look direction twice in 1994 in tandem with the X-Band Synthetic Ap- (west- and east-looking), and pulse bandwidth (10 MHz, erture Radar (X-SAR). SIR-C represents the first time 20 MHz, and 40 MHz) affect the geologic interpretations that a multifrequency polarimetric imaging radar em- drawn front these data. We examine differences in spatial ploying a phased array mltenna was flown in space (Stofan distortio~ of features near the summit of Nyiragongo vol- et al., 1995). The ability to select the incidence angle, cano, finding that opposite look direction data are helpful swath width, spatial resolution (pulse bandwidth), wave- in interpreting volcanic features, such as the shape of length(s), and polarization(s) provided the SIR-C Science small craters', in areas ~f high relief The relative bright- Team with a myriad of options in planning specific data hess ~f lava flows and vegetation change (and actually takes (Table 1). Often during mission planning the trade- reverse) fi~r different wavelengths and polarizations. Al- offs between these various parameters were difficult to though L-band data are best for mapping the outline of evaluate on the basis of real data in-hand, because of the the flows, C- and X-band cross-polarization data show great diversity of targets that SIR-C would image, and differences in backscatter intensity between and within the absence of similar orbital radar data from free-flyers flows. In addition, our ability to discriminate lava flows such as Seasat, the European ERS-1 or the Japanese and vents from surrounding vegetation is strongly influ- JERS-1. enced by pnlse bandwidth (resolution) and angle (f inci- The overriding objective for each of the SIR-C geo- dence. Pulse bandwidth is the most important paranwter logic experiments was to image a specific point or area in mapping small topographic features such as volcanic on the ground. A number of "Supersites" were selected cones and a set ~f particularly thick flows on the flanks for geology, ecology, hydrology, and oceanography exper- ¢f Karisimbi volcano. These examples illustrate important iments (Evans et al., 1993), and the orbit of the Shuttle considerations of the tradeoffs and priorities in acquisi- was optimized so that as many of these sites as possible tion parameters for fitmre missions such as ENVISAT, were imaged at least once during each flight. Once these as ,cell as potential pitfaUs fi)r interpretation of existing Supersites were identified, an addition~d 380 sites were data. OElsevier Science Inc., 1997 added in order to provide resilience in the mission plan and to cover a greater diversib~ of the Earth's surlhee. This mission plan nevertheless required great flexibility in the site-specific science objectives, and the system parameters were selected for each target on the basis of ° Hawaii Institute of Geophysics and Planetology, University of past experience with aircral?c data and from engineering Hawaii, ttonolnlu models of the performance of SIR-C. Furthermore, due Address correspondence to Mary E. MacKay, Hawaii Institute of to the excellent performance of SIR-C during each mis- Geophysics and Planetology, University of Hawaii, 2525 Correa Road, Honoluhl, HI 96822. sion, additional targets of opportuni~ were added during Received 24 Januar~j 1996; revised 20 May 1996. the missions as real-time confidence grew in the sensor

REMOTE SENS. ENVIRON. 59:321-336 (1997) ©Elsevier Science Inc., 1997 0034-4257/97/$17.00 655 Avenue of the Americas, New York, NY 10010 Pll S0034-4257(96)00144-7 322 MacKay and Mouginis-Mark

Table 1. SIR-C Capabilities This study describes our analysis of five SIB-C data Bands Polarizations IncidenceAngles Pulse Bandwidths takes that were collected over the Virunga Volcanic Field, Africa. The primal" target ibr four of the five SIR-C data L, C HH, HV, VH, \~7 20_60° 10, 20, 40 MHz" takes was the eastern part of the Virunga mountain chain Abbreviations: HH = horizontallypolarized transmit, horizontallypolar- on the borders of Zaire, Rwanda, and Uganda (Fig. 1), ized receive, HV=horizontally polarized transmit, vertically polarized receive, VH =vertically polarized transmit, horizontally polarized receive, which is the home of endangered mountain gorillas \~'=vertically polarized transmit, vertically polarized receive. (DT58.61 specifically targeted Nyiragongo as part of the " lnterferometric mode, VV polarization only. Decade Volcanoes effort). The aetive Nyiragongo and Nyamuragira volcanoes lie just to the west of this area, and were both imaged on two separate occasions. Six performanee and as transient phenomena (e.g., volcanic other volcanoes in the eastern part of the chain were also eruptions, rain storms, and a tsunami) occurred. imaged on all five occasions. As a result of focusing data As part of an analysis of basaltic shield volcanoes acquisition on the gorilla habitat, some of the data takes (Mouginis-Mark, 1995), our SIR-C experiment investi- were less than optinmm for studying Nyiragongo and gates the structure and geologic evolution of lava-produc- Nyamuragira, but nevertheless highlight the tradeofIis as- ing volcanoes such as those located in Hawaii, the Gala- sociated with the nmltiparameter SIB-C. By looking at pagos Islands, and Reunion Island. The results from the differences between four SIB-C data takes in the these geologic studies will be described in forthcoming Virunga Volcanic chain, we can identit~ the effect and papers. However, during the course of studying the data importance of several key acquisition parameters in an for other volcanoes that were imaged as "targets of op- area of mixed volcmlics and tropieal vegetation. This type portunity,'" we have 'also been able to evaluate the signifi- of comparison was not possible prior to SIR-C, but is im- cance of the compromises that were made in data acqui- portant in understanding the choices that might be faced sition, and how these compromises affect the resultant in the selection of operating modes for the BADARSAT quality of the geologic interpretations. In reconnaissance radar (launched in November 1995) or the ENVISAT mapping of volcanic areas, it is important to distinguish ASAR (scheduled tbr a 1998 launch). In particular, this between barren lava flows (young) and surrounding vege- work has relevance to the acquisition of radar data that tation (or old flows), discriminate dif{~rent flow units, will be used for mapping young volcanic terrains in tropi- identify vents and cones, identif~ the location of ash de- cal environments such as Cameroon, Indonesia, the Phil- posits, map strnetural features and lineaments, and, where ippines, and Central America. Where appropriate, we possible, distinguish between a'a and pahoehoe lavas. All also eompare the SIR-C data to a digital elevation model of these attributes provide important information rele- (DEM) created by digitizing an existing topographic map vant to the overall structure of the volcano, the distribu- (Thonnard et al., 1965) and visible wavelength SPOT tion of vents, and the diversity and relative importance data acquired in June 1995. of different types of eruption. VOLCANOLOGY OF THE VIRUNGA CHAIN Figure 1. Regional location map showing the main volcanoes The Virunga volcanic chain lies on the western arm of of the Virunga volcanic field. Inset shows the location of the the East African Rift. The two westernmost volcanoes, study area in Afriea. Nyiragongo and Nyamuragira (Fig, 2) have erupted re- peatedly throughout the last 40 years, most recently in 1994 (GVN, 1994). The summits of these volcanoes , zAI, E reach elevations of 3470 m and 3058 m, respectively, and Nyiragongo stands~2000 m above the level of" nearby Lake Kiv~l. The slopes of this volcano are therefore quite steep, exceeding 35 ° on tile upper slopes (Thonnard et al., 1965). Because of its extremely fluid lavas and prox- imity to the city of Goma, Nyiragongo has cost lives as ,,,yar,',ur,,o,r,. I recently as 1977 (Tazieff, 1977) and represents an ongo- ing threat to the area, earning its distinction as a Decade Volcano (Newhall, 1994). This natural hazard potential )fL~ake Nytragongo,-...A )..'K&1,=,mb, -/ I has been further heightened in 1994 by the establish- ment of many refugee camps on the eastern side of" the volcano. Few detailed physical volcanology studies have been conducted a either Nyiragongo or Nyamuragira, and only reconnaissance mapping has been done for the eastern ii ¸

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Table 2. Virunga SIR-C Data Range Bandwidth Incidence Look Swath Resohttion Data Take Mission" Polarizations (MHz) Angle Dir. Width (kin) Looks 58.60 SRL-1 HH, HV 20 36.7-42.7° Left 36.5 24.4 12.5-11.1 58.61 SRL-2 HH, HV 10 37.7-48.1 ° Left 71.4 12.1 24.5-20.2 154.90 SRL-2 VV 40 20.0-30.8° Right 46.4 37.8 11.0-7.3 170.90 SRL-2 VV 40 20.0--30.8° Right 46.4 37.8 11.0-7.3 171.10 SRL-1 HH 20 34.5--46.4° Right 71.6 25.4 13.2-10.4 X-SAR and SIR-C data were collected simultaneouslyfor all Virunga data takes. Only DT58.61 is shown here. 58.61 X-SAR X-VV 10 41.6-47.1 ° Left 60.2 10.8 23.8-21.6 "SRL-I: Shuttle Radar Lab-l, April 1994 flight. SRL-2: Shuttle Radar Lab-2, October 1994 flight. ;' Note that all data are processed (and presented here) at 25-m resolution. surrounding the higher volcanoes such as Karisimbi and DATA ACQUISITION AND PROCESSING Mikeno (both over 4000 m). Longer wavelength radar The Virunga data were collected during both flights of data "see" through the clouds and therefore provide an SIR-C/X-SAR. Some of the key acquisition parameters excellent opportunity to study these remote volcanoes. for these data are shown in Table 2. The missions simul- In conducting reconnaissance geology of a poorly taneously collected SIR-C L- and C-band data (24.0 cm studied volcanic field such as the Virunga chain, the and 5.7 em wavelengths, respectively) at several different same approach as that employed in the analysis of volca- polarizations; and X-SAt/ X-band data (3.1 em wave- noes on other planets is often appropriate (e.g., Carr et length) at VV polarization. Although all five Virunga data al., 1977; Head et al., 1992). In such studies, it is impor- takes include both SIR-C and X-SAR data, X-SAg pa- tant to identify the source area for each lava flow in or- rameters in Table 2 are shown only for DT58.61, which der to determine which part of the volcano is most pro- is discussed in the text. Two of the five data takes, DT ductive. Typically, basaltic volcanoes such as Nyiragongo 154.9 and DT170.9, were collected with identical acqui- and Nyanmragira erupt lava flows that either have a ra- sition parameters as part of the repeat-path interferome- dar-smooth surface texture (i.e., palmehoe lava), or a ra- try experiment; only DT154.9 is used in the discussion. dar-rough surface texture (a'a lava). Fissures or faults The processed data that we present here are the stan- may also be formed during the growth of a volcano. dard multilook images (12.5 m pixel) produced by the More silicie volcanoes (such as those found in the east- Jet Propulsion Laboratory (JPL), with an additional ern Virunga chain) will have more viscous lavas, and so multilook to produce a 25 m pixel image. Although many may produce very thick (tens to hundreds of meters techniques are available for further enhancing radar im- thick) lava flows with pressure ridges ("ogives") on their age quality, we chose to present the data in this standard surface that can be recognized in radar data due to their form so that the discussion might focus on basic acquisi- topography. It is also possible that explosive eruptions tion parameters rather than our processing skill. may take place, thereby generating radar-smooth or radar-absorbing ash deposits. Maps of the distribution of cinder cones on the flanks are also important because THE EFFECT OF DIFFERENT cones form as a result of intrusions along structural ACQUISITION PARAMETERS ON weaknesses within the volcano, and therefore serve as in- GEOLOGIC INTERPRETATION dicators of the deep-seated structural control on volcanic Evans et al. (1986) reviewed the general advantages of processes (Nakamura, 1977). The identification of the multipolarization over single-polarization radar data tbr relative occurrence and spatial distribution of a'a and pa- mapping, and demonstrated that cross-polarization (i.e., hoehoe lava flow types also permits the rate of eruption HV) data are particularly useful when used in conjunc- of lava on different parts of the volcano to be deter- tion with copolarization (i.e., HH) data for mapping dif- mined, since a'a is associated with relatively high-volume ferent geologic and vegetation units. In a similar manner, eruption rate and pahoehoe forms during low-volume Blom et al. (1987) investigated the optimum choice of eruption rate activity (Rowland and Walker, 1990). Rec- radar wavelength and incidence angle tbr discriminating ognizing the distribution of these flows, cones, fissures different lava flow's at the Craters of the Moon, Idaho and ash deposits is a task that is well-suited to the SIR-C using scatterometer data. More applicable to this study, data, due to the striking differences in radar baekscatter Gaddis (1992) used multiwavelength, multipolarization of the lava flow types, the sharp topographic expression Airborne Synthetic Aperture Radar (AIRSAR) data at of faults and cones, and the low radar returns associated multiple incidence angles to examine the use of" multipa- with ash deposits (Gaddis et al., 1989, 1990). rameter radar for discriminating between different flow ~ ~,, ! il ¸¸r

r 326 MacKay and Mouginis-Mark

length of a single flow (Gaddis et al., 1990), so that it is tation than the copolarized data, we have chosen to re- the difference between the flow and the surrounding strict our discussion to HH (or VV for DT154.9) in order area that enables the flow to be identified. Conversely, to compare the effects of other acquisition parameters cinder cones are most readily seen because of their topo- between data takes. graphic expression rather than their low radar backscat- Although spatial resolution is an important parameter (Sehaber et al., 1980). All of the Virunga SIR-C data ter governing the information content of orbital radar takes collected copolarized backscatter data (HH or VV); data, the effect of spatial resolution on geologic mapping DT58.60 and DT58.61 also collected cross-polarized data of volcanic fbatures has not, to our knowledge, been (HV). The various SIR-C polarizations were collected for evaluated. In the case of SIR-C multilook data, each data both L- and C-band data in addition to the X-SAt/ VV take is processed by JPL with a 12.5 m pixel; however, polarization X-band data. the image quality varies markedly due to differences in Figure 4 shows an area of recent lava flows and pulse bandwidth, a seldom-discussed parameter of im- dense tropical vegetation at multiple wavelengths and aging radar. Together, pulse bandwidth and angle of inci- polarizations. At HH polarizations the lava flows are dence determine the resolution (or ability to discriminate brighter than surrounding vegetation, a relationship that two neighboring targets) in the range direction; Rr=c/(2B is reversed for HV and VV polarizations; ash (stipple in sin 0), where B is pulse bandwidth, 0 is angle of" inci- Fig. 4) remains dark at all wavelengths and polarizations. dence, and c is the speed of light. The wider the range The contrast between recent lava flows on Nyamuragira of frequencies (i.e., the bandwidth), the better the reso- and vegetation is greatest at L-band, which clearly shows lution and the finer the detail that can be resolved in the outline of the flows. The flows themselves, however, the image. However, higher resolution comes at a price. are almost uniform in brightness at L-band. The shorter Perhaps most significantly, a twofold increase in pulse wavelength C- and X-band data show variation in back- bandwidth to improve spatial resolution results in either scatter between flows and within individual flows, provid- the loss of multiple polarizations or a reduction in swath ing a valuable addition to both mapping and interpreta- width of up to 50% (at incidence angles greater than 35 ° ) tion of the flows. The upper (near vent) portion of the because of data rate limitations of the SIR-C system; for 1981 flow is markedly darker than the 1980 and 1989 example, compare DT58.61 with DT58.60 (1/2 swath flows in the C- and X-band data; the more distal portion width) or DT171.1 (single-pol) in Table 2. Therefore, the of the flow lightens, appearing almost the same intensity choice of pulse bandwidth, like other acquisition parame- as the adjacent 1989 flow in the C-HV image, but slightly ters, is always a compromise. darker than the 1989 flow in the X-W image. More sub- We now present four cases which illustrate the dif- tly, the lower (downslope) 1989 flow is slightly darker ferences in the perceived geology of the Virunga volcanic than the separate upslope 1989 flow (originating near the chain based on changes in look direction, incidence caldera) in the C- and X-band data; the upper flow disap- angle, and pulse bandwidth of fbur of the SIR-C data pears completely in the X-VV image, lacking any contrast takes. with the surrounding vegetation. In a densely vegetated tropical region such as this, regrowth on lava flows occurs Look Direction and Incidence Angle: rapidly. Without additional information, it is difficult to Nyiragongo Volcano know whether the differences seen in Figure 4 reflect The effect of look direction on the geologic interpreta- primary flow textures or the regrowth of vegetation tion of the area can be seen by comparing the spatial (Gaddis, 1992); this may be particularly true for the distribution of features on Nyiragongo Volcano in data longer wavelengths, which are better able to penetrate takes DT58.61 and DT171.1 (Fig. 5). The pit craters vegetation. A number of excellent studies have examined called Shaheru and Baruta (each ~1 km diameter) and the effect of wavelength and polarization at various inci- the summit crater of Nyiragongo have strikingly different dence angles (Blom et al., 1987; Blom, 1988; Gaddis, relative positions in the two scenes due to foreshortening 1992; and the references contained within). In this arti- ill the radar image. Such planimetric displacements of cle we focus our discussion on the L-band data since features are also seen in Magellan radar images of Venus they most strongly discriminate between the recent flows (Connors, 1995), and have been used to determine the and vegetation. In addition, although the cross-polarized heights of fault scarps and other surface features. For HV data are generally of more value in geologic interpre- N~ragongo, where existing topographic maps give eleva-

Figure 4. (At right) DT58.61 showing SIR-C L- and C-band data (23.9 cm aud 5.7 em wavelengths, respectively) at HH and HV polarizations; and X-SAIl X-band data (3.1 em wavelength) at \W polarization. The SIR-C and X-SAIl data were acquired simultaneously with similar parameters (Table 2). Ages of flows shown in the map are from Global Volcanism Network Bulletin (1994). Ash is shown in stipple. Note the reversal in relative brightness between reeent flows and surrounding vegetation with ehange in polarization. Also note the changes in relative brightness between the 1981 and 1989 flows, and along the length of the 1981 flow with changes in wavelength. Effect of Vartji~g Acquisition Paraltwte~s on SIR-C Data Iaterpretatiot~ 32 7

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DT58.61 DT171.1 DEM

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Figure 5. Comparison of spatial distortion of the summit area of Nyiragongo volcano caused by changes in SIIt-C look direction and volcano topography. The locations of Baruta (top) and Shaheru (bottom) craters relative to the smmnit crater (the middle of the three large craters) shift due to their lower elevations and the opposite look direction of the radar data (DT58.61 is west-looking and DT171.1 east-looking). Each radar image shows the L-HH band data. The correct relative position of the features is shoval in the DEM, which was digitized from the map of Thonnard et al. (1965) and shows contours at 50 m intervals. Note tile steeper slopes on the left side of the volcano, which contribute to the distortion of DT171.1. Effect of Varying Acquisition Paranwters on SIR-C Data Interpretation 329

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Figure 6. (;eometric distortion of two points in a radar image (distortion occurs in the range direction only). The two points are shown as dark gray circles; their positions in the radar image are sho~a as light gray circles. The horizontal displacement (Ax) of a feature in the image is a function of its height (h, above some arbitrary datum or Ah, relative to the elevation of another feature) and the incidence angle of the data take (0), such that &=h/tan 0. In each case the displacement is towards the radar. The relative horizontal displacement of two points of different elevations (in the range direction) is Axe-&2, or Ah/tm~ 0. tion, we can easily calculate the horizontal displacement eru have proportionately less displacement toward the and resulting distortion of features on Nyiragongo Vol- radar, resulting in an apparent shift (relative to Nyira- cano. The horizontal displacement (A~c) of a feature in a gongo) away from the radar. Although the incidence radar image is a function of its height (h, above some angles for the two Nyiragongo data takes are comparable arbitrau datum or relative to the elevation of another (41.5 ° vs. 36.6 °, for DT58.61 and DT171.1, respectively), feature) and the incidence angle of the data take (0), the relative displacements of the satellite cones are such that Ax=h/tan 0; in each case the displacement is slightly larger for DT171.1 because of its lower incidence towards the radar (Fig. 6). Topographic data extracted angle, A more subtle example of this same effect is seen from the map of Thonnard et al. (1965) show that the in the distortion of the shapes of the two pit craters. For rim of Nyiragongo is 470 m above the summit of Baruta both craters, the west (left) side of the crater rim is and 770 m above Shaheru (Fig. 7). As a result of this dif- slightly higher than the east side, resulting in the appar- ference in elevation, Baruta is offset, relative to Nyira- ent elongation of the craters in the range direction. gongo, 531 m to the left or 633 m to the right (for left- Despite the small differences in incidence angle and looking DT58.61 and right-looking DT171.1, respectively); horizontal displacement between the two data takes, fea- Shahern is similarly offset 870 m to the left or 1037 m to tures in DT171.1 appear significantly more distorted the right (Fig. 5). Because the summit of N~ragongo is than in DT58.61 (Fig. 4). Although the snrface slope the highest of the three features, it has the greatest hori- does not play, a role in the horizontal displacement of zontal displacement toward the radar; Baruta and Shah- features, it does affect the appearance of the resulting 330 MacKayand Mouginis-Mark

Nyiragongo

Figure Z Oblique projection (looking west) of the DEM showing relative elevations of the three cones that make up Ny- iragongo volcano. Vertical exaggeration is approximately 4×. DEM is digitized from Thonnard et al. (1965). image; for greater surface slopes the horizontal displace- gree of "speckle" present in DT58.61, which has the ef- ment will be a higher percentage of the true horizontal fect of obscuring smaller-scale or low contrast features. distance resulting in a more distorted appearance. The For example, two small vent areas ~500 m diameter topographic map of Thonnard et al. (1965) shows that (Fig. 8), are surrounded by radar-dark deposit that we the NW-facing (left) flank of Nyiragongo has a slope of interpret to be ash. The shapes of these cones are clear ~30 ° and the SE-flank ~18 ° (Fig. 5, DEM). This means in DT171.1, but they are uninterpretable in DT58.61; that the right-looking radar in DT171.1 illuminates the and flow boundaries are indistinct and difficult to follow steeper slope of the volcano, resulting in a more dis- in DT58.61 (Fig. 8). In DT171.1, highlighting of the ra- torted image. dar-facing margin of thicker lava flows can also be seen, giving an indication of topographic relief. Although some Incidence Angle and Pulse Bandwidth: small cones can be easily seen in the SPOT image, the Nyamuragira cone shown in the inset (Fig. 8), its associated ash depos- Recent lava flows that have erupted from the northern its, or the relative thickness of the flows cannot be seen flank of Nyamuragira stand out in sharp contrast to the in SPOT multispectral (visible wavelength) data with a surrounding jungle in DT58.61, but show much less con- resolution of 20 m/pixel (Fig. 8). The flow boundaries are trast in DT171.1 (Fig. 8). The two data takes span about nevertheless more easily seen in the SPOT data, indicat- the same range of incidence angles; however, the flows ing the complementary nature of the radar and visual are imaged in the near-range section of DT171.1 (with data sets. incidence angles from 34.5 ° to 37.0°), as compared to the Although "all of the images were processed to a 25 m midrange section of DT58.61 (incidence angles from pixel, the higher pulse bandwidth (and better range reso- 42.0 ° to 43.5°). Variation in backscatter with incidence lution) of DT171.1 produces a more interpretable image angle varies greatly among different surfaces, in some than DT58.61. Because in DT171.1 preprocessing range cases causing surfaces of differing roughness to have sim- resolution (10.4-13.2 m) is less than that required for re- ilar intensity (hence low contrast) or to reverse relative processing (to 25 m for "all images), several independent brightness (particularly at much lower incidence angles) samples ("looks") go into each 25 m pixel and speckle is (Ulaby et al., 1982; Blom, 1988). This dependence of in- greatly reduced, enhancing the interpretation of small tensity on incidence angle is important to bear in mind and subtle features (Ulaby et al., 1982). when making any qualitative comparison between surfaces. Pulse Bandwidth and Look Direction: The effect of different pulse bandwidths on geologic Visoke-Sabinyo Cone Field interpretation can also be seen by comparing DT58.61 The field of cinder cones between Visoke and Sabinyo and DT171.1 (pulse bandwidths 10 MHz and 20 MHz, volcanoes provides another example of the importance of respectively). The most apparent difference is in the de- sufficient pulse bandwidth in mapping smaller volcanic DT58.61 DTi71.1 SPOT

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Figure 8. Comparison of the ability to discriminate lava flows to the north of Nyamuragira volcano due to changes in incidence angle and pulse bandwidth. Note the decrease in contrast between lava flows and surrounding vegetation in DT171.1, particularly towards left side of image (near range). Also note that the vents which are clearly seen in DT171.1 (arrowed) are difficult to identify in DT58.61 and indistinguishable in the SPOT image, which is not as sensitive to topographic relief. Slt/-C data are L-HH, SPOT image is Band 1 (0.50-0.59 ¢tm). SPOT image was obtained on 24 June 1995.

Go C.,3 332 MacKayand Mouginis-Mark

features (Fig. 9). In DT58.61 the cinder cones appear larly for low angles of incidence). Increased pulse band- almost randomly distributed. In DT171.1, where higher width, together with a high angle of incidenee, improves pulse bandwidth allows us to identify cones as small as range resolution resulting in a better ability to see small 100 m diameter, we see that cones are preferentially ori- vents and cones and the detailed outline of lava flows ented along a line from the easternmost cone (west of [which is helpihl in discriminating a'a and pahoehoe Sabinyo) towards Mikeno, with a seeond, more scattered, (e.g., Bruno et al., 1992)]. All of this is to little avail un- trend deviating from this line towards Visoke. The linear less we have adequate swath width to image the area of nature of the main cone field is confirmed by inspection interest. In the case of Virunga Chain, we find the in- of the SPOT data, and most likely implies that there has creased resolution of 20 MHz data (compared to 10 been an active fissure system or rift zone between the MHz) to be well worth the resulting decrease in swath main volcanoes. In addition to the greater pulse bandwidth; however, the loss of multipolarization capability in width of DT171.1, look direction and incidence angle 40 MHz data is probably not worth the tradeoff. In this may also contribute to the recognition of the linear cone geologic setting, we [like Caddis (199'2)] prefer the higher field. The cinder cones directly between Visoke and Sa- incidence angles, both for reduced distortion (which af- binyo lie in a relatively flat saddle, so that those cones feets even small features) and the increased resolution, on the main linear trend lie on a left-dipping slope. This but recognize that Blom (1988) found the opposite would suggest that there would most likely be larger choice to be preferable. Having a variety of incidence (and hence brighter due to near specular returns) down- angles ean be a powerful tool in some circumstances, but slope sides to the eones on the east-looking DT171.1 in the case of these SIR-C data acquisitions, obtaining compared to the up-slope sides of cones on the west- a wide range of incidence angles over a wide area was looking DT58.61. not possible.

Look Direction and Pulse Bandwidth: IMPLICATIONS FOR FUTURE MISSIONS Karisimbi Lava Flows With the late-1995 launch of RADARSAT, and the Eu- Several clearly defined lavas flows can be seen on the ropean ENVISAT ASAR radar mission in 1998, there is east side of Karisimbi volcano (Fig. 10). Although the a growing need to evaluate the tradeoffs associated with thickness of each flow is not known, these flows are best different wavelengths, polarizations, incidence angles, imaged where the look direction is in the same sense as and spatial resolution. The examples above demonstrate the slope (down slope to east, east-looking radar), as seen that while some parameter combinations are more useful by comparing DT171.1 with DT58.60. The increase in than others for the identification of certain volcanic fea- pulse bandwidth from DT58.60 and DT171.1 (20 MHz) tures, no single data take satisfies all of the requirements to DT154.9 (40 MHz) produces very little improvement while at the same time providing an image swath width in resolution or imaging of the flows because of the low that is adequate to cover the entire volcanic chain. incidence angle of DT154.9 (Fig. 11; Table 2). Although Choices must thereibre be made based on the science DT154.9 eolleeted only W-polarized data (vs. HH for objectives associated with reconnaissance mapping or the the other two data takes), changes in baekseatter inten- analysis of specific processes such as lava flow emplace- sity in the area of the flows appear to primarily reflect ment or the distribution of ash deposits. topographic relief; therefore, the differences due to po- This need to identify optimum parameters for data larization should be much smaller than the other factors collection becomes even more important when viewed in being compared. the context of the recent developments in radar interferometry, where the topography and surface deformation Interrelationships and Tradeoffs of volcanoes can be studied using multiple ERS-1 radar Each acquisition attribute varies in importance depend- images (Massonnett et al., 1995). In order to obtain data ing on the nature of the target and the scientific objec- suitable for interferometrie measurements, the orbit of tive. For example, the angle of incidence controls our the spacecraft must be precisely controlled and all of the ability to distinguish between lava flows and tropical veg- operating parameters described above held constant. For etation; for this type of study it is particularly important instance, the characterization of the volcano using multi- to avoid the angles at which intensity curves converge incidence angle radar data is precluded if radar interfer- and radar backscatter contrast is very low. However, we ometry is the prime objective of the mission. The SRL- note that multiple wavelengths and polarizations may aid 2 (Space Radar Lab-2; October, 1994 SIR-C/X-SAR mis- in discriminating the different flow units (Blom, 1988; sion) interferometry experiment provides a limited, but Gaddis, 1992). Look direction significantly influences the nevertheless intriguing, data set from which to draw con- mapping of volcanic features, particularly when studying clusions and make reeommendations for future missions. volcanoes such as Nyiragongo where great vertical relief The interferometry technique uses two phase-coher- causes the spatial distortion of vents and cones (partieu- ent radar images obtained with a very small baseline sep- DT58.61 DT171.1 SPOT

,3"¢a

,.,, ...... ,-,,, ,. 10 km ~N ¢3 ¢Z Figure 9. Comparison of ability to map smaller volcanic f>atures caused by changes in resolution (a function of pulse bandwidth and incidence angle). Note the greater number of identifiable cinder cones in DT171.1. Also note the difficulty in mapping cones in the SPOT image, despite its higher resolution (20 m/pixel vs. 25 m/pixel). SII/-C data arc L-HH, SPOT image is Band 1 (0.50-0.59/tin). SPOT image was obtained on 24 June 1995.

¢-0 Go 334 MacKay and Mouginis-Mark

DT58.60

DT171.1

DT154.9

5 km

I i ,i ...... n i ii II i Ii l 10km Figure 10. Identification of thick lava flows on the eastern flank of Karisimbi volcano is affected by changes in look direction and pulse bandwidth. DT58.60 is left-looking; DT171.1 and DT154.9 are right-looking. DT58.60 and DT171.1 are 20 MHz pulse bandwidth; DT154.9 is 40 MHz, but shows very little improvement in resolution due to the low incidence angles (Table 2). Arrow points to the same point on the flow margin for all three data takes. aration (which is the distance between the sensor on two acquired at a 40 MHz pulse bandwidth and single VV successive orbits) to produce a topographic map. In the polarization; DT154.9 is one of such a pair. VV polariza- case of SIR-C, these data were acquired by flying the tion was chosen as the sole polarization because VV usu- Shuttle in a repeat-orbit configuration for the last four ally has higher signal-to-noise (S/N) ratio than HH or days of the SRL-2 mission, with the spacecraft ground- cross-polarized data. When the SIR-C interferometry ex- track repeated every 24 h. The repeat-pass data were all periment was planned, it was expected that repeat-orbits Effect of Varying Acquisitiot~ Parameters on SIR-C Data Interpretation ~

50 CONCLUSIONS 1. Opposite look-direction data are of great value when studying areas with large topographic relief, 40 such as steep volcanoes where distortion of sec- fX\XX\ X ondary features may affect their interpretation. A v X high resolution DEM is needed to validate in- X% e- X X stances where this distortion is present if only a •£ 30 single radar data take is obtained. The geometric o distortion of the southern flank of Nyiragongo is n" an example of this phenomenon. 20 -, ""--30 MHz 2. The use of an incidence angle of~35 ° (DT171.1) leads to an inability to recognize lava flows when n" they are surrounded by tropical vegetation, due ""'" ..... 20 MHz to the crossover in their backscatter curves at this 10 -""-._40 MHz ...... incidence angle. When mapping similar volcanic terrains, incidence angles greater than~40 ° would be preferable. 0 I I I 3. Increasing the swath width, or adding a second 20 30 40 50 60 polarization or wavelength at the expense of go- Incidence Angle (deg) ing from 20 MHz to l0 MHz, produces an unac- ceptable degradation in signal-to-noise. Many Figure 11. Range resolution vs. incidence angle for dif- smaller volcanic features, in particular cinder ferent pulse bandwidths. Range resolution, &=c/ cones and vent systems, could only be identified (2B sin 0), where B is pulse bandwidth, 0 is angle of using the 20 MHz bandwidth. incidence, and c is the speed of light. 4. For the interferometric experiment, increasing the pulse bandwidth from 20 MHz to 40 MHz produced a regrettable decrease in the swath could only be achieved to within 1 km, so that every ef- width for incidence angles greater than about 35 °, fort was made to achieve phase-correlated images. This If a comparable interferometric mission were to included requiring 40 MHz pulse bandwidth because the be flown again using the repeat-orbit interferome- higher spatial resolution of 40 MHz data (rather than 20 try technique employed during SRL-2, 20 MHz MHz data) would be more tolerant of long baselines, data would be preferable. This conclusion is predi- which could cause decorrelation of lower resolution data. cated on the expected performance of the Shut- During the actual mission, repeat-passes were sometimes tle, wherein repeat orbits to within a tbw hun- achieved with baselines of less than 100 m (H. Zebker, dred meters were flown. personal communication, 1995) so that a smaller pulse 5. Multiwavelength, dual-polarization data provide bandwidth could have been used. This would have al- use{hl inibrmation on the distribution of lava lowed tbr either wider swaths or additional polarizations. flows from Nyamuragira volcano. Without the two Because the Shuttle orbit was fixed for these 4 clays different wavelengths (C- and L-band; Fig. 3) it in order to provide repeat coverage, the look direction would be difficult to map the spatial distribution and angle of incidence for a given target were also fixed. of these flows because lava textures va~ across For DT154.9 the chosen orbit resulted in very low angles the scene, thereby precluding the use of a single of incidence (20-30°), which combined with the restric- wavelength/polarization combination. tions on data rate resulting from the 40 MHz pulse bandwidth, produced a narrow swath width (46,4 kin). At This work was supported by JPL Contract No. 959457. This low incidence angles, interferograms are more difficult to is Hawaii Institute of Geophysics and Planetology Paper No. produce aud areas of high relief will have significant data 927 and SOEST Contribution No. 4180. We thank Harold gaps when converted to ground range projection and ter- Garbeil fi~r computer assistance, Scott Rowland fi~r assistance rain corrected. Low incidence angles also produce a spa- with Figure 4, and Cynthia Wilburr~ f~r producing the DEM tial resolution that is not much better than that obtained in Figure 8. We also thank Jeff Plaut and two anony~rums re- viewers fi~r their helpfid comments. at higher incidence angles and lower pulse bandwidth (e.g., Fig. 10 shows little difference between DT171.1 and DT154.9). For areas of higher incidence angle, 20 REFERENCES MHz data offbr the choice of greater swath width or additional polarization while retaining sufficient resolution Blom, R. G. (1988), Effects of variation ill look angle and wave- for intcrferometry. length in radar images of volcanic and aeolian terrains or 336 MacKay and Mouginis-Mark

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