Airborne SAR and Landsat MSS as complementary information source for geological hazard mapping

28

ISPRS Journal of Photogrammetry and Remote Sensing

B.N. Koopmans i and G. Forero R. 2

Airborne SAR and Landsat MSS as complementary information source for geological hazard mapping A comparative study has been made of the usefulness of Landsat and airborne radar images. The study area is situated in the Middle Magdalena Valley of Colombia. It consists of a folded sedimentary sequence of Upper Cretaceous to Lower Tertiary rocks, partially covered by extensive volcanic lahar and alluvial fan material. To obtain the full benefit of the spectral information from Landsat and the textural and pattern information from radar, a combined image was produced using the hue and saturation information from Landsat data and the intensity values from radar data. A clear differentiation between old lahar deposits and the recent one caused by the Nevado de| Ruiz eruption of 1985 was possible on SAR images. The synergistic radar imagery, particularly used in stereo, is very useful for prediction of future lahar routes and volcanic risk evaluation.

I. Introduction The Magdalena Valley, with an elevation of about 400 m above sea level (a.s.1.) in the study area, separates the Cordillera Oriental (4500 m a.s.1.) from the Cordillera Central (5400 a.s.1.) of the Andes Mountains in Colombia. In the central Cordillera, a number of snowtopped active to dormant volcanos form the highest peaks. Although the area is situated in the tropics (4.5°N), the permanent snow line is located at around 5000 m. The distance from Nevado del Ruiz (5432 m a.s.1.) to the foothills at about 600 m altitude along the western side of the Magdalena Valley is about 47 km as the crow flies. The Magdalena Valley has an average width of 25 km in the study area, with the present river bed situated asymmetrically towards the eastern part of the valley. In the valley, some older remnants of folded Cretaceous and Early Tertiary rock outcrops are evident, particularly in the area between Girardot and Ibague. Near-horizontal fluvial sediments of the Honda and Mesa Formations of Miocene and Pliocene age represent the early fluvial valley fills of the ~lnternational Institute for Aerospace Survey and Earth Sciences (ITC), P.O. Box 6,7500 AA Enschede, The Netherlands. 2 lnstituto de lnvestigaciones en Geosciencias, Minera y Quimica, Apt. Aereo 4865, Bogot~i D.E., Colombia.

proto-Magdalena Valley. These deposits are locally gently folded and tilted due to faulting. However, for the most part, they form horizontal mesas, especially in the northern part of the study area. The Pleistocene and Holocene deposits are represented by extensive fan deposits, river alluvium and terraces. The influence of neotectonic movements are also evident in these younger deposits. In this paper, particular attention will be paid to these Quaternary deposits.

2. Remote sensing and image processing In this study, use has been made of Landsat MSS and Landsat TM data and of airborne synthetic aperture radar data (STAR). In localised parts, black and white aerial photographs at a scale of 1 : 25,000 were used. The Intera radar data were fully focussed, high resolution data of X-band, H H polarization, with a range and azimuth resolution of 6 m and "7 looks processing" (the result of a process of segmenting and averaging pixel values from seven uncorrelated images of the same scene). Stereo radar coverage was available for the entire area. For the northern part, separate use has been made of the stereo radar and the Landsat TM data for the geomorphological/geological interpretation. For the southern part, a combined image was produced, based on a radar/Landsat MSS corn-

1SPRS Journal of Photogrammetry and Remote Sensing, 48(6): 28-37 0924-2716/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

Volume 48, number 6, 1993

bination (Landsat TM or SPOT images were not available to the authors), to extract and combine the optimal information content of both images for our interpretation. The large difference in spatial resolution of the Landsat MSS (80 x 80 m) and the airborne synthetic aperture radar (6 x 6 m) makes extensive resampling necessary. Multiband data in the visible and infrared will give particularly information in the spectral domain. Colour differences expressed at the surface as a result of variation in rock colours, soil variation or vegetational differences may be captured in a natural colour image. Extending the sensitivity range into the infrared domain allows additional information to be obtained, particularly with respect to health and type of the natural vegetation. Side-looking radar images obtained in the microwave domain are particularly sensitive to surface roughness. Moreover the shadow effect be-

29

hind morphological features is much stronger for radar imagery than for images obtained in the optical domain. Radar shadow areas are completely black. The SAR is an active remote sensing instrument using its own emitted energy. An object or mountain obstructing the radar beam will form a radar shadow where no microwave energy will reach the ground. These dark shadows strongly enhance the relief impression used by the interpreter. However, no information is obtained in the shadow area. Small gullies, hatches, single trees or bushes are well expressed on the radar as strongly contrasting light to dark tonal differences, caused by strong radar returns from near perpendicular incidence and low to nil return from grazing angles or shadow zones (Fig. 1). For synergistic use of both data sets, a combination should be made of the optimal information content of the Landsat, which is repre-

Figure 1. Radar/Landsat synergism. From right to left: radar STAR image, Landsat MSS coiour composite, Landsat (hue and saturation)/radar (intensity) combination.

30

sented by the spectral (colour) information, with the optimal information content of the radar, which is represented by the intensity information. Texture and patterns such as drainage morphology, sedimentary strike ridges, fault lineaments and other morphologically expressed features are generally well visible and interpretable on radar images (Koopmans, 1983). Using hue and saturation from Landsat and intensity from radar will give an optimal image combination for geological/ geomorphological application. The steps in image processing are briefly outlined below. (1) A haze correction and correction for radiometric errors was performed on the Landsat data. (2) Geometric correction of Landsat and digital SAR. Both data sets were resampled to 10 × 10 m pixels. For Landsat this means a change from 80 to 10 m, whereas for the SAR data this was from 6 to 10 m. Sufficient ground control points in both data sets were present for a geometric correction and a very good image fit was obtained. (3) Extraction of relief/sun angle effect from Landsat data. The effect of relief and sun angle is strongly related to the total intensity of the spectral bands. Normalisation of the original spectral bands removes this effect. (4) Saturation enhancement of Landsat data. The normalised data are now present in a twodimensional red, green, blue, colour plane, with an M1 axis from the centre of the red/green/blue triangle to the maximum red and an M2 axis orthogonal to it also going through the centre (Fig. 2). We used ml and rn2 as descriptors of the axes. Saturation enhancement can then be defined as a scaling transformation on m l and m2. Maximum separation on hues is obtained without losing the initial colour input. (5) IHS SAR-Landsat synergism. The huesaturation information from Landsat is now combined with the intensity information from the radar data. This is accomplished by the multiplication of the saturated results with the Dn values of the radar. To get a better result, the radar values are first transformed to the original intensity range of the Landsat data, so that radar shadow areas will not become zero, but will include information from Landsat. Further shift or stretch of the radar intensity range for the synergistic image will affect the sharpness of the shadows and consequently the

ISPRS Journal of Photogrammetry and Remote Sensing

,,,,,,,~

~.. Zm 2

]G

m 1. m 2

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ml. m2 transformed

Figure 2. Transformation from colour cube to two-dimensional (M1, M2) plane, followed by a scaling transformation (saturation) and a return to the RGB cube.

morphological expression of relief. On the other hand, radar shadow areas may give more information derived from Landsat (Fig. 1). The advantage of a strong relief impression obtained by dark shadows from radar against slope information in the shadow area from Landsat is a trade off and the right balance should be found by interactive processing.

3. Information extracted from the synergistic radar/Landsat image The strike ridges of the Cretaceous La Tabla Formation and Olini Group and the Tertiary Gualanday Formation can easily be mapped on the synergistic image (Fig. 3). The dip slopes can be determined and flatirons and V shapes are clearly indicative for the bedding plane dips. Fold noses are seen and axial plunges may be determined. In the lower right hand part of the radar/Landsat combination (Fig. 1), some anomalous blue colours can be seen. These represent a cloud-covered area on the Landsat; the morphological terrain expression here is clearly seen as intensity information derived from the radar. The radar shadow areas are rather dark in order to keep sufficient relief impression and small surface morphology. However, some colour inforr~mtion is present, coming from Landsat in these radar shadow areas. This enables the interpreter to map the slope break in the shadow areas between the rock slopes and the flat fan deposits. In the upper part of the image the Ibague fan deposits

Volume 48, number 6, 1993

31 75* I

5°-

4~,30-

pAy~I~cE S ~LLU. HICORAL"~ " ~

Figure 3. Geological interpretation of the synergistic Radar/ Landsat image over the Gualanday area (after G. Forero R.). A = Ibague fan; B = Espinal fan; C = Gualanday Formation, upper member; D = Gualanday Formation, middle member; E = Gualanday Formation, lower member; F = Seea Formation (argillaceous); G = La Tabla Formation (arenaceous/ argillaceous); H = Grupo Olini (multicoloured shales); I = Loma Gordo Formation (black shales); J = Hondita Formation (shale-list). are blocked by the Tertiary strike ridges. The radial to sub-parallel drainage pattern in these deposits is clearly visible.

4. Fan deposits in the Middle Magdalena Valley In the study area, a number of fan deposits are visible. These are, from north to south, the Rio Lagunilla mudflow deposits, the Recio fan, the Totare fan and the Ibague/Espinal fan (Figs. 1 and 5). In the imagery, the fan deposits show clearly different morphological characteristics, which are related to the presence of volcanic activity in the upper drainage areas. The Rio Lagunilla has its source on the northeastern upper slopes of the active Ruiz volcano (Fig. 4). The recent mudflows strongly dominate the spectral characteristics in the different images (Fig. 5). The fan has a limited extent because of the presence of the flat-topped hills of the H o n d a Formation in the Magdalena Valley.

G,IRARDOT I

i 75 °

Figure 4. Drainage map of the eastern slopes of the Central Cordillera volcanos Ruiz and 'Iblima towards the Magdalena Valley. The Rio Recio, feeding the fan of the same name, has its source on the southeastern slopes of the Nevado del Ruiz. Two clear fan levels are present: a higher one, directly north of Venadilla, and a lower level further north on which La Sierra and Lerida are situated (Fig. 6). Fan material is coming close to the Magdalena River from both levels. The Totare fan is of limited extent, and not much incised by superficial drainage. The Totare River has its source on the southern slopes of the Nevado Sta. Isabel. In contrast to the Recio fan which has been build up of large masses of volcanic material derived from the active Ruiz volcano, the Totare River has been transporting relatively little volcanic material and the Sta. Isabel volcano has been contributing little to the fan formation. The Ibague fan, on the other hand, has been build up by large masses of volcanic material in the form of mudflows and lahar deposits derived from the slopes of the Tolima volcano. The Rio Combeima is the principal feeder channel of this fan. Towards the east the fan material is blocked by

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ISPRS Journal of Photogrammetty and Remote Sensing

Figure 5. Synthetic aperture radar image (STAR) of the Totare, Recio and Lagunillas fan deposits,

the strike ridges of the folded Tertiary Gualanday Formation, but to the northeast the deposits extent down to the Magdalena River and the Totare fan. Rivers are deeply incised in the fan materials. The Landsat images do show a typical culti-

vation pattern on the gently sloping fan deposits, but elevation differences are difficult to establish. However, the radar images give indications of different levels because of the drainage incisions. For interpretation of these levels and for correla-

Volume 48, number 6, 1993

33

•

Armero Lahar deposits 1985

~

Faults Drainage channels

_ _

Recio Fan deposits lower level Recio Fan deposits uppe[ level

Figure 6. Quaternary fan deposits interpreted from stereo radar imagery in the Armero-Venadilla area (Fig. 5).

tion over larger areas, the use of stereo radar is considerably increasing the confidence level of interpretation (Koopmans, 1974). The synoptic view of radar, together with the stereo information and detailed morphological and textural information of the drainage characteristics on the different terrace levels, facilitates mapping. 5. The Recio fan

The Recio fan can be subdivided into two levels on the basis of radar interpretation. The apex of the upper level fan material occurs at an elevation of 625 m near la Sierrita at the point where the Recio River flows out from the mountains into the wide Magdalena Valley. The river bed is deeply incised to an elevation of just over 400 m. The altitude difference of 225 m is partly due to neotec-

tonic movements which have affected the foothills and tilted the fan material. The Central Cordillera have been uplifted with respect to the Magdalena Valley along the N-S oriented Perico fault. The fault has also an important dextral component. The edge of the Recio fan has been affected by this faulting and the upper part of the fan has been tilted 8 degrees (Fig. 7). Another neotectonic active fault is the Megu6 fault. It runs in a WNW direction, slightly affects the upper fan surface, and has caused the well aligned valley depression for a short stretch of the Recio Valley and its tributary the Quabrada Megu6. From La Sierrita, the fan slopes gently under an angle of 2.7° in the upper part to 1.2° for the lower part towards the Magdalena River (Fig. 8). The upper fan surface is smooth and little gully pattern has developed on it. The Recio River is

34

ISPRS Journal of Photogrammetry and Remote Sensing

Figure 7. The Reciofan, upper level(foreground)with the tiltedpart (background)upliftedalongthe Perico and Megu6faults. deeply incised and shows backward erosion in the near-vertical fan cliffs. The fan material is over 150 m thick in most places. Along the main road from Venadillo to La Sierra, a good section can be seen. At the base, lahar and mudflow deposits occur. 75% of the boulders are of volcanic andesitic composition, but also schists, gneisses and quartzite boulders occur and a minor amount of igneous origin. The basal mudflow unit is followed by a sequence of fluviatile coarse sands, poorly sorted, containing quartz, hornblende, biotite and plagioclase grains among others. Cross-lamination is well developed and channelling may also be seen. The presence of a paleosol, of a calcrete horizon, some horizons with some organic material, and concretions of calcareous enriched sandy material are indicative of a stable geomorphic condition during deposition, contrasting strongly with the torrential mudflow deposits below and towards the top. The upper part of the sequence, 80 m thick, is made up of two thick boulder beds very similar to the torrential mudflow/lahar deposits of the base, separated by a 5 m thick sandy fluviatile horizon, forming the upper level fan. This upper level of the Recio fan represents a material outflow towards the east; the lower level represents outflow towards the north and northeast. The apex of the lower level fan is at 500 m near la Sierra. The surface has a much gentler gradient than the upper level fan and dips less than 1° towards the northeast.

The fan materials here consist principally of ftuviatile deposits and contrast strongly with the volcanic dominated upper level deposits. Near the bridge over the Recio River, fluviatile terrace conglomerates can be seen overlying the irregular topography of the Ibague batholith, with a saprolite of 1 to 2 m thick at the base. The age of these fan deposits is difficult to establish. Jungerius (1976) tried to correlate different horizons in the Magdalena Valley with dated deposits in the Sabana de Bogot~i. Periods of masswasting and landscape instability produced the glacis formation in the Magdalena Valley in the Early to Middle Pleistocene. He dated the Ibague fan deposits on the basis of the probable age of the overlying interglacial soil as at least of Penultima glacial age. H.H. Acosta and J.R. Ramirez (in Vergara, 1989) gave a Late Pleistocene age to the Ibague deposits. The Ibague fan materials and Recio fan (upper level) are clearly influenced by strong volcanic activity, in contrast to the Totare fan and the lower level of the Recio fan which are principally formed by fluviatile waste materials of the Cordillera Central. 6. Neotectonic movements

Along the Ibague fault, which runs in a ENE direction, small fault scarps, several meters high are observed, and sag ponds and pressure ridges

35

Volume 48, number 6, 1993 Fan Apex

1300 Buenos Aires

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are reported (Vergara, 1989). The scarp is visible by shadowing on the radar image in the upper and middle part of the Ibague fan. Towards Piedras near the Rio Opia, pressure

ridges are visible along the fault line on the radar image. On the fault plane, locally slickensides were observed of a very irregular structure. The slickensides were strongly curved from horizontal to

36 nearly vertical, indicating that apart from horizontal dextral movement, rotation and upwards movement also occurred. The Holocene movements along the Perico fault have already been mentioned. Uplift of the Central Cordillera has caused tilting of the upper and lower levels of the Recio fan, but also uplift of the Totare fan where fan conglomerates have been displaced upwards several tens of meters along the fault line in a narrow strip. Further north, the depression of the Quebrada de Aqua Fria marks the fault line. Another neotectonic line is present along the escarpment of the Honda Formation north of the Rio Totare near its confluence with the Magdalena River. This lineament, aligned with the Megu6 fault, is not expressed either on the radar image or in the field in the Totare fan deposits. However, it seems to branch off in a northern direction, where it can be followed on the radar image, affecting the Recio fan deposits. The upper level of the Recio fan has been uplifted and tilted, and dips locally 20 ° towards the east. Local uplift has been several tens of meters. Small SWNE and N W - S E oriented transcurrent faults are present, displacing the uplifted ridge. Horizontal slickensides on these vertical fault planes in the fan material indicate a dextral horizontal displacement in a S W - N E direction. This uplifted ridge has been previously mapped as an anticlinal structure in the Honda Formation (Barrero and Vesga, 1976). Field evidence already indicated that the uplifted rock belongs to the layered fluviatile boulder beds of the Recio fan sequence. At the base of the fault, white-toned lahar beds with volcanic ash material and some large strongly weathered granite boulders were found. 7. Environmental risk factors

The two principal risk factors in the area are: (1) volcanic mudflow deposits; and (2) seismic hazards related to neovolcanics The 1985 Armero volcanic mudflow deposits clearly demonstrated the large risk of this zone (Table 1). On the radar image, the recent mudflow clearly stands out due to the specular reflection of the relative smooth and non-vegetated surface. A high risk area may be outlined. But also the Recio fan demonstrates that in the past this area has been strongly affected by torrential lahars.

ISPRS Journal of Photogrammetry and Remote Sensing

TABLE 1 Volcano Nevado del Ruiz eruption t985 Plinian eruption, 13-11-1985 Over 25,000 persons killed by volcaniclahars Volcanic production:

3.5 x t0 j° kg daeitic to andesitic pyroclasticsand ash Pyroclasticsurge flows up to 2 km from the crater Approximately 10% of the ice cap has been melted three major lahar flows: Marequita-Armero-Chinchinfi Annero laharflow:

volume 2-4 x 1 0 7 m 3 area coverage42 km~ thickness 0.5-1 m max. distance to crater 60-70 km min. velocity38 km/h between cratcr and Armero A change in the volcanic configuration of the Nevado del Ruiz eruptions, lately directed in a N E direction, may affect the southeastern slopes in the future. If so, most lahar deposits will be channelled through the Recio Valley instead of the Lagunillas River and consequently wilt affect the Recio fan area. Also the Ibague fan shows the effect of extensive volcanic activity in the past of the Nevada del Tolima. Through the Rio Combeima, draining the southern slopes of the Tolima, many torrential volcanic mudflows must have passed between 14,000 and 3600 BP (Murcia and Vergara, 1986), building up the extensive Ibague fan. Ibague town with 300,000 inhabitants is situated at the apex of the fan at an extremely critical position. The Armero disaster, with at least 25,000 persons killed, was the effect of 0nly a small Plinian eruption (Naranjo Henao et al., 1986). A seismic hazard is also present here. The Ibague fault has shown Holocene activity and runs through the eastern part of the town. Also the N - S faults in the Magdalena Valley show Holocene displacements. The historical record (Ramirez, 1969), on the other hand, does not show many seismic epicentres in the area. Acknowledgements

The digital high resolution SAR data were kindly provided by Intera Information Technologies of Canada. The image processing was carried out at the image processing laboratory of ITC by Mr. J.P.G. Bakx. Local transport for field checking was facilitated under the ITC-IGAC project.

Volume 48, number 6, 1993

References Barrero L., D. and Vesga O., C.J., 1976. Mapa Geologico del Cuadrangulo K.9 Armero y Parte sur del J.9 La Dorada. Ingeominas, Bogoui. Jungerius, P.D., 1976. Quaternary landscape development of the Rio Magdalena Basin between Neiva and Bogot~i (Colombia). A reconstruction based on evidence derived from paleosols and slope deposits. Palaeogeogr., Palaeoclimatol., Palaeoecol., 19: 89-137. Koopmans, B.N., 1974. Should stereo SLAR imagery be preferred to single-strip imagery for thematic mapping? ITC J., 3: 424-445. Koopmans, B.N., 1983. Side-looking radar, a tool for geological surveys. Remote Sensing Rev., 1: 19-69.

37 Murcia L., A. and Vergara S., H., 1986. Riesgos geologicos potentiales en ia ciudad de Ibague, departamento del "Iblima. Rev. CIAF, 11 (1-3): 342. Naranjo Henao, J.L., Carey, S., Sigurdsson, H. and Fritz, W., 1986. La Erupcion del Volcan Nevado del Ruiz en Colombia el 13 de Noviembre de 1985: Caida de Tefra y Lahares. Rev. CIAF, 11 (1-3): 56-71. Ramirez, J.E., 1969. Historia de los Terremotos en Colombia. Inst. Geografico '~,gustin Codazzi". Vergara S., H., 1989. Actividad neoteconica de la Falla de Ibague - - Colombia. Memorias V Congreso Colombiano de Geologia, pp. 147-168. (Received December 15, 1992; revised and accepted July 21, 1993)