Combined seismic reflection and refraction profiling in southwest Germany - detailed velocity mapping by the refraction survey

Transcription

1 Geophys. J. R. astr. SOC. (1987), 89, Combined seismic reflection and refraction profiling in southwest Germany - detailed velocity mapping by the refraction survey 1. Introduction D. Gajewski, W.S. Holbrook and C. Prodehl Geophysical Institute, University of Karlsruhe, Herfzstr. 16, Karlsruhe 21, FR. Germany Summary. Within the framework of site survey studies of the Deep Drilling Program of the Federal Republic of Germany (KTB), coincident deep-seismic reflection and refraction experiments in the Black Forest, southwest Germany, were carried out. The simultaneous interpretation of the reflection and the refraction data reveals in particular both a strong velocity reduction in the upper crust and a laterally varying laminated structure of the lower crust. Additional refraction lines result in a three-dimensional crustal model which shows two distinct crustal types of different seismic properties. These crustal types seem to correlate with the major geologic units of Southwest Germany. Variations of Poisson's ratio derived from clearly recorded shear wave data show a similar trend. In the framework of multi-disciplinary investigations of the Urach geothermal anomaly (Haenel 1982) and a proposed deep continental dnllhole (Alfred-Wegener-Stiftung 1985) seismic reflection and refraction surveys were carried out in Southwest Germany in 1978 and 1979 in the Urach area (Bartelsen et al. 1982; Gajewski & F'rodehl 1985; Walther et al. 1986) and in 1984 in the Black Forest and Hohenzollemgraben and adjacent areas (Fuchs et al. 1986; Gajewski et al. 1986; Gajewski & Prodehl 1986, Liischen et al. 1985; KTB Research Group Black Forest, this issue; Wenzel et al., this issue). This contribution will deal with the seismic-refraction survey of 1984 (Fig. 1). 2. The upper crust of the Black Forest The data, of which an example is shown in Fig. 2, will be discussed in some detail for the Black Forest line S, but the same basic features hold also for the other two lines U and Z (Fig. 1). The traveltime curves shown in the record sections (Fig. 2) are re-calculated from the corresponding velocity-depth cross section (Fig. 3a). The whole data set is represented and discussed in more detail by Gajewski & Prodehl(l986). In order to enable immediate comparison with the seismic-reflection data of KTB Research Group Black Forest (this issue), a simplified line drawing after Liischen et af. (1985) is shown in Fig. 3c onto which the main features of the seismic-refraction model (Fig. 3a) have been superimposed. The first-arrival phase a, refracted in the basement with a velocity of 5.9 km/s is followed by a reflection (phase a,) from the upper boundary of a low-velocity zone (dotted area in Fig. 3a and c). That zone is very accentuated under the northern and central Black Forest and is characterized by a velocity decrease from 6.2 to 5.4 km/s between 8 and 14 km depth. Toward the south, the velocity inversion gradually loses its intensity.

2 334 D. Gajewski, W.S. Holbrook and C. Prodehl Plutonihs Alpinr Rocks fiuotwnarj and Twtiofy Shot points Figure 1. Simplified geology and location map of recent seismic profiles in Soutwest Germany. Cities: A Augsbury, Ba Basel, Fr Freiburg, Ka Karlsruhe, Ra Rastatt, S Stuttgart, St Strasbourg, U1 Ulm. Under the central part of the Black Forest near the town of Haslach, within the lowvelocity zone a local reflector (phase a, in the record sections of shots S2 and S3, see e.g. Fig. 2) with a horizontal extension of some ten kilometres can be traced. Its position coincides with the location of a so-called bright spot in the reflection data as is indicated in the line drawing of Fig. 3c. Citin d W 0 E cm t0 60 eta VRED- 6.00KWSEC VERT I CAL DISTANCE IN KM Figure 2. Record section of the seismic refraction profile S observed from S3. North direction is to the left and south direction to the right of S1. The plotted traveltime curves are calculated from the model of Fig. 3. Major geologic units are indicated on the top. The position of the other shotpoints are marked by arrows.

3 3. The lower crust Seismic reflection and refraction profiling in Southwest Germany 335 The lower crust starts at the lower boundary of the low-velocity zone and shows a very complicated response in the record sections (phase b in Fig. 2). The combined interpretation of the near-vertical (Fig. 3c) and the wide-angle (Fig. 2) reflection data results in a refined model for the lower crust assuming that it exists of a stack of thin layers with alternating high (2 7.2 km/s) and low (5 6.2 kds) velocities. Sandmeier and Wenzel(l986) have computed synthetic seismograms for such a laminated lower crust assuming an average thickness of the lamellae of 120 k 30 m and a Poisson's ratio of 0.25 k 0.05 (see Fig. 1 of Sandmeier el al., this issue), using the reflectivity method (Fuchs & Miiller 1971). As the reflectivity method cannot take into account horizontal lateral variations, a more crudely-laminated model using lamellar thicknesses of about 800 m has been used to calculate synthetic seismograms for a laterally-varying structure (Fig. 3b) using the ray method (Cervenf et al. 1977). The southward-decreasing reflectivity of the lower crust indicated by the reflection data was taken into account by gradually decreasing the velocity contrast between high- and low-velocity lamellae starting at S3 until a zero-contrast is reached south of S5. The resulting synthetic record sections (Gajewski & Prodehl 1986) qualitatively mirror the observed data (Fig. 2) surprisingly well. The comparison of the seismic-reflection data (Fig. 3c) and the seismic-refraction model (Fig. 3a, b) shows remarkable agreements. The low-velocity zone corresponds with the area of low reflectivity in the upper crust, the Conrad discontinuity is identical with the top of the highly reflective lower crust, the decreasing reflectivity of the lower crust towards the south (Fig. 3c) agrees with the decreasing velocity contrast of the laminated lower crust (Fig. 3b) as is indicated by the laterally varying characteristics of the observed seismic-refraction "phase" b. The reflection from the Moho, the crust-mantle boundary, is the most prominent phase at epicentral distances greater than 60 km. The Moho is a first-order discontinuity with a velocity jump from 6.7 to kmls. Beneath the entire Black Forest it is almost flat at a depth of km. Only south of the Rhine does it descend towards the Alps, reaching 30 km depth south of the Swiss Jura near Olten in the Swiss Molasse Basin. Its position corresponds to a clear reflection band in the seismic-reflection data (Fig. 3c, KTB Research Group Black Forest, this issue). 4. Crustal structure of Southwest Germany As mentioned above, the other two lines U and Z (Fig. 1) show the same basic features as the Black Forest line S, but they in particular demonstrate that crustal structure varies considerably throughout southwestern Germany (Fig. 4). The lateral variations of crustal structure evidently follow a distinct pattern characterizing two specific "crustal types" (Gajewski et al. 1986): Type I contains a mid-crustal low-velocity zone ( kmts) over a thick (> 10 km) high-velocity lower crust ( kds). Type 11, in contrast, has no prominent mid-crustal low-velocity zone and a thin (< 10 km), low-velocity lower crust ( kds). Although the two crustal types have similar average crustal velocities ( km/s) and traveltime curves, they are distinguishable by their amplitude characteristics: in Type I the top of the lower crust (the so-called Conrad-discontinuity) is found at shallow depths of km, has a high reflectivity and produces correspondingly high "phase"-b amplitudes, indicating a strongly laminated lower crust. In Type I1 the converse is true: the Conrad-discontinuity is found at depths of km and the lamination of the lower crust may be less pronounced. Furthermore the "crustal types" seem to correlate roughly with the major geologic provinces of southern Germany. Type I (with an upper-crust low-velocity

4 336 D. Gajewski, W.S. Holbrook and C. Prodehl a 1. N BLACK FOREST SWISS JURA s Figure 3 (a) Crustal model of the Black Forest and Swiss h a, derived from refraction seismic data. (b) Velocity-depth sections along line S. (c) Line drawing of the reflection section along the Black Forest (from Lirschen eta/., 1985) zone) occurs in the Black Forest and most of the Swabian Jura, while Type I1 dominates in areas which are covered by Triassic sediments. It is also noteworthy mentioning that the Moho is an almost flat boundary at km depth beneath the entire area of investigation including the area of the geothermal anomaly of Urach (Haenel 1982). The results of the interpretation of the new data of 1984 agree very well with the structure and conclusions derived in earlier investigations (Bartelsen et ul. 1982, Gajewski & Prodehl 1985, Walther et ul. 1986). 5. Shear Wave observations Besides the P-wave observations (Fig. 2), extremely high-quality S-waves were recorded. Shear-wave models for lines U and Z (Fig. 1) indicate clear deviations of Poisson's ratio

5 Seismic profiling in Southwest Germany 337 Figure 4 Fence diagram showing crustal structure of Southwest Germany. Hatched areas: velocity inversions, M Moho. Depths in km. from 0.25 for the mid-crust ( ) and the lower crust. The average Poisson's ratio of the lower crust correlates roughly with the crustal types mentioned above: Type I (Black Forest and Urach area) has a high ratio ( ), while Type I1 has lower values ( ) (Holbrook et al. 1986). References Alfred-Wegener-Stiftung, ed., nd international symposium on observation of the continental crust through drilling. 4th Alfred-Wegener-Conference, Oct. 4-6, 1985, Seeheim. Abstracts. Bartelsen, H., Luschen, E., Krey, Th., Meissner, R., Schmoll, H. & Walter, Ch., The combined seismic reflection-refraction investigation of the Urach geothermal anomaly, pp , in The Uruch Geothermal Projecf, ed. Haenel, R., Schweizerbart, Stuttgart. hen9, V., Molotkov, I.A. & PSenEik, I., Ray methods in seismology, Univerzita Karlova, Prague. Fuchs, K., Bonjer, K.-P., Gajewski, D., Liischen, E., Prodehl, C., Sandmeier, K.-J., Wenzel, F. & Wilhelm, H., Crustal evolution of the Rhinegraben area, I. Exploring the lower crust in the Rhinegraben rift by unified geophysical experiments, (submitted to) Tectonophys. Fuchs, K. & Milller, G., Computation of synthetic seismograms with the reflectivity method and comparison with observations. Geophys. J. R. astr. SOC., 23, Gajewski, D., Holbrook, W.S. & Prodehl, C., Three-dimensional crustal structure of Southwest Germany, derived from seismic-refraction data (submitted to) Tectonophys. Gajewski, D. & Prodehl, C., Crustal structure beneath the Swabian Jura, SW-Germany, from seismic refraction investigations. J. Geophys., 56,6940. Gajewski, D. & Prodehl, C., Crustal evolution of the Rhinegraben area. 11. Seismic-refraction investigation of the Black Forest, (submitted to) Tectonophys. Haenel, R. (ed.), The Urach geothermal project (Swabian AIb, Germany). Schweizerbart, Stuttgart. Holbrook, W.S., Gajewski, D. & Prodehl, C., Shear-wave velocity and Poisson's ratio structure of the upper lithosphere in Southwest Germany. J. geophys. Res. Letf. (in prep.) Uschen, E., Menges, D., Riihl, Th., Sandmeier, K.-J., Gowin, J., Janoth, W., Keller, F., Stiller, A., Sollner, R. & Trappe, H., Presite seismic reflection survey: Abstracts of the 2nd internutiom[ symposium on observations of the continenfuf crust throughdriffing, Bonn, ed. Wegener Stiftung, A. Sandmeier, K.-J. & Wenzel, F., Synthetic seismograms for a complex model. J. geophys. Res. Letters, 13, Walther, Ch., Trappe, H. & Meissner, R., The detailed velocity structure of the Urach Geothermal anomaly, J. Geophys., 55,