The Heidelberg Basin drilling project : Geophysical pre-site surveys

Currently, the Heidelberg Basin is under investigation by new cored research boreholes to enhance the understanding concerning the control on Pliocene and Quaternary sedimentation by (neo)tectonics and climate. The Heidelberg Basin is expected to serve as a key location for an improved correlation of parameters that characterise the climate evolution in North Europe and the Alpine region. The recovery of sediment successions of high temporal resolution that are complete with respect to the deposition of Pleistocene glacials and interglacials in superposition is of special importance. Prior to the new research boreholes in Viernheim and Heidelberg geophysical pre-site surveys were performed to identify borehole locations that best achieve these requirements. In the area of the Heidelberg Basin the strongest negative gravity anomaly of the entire Upper Rhine Graben is observed (apart from the Alps), hinting at anomalously thick sediment deposits. However, especially refl ection seismic profi les contributed signifi cantly to the decision about the borehole locations. In the city of Heidelberg for the fi rst time, the depocentre of the Heidelberg Basin, as indicated by additional subsidence compared to its surroundings, was mapped. In this area, sediments dip towards the eastern margin of the Upper Rhine Graben. This is interpreted to represent a rollover structure related to the maximum subsidence of the Upper Rhine Graben in this region. At the Viernheim borehole location the seismic survey revealed several faults. Although these faults are mainly restricted to depths greater than 225 m, the borehole location was fi nally adjusted with respect to this information. [ Das Bohrprojekt Heidelberger Becken: Geophysikalische Voruntersuchungen] Kurzfassung: Das Heidelberger Becken wird aktuell durch neue Kernbohrungen untersucht, um das Wissen hinsichtlich der Steuerung der pliozänen und quartären Sedimentation durch Klima und (Neo)Tektonik zu erweitern. Es wird erwartet, dass das Heidelberger Becken eine Schlüsselstelle für eine verbesserte Korrelation von Parametern darstellt, welche die Klimaentwicklung in Nordeuropa und im alpinen Raum charakterisieren. Besondere Bedeutung hat daher die Gewinnung von Sedimentsukzessionen hoher zeitlicher Aufl ösung, die im Hinblick auf die Ablagerung kaltund warmzeitlicher pleistozäner Sedimente in Superposition möglichst vollständig sind. Im Vorfeld der neuen Kernbohrungen bei Viernheim und Heidelberg wurden geophysikalische Vorerkundungen durchgeführt, um Bohrlokationen zu identifi zieren, die diesen Ansprüchen am besten genügen. Im Bereich Heidelberg wird die größte negative Schwereanomalie des gesamten Oberrheingrabens beobachtet (mit Ausnahme der Alpen), was auf ungewöhnlich mächtige Sedimentablagerungen hindeutet. Aber insbesondere refl exionsseismische Messungen haben zur Auswahl der Bohrpunkte beigetragen. Im Stadtbereich von Heidelberg ist zum ersten Mal das Depozentrum des Heidelberger Beckens kartiert worden, abgebildet durch eine zusätzliche Absenkung gegenüber der Umgebung. In diesem Gebiet fallen die Sedimente zum östlichen Grabenrand hin ein. Dies wird als ‘Rollover’ Struktur interpretiert, die in Verbindung mit der maximalen Subsidenz des Oberrheingrabens in diesem Bereich steht. An der Bohrlokation Viernheim konnten durch die SeisThe Heidelberg Basin drilling project: Geophysical pre-site surveys HERMANN BUNESS, GERALD GABRIEL & DIETRICH ELLWANGER* *Adresses of authors: H. Buness, Leibniz Institute for Applied Geophysics, Stilleweg 2, D-30655 Hannover, Germany. E-Mail: hermann.buness@liag-hannover.de; G. Gabriel, Leibniz Institute for Applied Geophysics, Stilleweg 2, 30655 Hannover, Germany. E-Mail: gerald.gabriel@liag-hannover.de; D. Ellwanger, Landesamt für Geologie, Rohstoffe und Bergbau im Regierungspräsidium Freiburg, Alberstraße 5, 79104 Freiburg im Breisgau, Germany. E-Mail: dietrich.ellwanger@rpf.bwl.de Eiszeitalter und Gegenwart Quaternary Science Journal 57/3–4 338–366 Hannover 2008


Introduction
The Heidelberg Basin is located in the eastern part of the Northern Upper Rhine Graben (URG; Fig. 1), bordered by the dominating master fault of the URG to the east.The boundary fault is assumed to extend deep into the crust, by 15 -24 km according to MAUTHE, The Heidelberg Basin drilling project: Geophysical pre-site surveys BRINK & BURRI (1993).The sedimentary fill of the URG is characterised by synthetic and antithetic faults which strike parallel or subparallel to this boundary fault.The area of the Heidelberg Basin was subjected to continuous and strong subsidence since late Oligocene (SCHUMACHER 2002).Up to 1500 m sediments were deposited in early Miocene alone (comprising Cerethia, Corbicula, and Hydrobia beds).The thickest succession of Quaternary sediments can be found here (up to 350 m according to BARTZ 1974;Fig. 2, top).The Heidelberg Basin constitutes therefore the most complete sediment archive of the whole URG.Sediments of different geosystems interfere with each other: the local system of the Neckar River (Odenwald), the regional Upper Rhine Graben-Highlands (Black Forest, Vosges) system, and the supra-regional Alps-Upper Rhine Graben system.Especially the distal deposits of the system Alps -Upper Rhine Graben are supposed to contain information about both the alpine and north European climate history, which can not be observed elsewhere (e.g.ELLWANGER et al. 2005).The term Heidelberg Basin stands for the Miocene to Quaternary depocentre of the northern URG with an extent of some tens of km.The term 'Heidelberger Loch', sometimes also found in literature, was introduced by SALOMON (1927) and denotes the locally, very delimited centre of subsidence around the city of Heidelberg.The exploration of this sediment archive is the aim of two new research boreholes sponsored by the Leibniz Institute of Applied Geophysics (LIAG -former Leibniz Institute for Applied Geosciences, GGA-Institut) and the geological surveys of Baden-Württemberg (LGRB) and Hessen (HLUG) (Fig. 2, bottom).One of the boreholes was drilled exactly in the depocentre of the basin, close to the outlet of the Neckar River, from the Odenwald to the plain of the Upper Rhine Graben ('Heidelberg UniNord ', ELLWANGER et al. 2008).The other one was drilled in the geographic centre of the Heidelberg Basin about 17 km to the northwest, near the city of Viernheim (HOSELMANN 2008).These two locations are complemented by two boreholes in Ludwigshafen that were drilled on the 'Parkinsel' island on the western margin of the basin (Fig. 2, bottom; WEIDENFELLER & KNIPPING 2008).Prior to drilling these boreholes, a number of seismic profi les were carried out to facilitate a profound drilling design (Fig. 2; bottom).This included an estimation of depths for the geological targets, i.e.Pliocene and Pleistocene strata as well as the detection of possible fault zones.The latter point was important because the boreholes were to serve as a stratigraphic reference for the Quaternary sediment succession.Furthermore, high resolution seismic profi les in general are able to reveal tectonic and sedimentological events caused by the continued subsidence of the basin.This article presents the results of the pre-site surveys.However, at the present stage, as there is not even a consistent and homogenous interpretation of all available borehole data, it is not possible to make conclusions about the basin dynamics (e.g. a seismostratigraphic interpretation).A more exhaustive analysis of the data could be done in the framework of a project, which was proposed to the German Research Foundation and is currently under evaluation.

Geological setting and nomenclature
Knowledge about the deeper subsurface of the URG is based mainly on seismic profi ling and evaluation of boreholes of the oil and gas industry, which was quite intensively done in this area.According to MAUTHE, BRINK & BURRI (1993), about 5000 km seismic lines were recorded between 1970 and 1992.An extensive presentation of these activities is not possible due to the lack of corresponding publications.However, isobath maps derived from these data were published, e.g. for the Tertiary by DOEBL & OLBRECHT (1974) or for the Quaternary by BARTZ (1974).Two deep-reaching refl ection seismic profi les running across the southern and northern URG were carried out in 1988 in the framework of the DEKORP-ECORS Project (BRUN & GUTSCHER 1992).The northern profi le crosses the URG HERMANN BUNESS, GERALD GABRIEL & DIETRICH ELLWANGER The Heidelberg Basin drilling project: Geophysical pre-site surveys about 20 km north of Heidelberg.The varying refl ector signature of the lower crust images the asymmetric structure of the URG.The largest Tertiary sediment thickness of 3400 m is observed close to the eastern rim of the graben, it reduces stepwise to 300 m below the western border.However, depths corresponding to the Quaternary, the Pliocene, and down to the middle Miocene units are poorly resolved.The interaction between tectonic and sedimentation in the northern URG was investigated by DERER by means of sequence stratigraphy (DERER 2003, DERER et al. 2003, DERER, SCHU-MACHER & SCHÄFER 2005), who found the area to be divided into two halfgrabens with opposing tilt directions by a transfer zone.The study was based on seismic profi les and geophysical borehole measurements made by the oil and gas industry.Seismic facies were assigned to several lithostratigraphic units from Eocene to the upper Miocene.About 190 km of industrial seismic lines where shot between 1981 und 1993 in the Heidelberg Basin.However, these lines do not extend to the depocentre of the Heidelberg Basin ('Heidelberger Loch').Furthermore, only little information about the structure of Plio-Pleistocene sediments can be derived from the hydrocarbon seismic lines, because these focused only on storage or sealing horizons, and hence deep structures.The existing gap in the refl ection seismic data, near to the city of Heidelberg, can be partly fi lled by information from boreholes related to hydrogeological investigations.In the Heidelberg area two fl ush boreholes are available that reveal large thickness of Quaternary sediments: the 350 m deep Entensee borehole from 1973 (CONRADS & SCHNEIDER 1977), about 1 km north, and the 1022 m deep Radium-Sol Therme borehole from 1918 (SALOMON 1927), about 1 km south of the new borehole Heidelberg UniNord 1 (Fig. 3).Interpretation of these two boreholes is controversially discussed.The main problem here are the confusing defi nitions and applications of the terms Quaternary and Pliocene (and some other stratigraphic terms) in publications, reports and archives related to the URG.The most frequently applied 'traditional' defi nition relates 'Quaternary' to the onset of alpine sediments, i.e. to a change in sediment provenance.'Pliocene' sediments come from local sediment sources, e.g.Black Forest, Vosges, and Odenwald; 'Quaternary' sediments include or correlate with sediments of alpine origin.This change of provenance is, of course, a matter of lithostratigraphy, although chronostratigraphic terms are used.This different use of the term 'Quaternary' is illustrated by the controversial interpretations of the Radium-Sol Therme borehole by SALOMON (1927), BARTZ (1951) andFEZER (1997).SALOMON and BARTZ suggest depths of '382 m' and 'almost 400 m', both referring to the lithostratigraphic version of 'Quaternary'; FEZER suggests a depth of 650 m which is based upon a calculation of sedimentation rates i.e. he applies the term Quaternary in its proper chronostratigraphic sense.The aim of this paper is not to solve problems of stratigraphic nomenclature but report seismic and gravimetric activities prior to the new drilling activities.Our references are the old data (publications, reports, and archive data) which are, at this state, not re-interpreted but used as they are.Stratigraphic terms are set in quotation marks where we feel the authors use the lithostratigraphic version (e.g.'Quaternary'), and without quotation marks when used as chronostratigraphic term (e.g.Quaternary).A refl ection seismic profi le that focused especially on the Quaternary deposits was published by HAIMBERGER, HOPPE & SCHÄFER (2005).This river seismic profi le -recorded along the Rhine River between Mainz and Mannheim, and parts of the Neckar River over a length of 150 km in total -revealed high-resolution information which was able to defi ne the base of 'Pleistocene' sediments mainly north of Mannheim.Maximum thickness of 'Pleistocene' was found in the area of Mannheim and confi rmed the map published by BARTZ (1974;Fig. 2).This map images an increase of 'Pleistocene' sediment thickness towards the eastern boundary fault of the Upper Rhine Graben, where it amounts to more than 350 m.The Heidelberg Basin drilling project: Geophysical pre-site surveys

HERMANN BUNESS, GERALD GABRIEL & DIETRICH ELLWANGER
The interpretation of the river seismic data revealed well the alternating sequence of coarsegrained layers (aquifers, so called 'Kieslager') and fi ne-grained layers (aquitards, so called 'Zwischenhorizonte') typical of the Heidelberg Basin.This hydrostratigraphic classifi cation (Table 1) is often used in the northern part of the Upper Rhine Graben due to a lack of geochronological data concerning Pliocene and Pleistocene strata.It is based on a macroscopic description of the sediments, their colours and carbonate content, and as complementary information on gamma logs, where available.The hydrostratigraphy was broadly introduced by BARTZ (1982).It is presently much used in hydrogeology (HGK 1999) and was last updated by WEIDENFELLER & KÄRCHER (2008).On a larger scale it distinguishes between three coarse-grained layers separated by two fi ne-grained layers, which are regionally distributed.In the system after BARTZ (1982) this alternating succession is separated from the underlying provenance change by another unit called 'Altquartär' ('Early Quaternary').This term again is used as description of a sediment type rather than a stringent chronostratigraphic classifi cation.In more recent publications and reports, the lowest coarse-grained bed is defi ned as Early Quaternary (WEIDENFELLER & KÄRCHER 2008).The thickness of each layer is quite variable in a lateral direction.Locally, additional fi ne-or coarse-grained layers of smaller extent can be intercalated.The occurrence of fi ne-grained sediments was traditionally related to interglacial periods, and the occurrence of coarse-grained layers to glacial periods (BARTZ 1982).One of the aims of this drilling project is to correlate the before mentioned hydrostratigra- phy with the lithostratigraphy of the southern part of the Upper Rhine Graben (SYMBOL-SCHLÜSSEL GEOLOGIE BADEN-WÜRTTEMBERG).
The upper gravel layer can be quite well correlated with the Mannheim Formation, and the upper interlayer (fi ne-grained layer) with the Ladenburg Horizon.The succession of the middle sand-gravel layer and lower sand-silt layer and the interlayer in between is only roughly equivalent to the Weinheim Beds (Table 1).

Gravimetry
Generally, gravity anomaly data is expected to refl ect a fi rst-order pattern of the shape of the Heidelberg Basin, especially varying thickness of (unconsolidated) Pliocene/Pleistocene sediments.The mean density of Plio-Pleistocene sediments should be reduced compared to Tertiary or even older and more compacted sediments.Therefore the anomalous thick sedimentary fi ll of the Heidelberg Basin should cause a The Heidelberg Basin drilling project: Geophysical pre-site surveys negative gravity anomaly that can be related to the the extent of the basin and its depth.But in fact the situation is more complicated.

Gravity data
Gravity data for the Upper Rhine Graben is available from several sources.ROTSTEIN et al. (2006) have compiled the most recent Bouguer gravity map of the Upper Rhine Graben comprising all the available data from France and Germany (Fig. 4; regional map).This map is based on about 33.000 Bouguer gravity values.About one third of the data was provided by the Leibniz Institute for Applied Geophysics covering the German part of the Rhine Graben.This dataset consists of data available from the Geophysikalische Reichsaufnahme, but also some local surveys.The French data is mainly based on two data sources, which are themselves compilations of numerous surveys.The fi rst dataset -about fi fty percent of the French data -was provided by the Bureau de Recherches Géologiques et Minières, the second one by Mines de Potasse d'Alsace.
A complete Bouguer anomaly was recalculated for the entire new dataset, using a density of 2670 kg/m 3 and considering newly-calculated terrain reductions.The distribution of the gravity stations is strongly inhomogeneous; their spatial coverage varies signifi cantly from about 0,25 stations/km 2 in some parts of the Vosges and Black Forest and 40 stations/km 2 in some parts of the graben itself.Note, the complete Bouguer map is based on a calculated grid of 1 km.The area of the Heidelberg Basin (Fig. 4; local map) is characterised by the strongest gravity anomaly of the entire Upper Rhine Graben (apart from the most southern part where the regional trend is strongly affected by the Alps).
It is in the order of -40 to -50 mGal, with the absolute minimum close to the outlet of the Neckar River, from the Odenwald in the plain of the Upper Rhine Graben (Fig. 4).

Preliminary Interpretation
When interpreting the anomaly apparently related to the sediment fi ll of the Heidelberg Basin some general knowledge regarding the source of gravity anomalies in the Upper Rhine Graben has to be considered.ROTSTEIN et al. (2006) performed some two-dimensional calculations along profi les approximately perpendicular to the strike of the graben.Although these profi les were restricted to the southern part of the graben and a two-dimensional approach is not suitable to investigate the complex graben geology in great detail, their general result showed, that the gravity anomalies are not only caused by the sediment fi ll of the graben, but that they are also strongly affected by density inhomogeneities in the crystalline basement.One of the profi les south of Karlsruhe revealed increasing sediment thicknesses from west to east accompanied by increasing Bouguer gravity values -the structure of the sediments was well constrained by refl ection seismic data and drilling information.Therefore, for this profi le the gravity effect of the sediments must be overcompensated by density contrasts in the basement.Density of the basement must be assumed to be of high lateral variation as that of rocks of the adjacent graben shoulders is.Therefore, some impact on the Bouguer anomalies observed in the area of the Heidelberg Basin must also be assumed.A preliminary 2-D profi le which crosses the Rhine Graben close to Heidelberg is shown in Fig. 5.Although the structural resolution of the sediments is rather low, the necessity to introduce at least lateral density contrasts at the western and eastern boundary faults and within the most western part of the basement is obvious.Nevertheless, the sediments of the Heidelberg Basin will undoubtedly contribute to the negative gravity anomalies observed in this region.Confi rming the map of 'Quaternary' thickness in the Upper Rhine Graben published by BARTZ (1974), the new refl ection seismic profi les recorded in the framework of the pre-site surveys reveal anomalously thick Neogene sediments.
From the downhole logging experiments conducted in the Heidelberg Drilling Project, low densities between 2000 kg/m 3 and 2300 kg/m 3 can be estimated for the Pleistocene sediments (HUNZE & WONIK 2008), whereas Tertiary sedi- The Heidelberg Basin drilling project: Geophysical pre-site surveys HERMANN BUNESS, GERALD GABRIEL & DIETRICH ELLWANGER ments in the Upper Rhine Graben are known to have densities between 2350-2450 kg/m 3 , strongly depending on the amount of evaporates in the Early Oligocene strata, at least in the southern part of the graben (ROTSTEIN et al. 2006).
Interpreting the observed gravity anomalies only in terms of varying sediment thickness, two main centres of deposition can be distinguished: one between Heidelberg and the Rhine River, and the second northeast of Landau (Fig. 4).Considering the information about Base 'Quaternary' (Fig. 2), the anomaly close to Landau is not related to anomalously thick Quaternary deposits.The source of this anomaly must be assumed to be at greater depth.(1943).Based on torsion balance data, models were derived that explain the observed horizontal gradient (in Eotvös, 1 E = 10 -9 s -2 = 0.1 mGal/km) running from the Odenwald in the east about 5 km into the Upper Rhine Graben in the west (Fig. 6).Therefore, only the local situation at the eastern graben boundary fault was investigated.The thickness of Pleistocene and youngest Plicocene sediments is estimated to be ~ 500 m at the Heidelberg Uni-Nord location.Furthermore in CLOSS's model the uppermost sediment unit dips with about 45° towards west -strongly contradicting the result of the new refl ection seismic surveys.Density contrasts in the crystalline basement of the URG in this region were not taken into account, although from the adjacent Odenwald a large variety of alkaline (high density) and acidic (lower density) rocks are known.

Seismic survey and data processing
Seismic measurements in urban areas often encounter considerable diffi culties.Restrictions exist for seismic sources (only low-energy seismic sources, services lines, endangerment of supply lines, services pipes, traffi c restrictions, etc.) as well as for the recording side (sealing of the surface, enhanced noise level, etc.).The design of seismic profi les is therefore more often dictated by logistics than by geological reasoning.
At the start of the project a borehole location in Heidelberg was proposed at the abandoned freight depot south of the Neckar River (Fig. 7).A 1.5 km long profi le (profi le 1) was therefore measured, that showed strong inclinations of deep refl ectors.To check the true inclination of the refl ectors, the data were supplemented by a second profi le later on (profi le 4).The profi les could not be connected to existing industry profi les (the next one being about 1.5 km apart to the SW), but the interpretation could be done by comparison of refl ection patterns.The borehole Radium Sol Therme could barely be The Heidelberg Basin drilling project: Geophysical pre-site surveys After some encouraging results, a longer north -south trending profi le (profi le 3) was registered.However, continuation to the exploration Abb.8: Seismischer Hydraulikvibrator, der für hoch aufl ösende oberfl ächennahe Messungen entwickelt wurde.
Profi The Heidelberg Basin drilling project: Geophysical pre-site surveys borehole 'Schriesheim', about 1 km further north (Fig. 2), could not be established.After fi xing the location of the research borehole 'Heidelberg UniNord 1', another short profi le (profi le 5) was shot to check for possible fault zones.In this area signal quality was degraded by waves originated from service pipes (called 'pipe waves' for simplicity further on).These are especially visible on profi le 2 and profi le 5.
The two profi les at the Viernheim location (profi les 6 and 7 in Fig. 2) were laid out perpendicular to each other, with the proposed position of the research drilling at their crossing point.The profi les were surveyed in a forested area without further problems.All profi les were shot by a small hydraulic vibrator (Fig. 8), which yields a maximum peak force of 30 kN and a frequency range of 20 -500 Hz (for recording parameters cp.Table 2).This vibrator was developed especially for high-resolution shallow profi ling (BUNESS & WIEDERHOLD 1999, VAN DER VEEN et al. 2000).It is very appropriate for use in an urban environment, due to its relatively low impact on the surface, compared to e.g.weight dropping or other impulsive sources.Data were processed using a commercial processing system (ProMAX, Landmark Corp.).Processing steps are listed in Table 3, details regarding the single steps are discussed e.g. in YILMAZ (2001).In addition, to the processing steps listed in Table 3, waves in profi les 2 and 5 were suppressed by trace-mixing algorithms.Trace mixing, although a very simple algorithm, proved to be most effective of a variety of other algorithms, including spatial 2D fi lters, f-k based fi lters and eigenvector fi ltering.However, the incoherency of this kind of noise prevents elimination without damaging the refl ection signal.The velocity fi elds for fi nite-difference migration and for the subsequent depth conversion were derived from the smoothed stacking velocity fi elds.The deviations from the velocities derived by the VSP (vertical seismic profi le) measurements carried out in the UniNord 1 borehole down to a depth of 180 m turned out to be very small, hence the former were kept during later processing.The structure of the uppermost low velocity layer down to a depth of 20 m could not be determined adequately, causing some uncertainty about the overall depth level of the profi les.As a consequence the depth level was calibrated using the VSP refl ections, which have a known depth.A reference level of 100 m a.s.l. was chosen for all profi les.FX deconvolution (60 -180 Hz) 13.
depth conversion

Heidelberg -the region north of the Neckar River
The profi le which is best tied to an existing deep borehole is the north-south trending profi le 3 in the area north of the Neckar River: the Schriesheim borehole is located about 1 km further north (Fig. 2).Although no direct connection between the newer seismic line and the borehole exists, the geological structures can be controlled by hydrocarbon seismic lines.
Both, E-W and N-S trending profi les that cross the Schriesheim borehole are available.

HERMANN BUNESS, GERALD GABRIEL & DIETRICH ELLWANGER
Heidelberg Basin investigated with the new refl ection lines no distinct information is provided by the velocity fi elds.An estimation of the velocity fi eld of profi le 3 (Fig. 10) reveals interval velocities that increase gradually from 1600 m/s near the surface to 3200 m/s at a depth of 1500 m.These velocities are deduced from stacking velocities, which do not constitute physical seismic velocities, since they are affected by layer dips, side refl ections, diffractions and other seismic particularities.The reliability of stacking velocities decreases strongly with depth, depending on the maximum offset of the seismic survey.The velocities cannot be controlled by other methods, e.g.VSP measurement, due to the lack of deep VSP (vertical seismic profi le) or sonic measurements.However, they coincide The Heidelberg Basin drilling project: Geophysical pre-site surveys quite well with the regional velocity trend derived from deep boreholes in the Heidelberg Basin.A remarkable feature is a zone of low velocity, which has a depth of approximately 180 -250 m at the northern edge of the profi le and of 300 -450 m at its southern edge.The upper limit of this zone corresponds therefore well with the lower boundary of the coarsegrained Weinheim beds (cf.Fig. 3).Profi le 2 is located between the Neckar River and the eastern boundary fault of the Upper Rhine Graben (see Fig. 7).The profi le approaches at its eastern edge the topographic border of the URG and hence the master boundary fault of the URG.

HERMANN BUNESS, GERALD GABRIEL & DIETRICH ELLWANGER
Both the unmigrated time section (Fig. 11) and the migrated depth section (Fig. 12) are presented, so the reader can better judge the infl uence of the migration processing step.Generally, the refl ections dip to the east, with dips becoming greater with increasing depth.A wedge-shaped zone at the easternmost position (marked by the dotted line in Fig. 11) shows no coherent signal.This wedge is probably due to the eastern master fault which juxtaposes the sedimentary infi ll of the URG against the crystalline basement of the Odenwald.A very rough estimation of its dip yields values of approximately 80°.Adjacent to this wedge, fl at or slightly westward dipping refl ectors are displayed at travel times greater 700 ms.This feature is interpreted as a drag fold.Below 700 ms, refl ections again dip towards east.Some hints for diffractions can be seen, that could be caused by the fault zone (e.g.CMP 260 at 800 ms).Areas of apparently low refl ectivity as observed in the youngest Quaternary, e.g. between CMP 110 and CMP 310, are caused by strong noise due to pipe waves.After migration (Fig. 12), the bending of the sediment strata next to the assumed boundary fault becomes more obvious.However, it is diffi cult to separate real refl ections from migration artefacts that always occur at the ends of a seismic profi le.Signals inside the marked triangle should not be considered as real refl ectors.The continuous subsidence of sediments beneath the central and western part of the profi le is revealed by an increasing dip angle with depth.The apparent dip of Base 'Quaternary' is 1.5° towards east and 4.5° for Base Pliocene.A fault with a distinct displacement is again only visible in the upper part of the Hydrobia beds.
Profi le 5 was recorded with modifi ed acquisition parameters (smaller CMP spacing, smaller offset, cp.Table 2), which yielded a higher resolution (Fig. 13).Again, in the eastern part the image of Quaternary deposits is signifi cantly affected by pipe waves.At depths between 600 and 700 m, reduced refl ectivity is observed at the western end of the profi le.The dip angles of Base 'Quaternary' and Base Pliocene, 2° and 6° respectively, are slightly increased with respect to those of profi le 2.
In the 'Heidelberg UniNord 1' borehole in 2006 (ELLWANGER et al. 2008), a vertical seismic profi le (VSP) was recorded (Fig. 14), as well as numerous other geophysical logging methods (HUNZE & WONIK 2008).To ensure comparability with the refl ection seismic profi les, the same vibrator and the same fi eld parameters  were used ( The VSP corridor stack represents the locally-refl ected wavefi eld and corresponds with the surface refl ection profi ling results.Small discrepancies, as seen in Fig. 14, can be explained by different fi eld geometries and recorded frequencies.The corridor stack was used to calibrate the total static of the refl ection seismic lines, since the depths of its refl ections are determined directly.

Heidelberg -the region south of the Neckar River
The new refl ection seismic profi les 1 and 4 (Figs. 15,16) are located south of the Neckar River (see Fig. 7).Therefore a tie to existing deep wells is hardly possible.The observed refl ection patterns cannot be easily correlated with those of the hydrocarbon seismic lines, as assumed prior to the surveys.Therefore, the information from the Schriesheim well was correlated along several seismic lines and transferred to the nearest seismic profi le recorded by the hydrocarbon industry, which is about 1,5 km south of profi le 4. But the interpretation of the youngest sediments, e.g.Quaternary, remains especially uncertain, because these were not imaged well in the industrial seismic lines.Both profi les 1 and 4 intersect each other at an angle of about 50°.Again, on both profi les the apparent dip of the sediment fi ll increases with depth.For Base 'Quaternary' it amounts to 3°, for Base Pliocene it amounts to 9° (profi le 1) and 11° (profi le 4).Therefore, real dip is about 4° for the Base 'Quaternary' and 11° for the Base Pliocene, in each case towards east.This is about twice as much as these beds north of the Neckar River have.Similar to profi le 3, Quaternary deposits show quasi-continuous refl ections (e.g.profi le 1 in 300 m depth), alternating with weak and partly sub-parallel refl ections.Within the Hydrobia beds again a strong refl ector occurs that is also visible on the other seismic sections.

Viernheim
In contrast to the seismic profi les recorded in the Heidelberg area the seismic lines recorded in the vicinity of the Viernheim borehole (Figs.17, 18) allow a distinct classifi cation of the sediment fi ll.Tops of zones of high refl ectivity are visible at both depths of 220 m and 570 m, whereas especially the depth interval between 450 m and 570 m shows low refl ectivity.
Considering the results of the research borehole Viernheim (HOSELMANN 2008) the refl ector at a depth of 228 m can be assigned to the transition between the 'Quaternary' and the underlying material of local provenance (reference height of the seismic line is 100 m above sea level, the drilling site is 97 m above sea level).The limnic-fl uviatile deposits of local provenance cause the observed high refl ectivity.Due to the high quality of the Viernheim data a complex fault pattern can be identifi ed.Especially on the E-W oriented profi le 6 (Fig. 17), a fault zone is observed that runs through all prominent refl ection horizons and penetrates deep into the Rhenish facies, e.g.into 'Pleistocene' sediments.Originally a drilling location close to the intersection of profi les 6 and 7 was favoured.Based on these seismic results it was shifted by about 500 m towards south where the sediment succession was expected to be less affected by faults.Due to the acquisition parameters and the processing of the seismic data, refl ectors above HERMANN BUNESS, GERALD GABRIEL & DIETRICH ELLWANGER  A precise and detailed stratigraphic interpretation of the refl ectors below 570 m depth was not possible, because these were not reached by the research borehole.Instead we intend to achieve this by using industrial refl ection seismic data, which is much denser than in the Heidelberg area.

Discussion and Conclusion
For the Heidelberg Basin thick sediment sequences are apparent, both in gravity and seismic data.Quantitative interpretation remain preliminary, as long as no deep boreholes can be included and 3-D models based on gravimetric and seismic data can be calculated, that additionally consider data from the hydrocarbon industry.
The The Heidelberg Basin drilling project: Geophysical pre-site surveys characteristics.Therefore a consistent seismic stratigraphy for the Plio-/Pleistocene sediments of the URG is not achievable within the limits of the presently applied inconsistent stratigraphic scenarios.
Concerning the location of the Viernheim boreholes, two new refl ection seismic profi les image a sedimentary environment that is affected by tectonics, as documented by several faults.But these faults are mainly restricted to older sediments, e.g. to depths greater than about 225 m (Pliocene and older according to HOSELMANN 2008).In contrast, during the Quaternary the tectonic activity seems to be quiet.
Along the entire profi le these sediments are horizontal.But on a smaller scale much more detailed information is imaged by the two seismic lines indicating a complex depositional system, especially during Pliocene or even older times.
Considering the results of the Viernheim research borehole, the top and the base of a fi negrained horizon, comprising the Ladenburg Horizon (Oberer Zwischenhorizont) at about 40 and 80 m, respectively, depths can be traced in the seismic section.Refl ections at a depth of about 180 m can be correlated with a sequence of regional distributed fi ne-grained sediments (Unterer Zwischenhorizont).
The top of the uppermost section of high refl ectivity can be correlated with the transition from the Rhenish facies (alpine) sediments to material with local provenance at a depth of 225 m, as revealed by heavy mineral analysis, carbonate content, and petrography (HOSELMANN 2008).Whether this seismic refl ector can be interpreted as the transition Plio-/Pleistocene must be investigated by additional palynological studies that are not yet available.
The most important result derived from the seismic pre-site survey at the Heidelberg UniNord location is the existence of a subbasin in the Heidelberg Basin close to the eastern margin of the Upper Rhine Graben.
The additional subsidence adds up to some hundred meters with respect to deeper strata, as imaged on profi le 3, e.g. about 400 m for the top of the Hydrobia beds.All recorded profi les do not show any disconformities, faults are restricted to early Miocene (Hydrobia beds or older) units.However, this observation does not exclude potential hiatuses, which cannot be imaged by refl ection seismic data.By tying the new refl ection lines to the Schriesheim well, the transition Pliocene to Pleistocene was predicted at depths between 400 and 500 m.This statement was based on the assumption that the extrapolation of information of the industry seismic lines across the gap between the northern end of the seismic profi le 3 and the location of the Schriesheim borehole is correct.Furthermore using the interpretation of Base 'Quaternary' from a hydrocarbon well is uncertain because information is not available on the kind of data it is based on.Work done in the framework of hydrocarbon exploration was focused on deep structures rather than on young sediments.The seismic lines reveal more or less continuous subsidence.The dip of sediments towards east increases with depth, showing a classical rollover anticline structure at the eastern URG master fault.The amount of subsidence increases from north to south: the Base Pliocene dips east by about 4.5 % on profi le 2, 6 % on profi le 5 and 11 % on profi le 1 and 4.These values make a major fault zone beneath the Neckar River plausible, of which indications can be found at the southern end of profi le 3. Differential compaction may have played a role by increasing the dip of the Miocene and younger strata.The rollover structure is not found to the north, as well as to the south of the 'Heidelberger Loch', as inferred from industry seismic profi les, which affi rms it as the area with the most rapid subsidence.To aim on a deeper understanding of the basin genesis, a crucial point in the chronostratigraphic interpretation of seismic refl ectors will be the proper defi nition of 'Base Quaternary'.A major step is also expected once the new drillings will be interpreted and correlated using the tools of sequence stratigraphy (ELLWANGER et al. 2008).We are convinced that this will also include a consistent seismic scenario.

Fig. 5
Fig.5: 2-D gravity profi le across the Rhine Graben in the Heidelberg area.Although the regional gravity fi eld correlates well with sediment thickness, additional density contrasts in the basement are required to explain the observed anomalies in the western part.

Fig. 11 :
Fig. 11: Stacked seismic refl ection profi le 2 in time domain, unmigrated.Areas with very low refl ectivity in the upper 300 ms in the central part of the profi le are due to strong noise and do not refl ect geology.A wedge-shaped zone at the eastern edge of the profi le, indicated by a dotted line, maybe caused by the transition of the sedimentary infi ll of the URG to the crystalline basement of the Odenwald.Abb.11: Gestapeltes refl exionsseismisches Profi l 2 im Zeitbereich, unmigriert.Die stellenweise sehr geringe Refl ektivität in den oberen 300 ms in zentralen Teil des Profi ls wird nicht durch die Geologie, sondern durch starke Störungen hervorgerufen.Eine keilförmige, durch eine punktierte Linie markierte Zone am Ostrand des Profi ls kann durch den Übergang von der Sedimentfüllung der ORG zum kristallinen Grundgebirge des Odenwaldes verursacht werden.

Fig. 13 :
Fig. 13: Migrated seismic refl ection profi le 5.No seismic information is available inside the transparent triangle for geometric reasons.

Fig. 16 :
Fig. 16: Migrated seismic refl ection profi le 4. No seismic information is available inside the transparent triangle for geometric reasons.The strongly west-dipping refl ectors close to the triangle zone are probably artefacts caused by migration.

Table 2 :
Recording parameters used in the refl ection seismic surveys.

Table 3 :
Typical data processing fl ow

Table 2
Due to the thick sequences of Plio-/Pleistocene sediments, the research boreholes Heidelberg UniNord reveals a unique high temporal resolution.From the fi rst Heidelberg borehole in 2006 a time marker of the Waalian stage is reported at 183 m -210 m depth (HAHNE, ELLWANGER & STRITZKE 2008).As the age of the Waalian is considered to amount to 1.5 Ma, and assuming continuous sedimentation, the Top of the Pliocene at 2.6 Ma has to be expected at only 365 m depth.However, the preliminary interpretation of the new refl ection seismic HERMANN BUNESS, GERALD GABRIEL & DIETRICH ELLWANGER profi les reveals Base 'Quaternary' fi rst at about 430 m depth.Consequently, concerning the early Quaternary, increased sedimentation rates can be expected.