GEOPHYSICS,VOL.66,NO.1(JANUARY-FEBRUARY 2001);P .78–89,9FIGS.
Ca History
逃避心理
The u of geophysical prospecting for imaging active faults in the Roer Graben,Belgium
Donat Demanet ∗,Fran¸c ois Renardy ∗,Kris Vanneste ∗∗,Denis Jongmans ‡,Thierry Camelbeeck ∗∗,and Mustapha Meghraoui §
ABSTRACT
As part of a paleoismological investigation along the Bree fault scarp (western border of the Roer Graben),various geophysical methods [electrical profiling,elec-tromagnetic (EM)profiling,refraction ismic tests,elec-trical tomography,ground-penetrating radar (GPR),and high-resolution reflection ismic profiles]were ud to locate and image an active fault zone in a depth range between a few decimeters to a few tens of meters.The geophysical investigations,in parallel with geomorpho-logical and geological analys,helped in the decision to locate trench excavations exposing the fault surfaces.The results could then be checked with the obrvations in four trenches excavated across the scarp.Geophysical methods pointed out anomalies at all sites of the fault po-sition.The contrast of p
hysical properties (electrical re-sistivity and permittivity,ismic velocity)obrved be-tween the two fault blocks is a result of a difference in
the lithology of the juxtapod soil layers and of a change in the water table depth across the fault.Extremely fast techniques like electrical and EM profiling or ismic refraction profiles localized the fault position within an accuracy of a few meters.In a cond step,more detailed methods (electrical tomography and GPR)more pre-cily imaged the fault zone and revealed some struc-tures that were obrved in the trenches.Finally,one high-resolution reflection ismic profile imaged the dis-placement of the fault at depths as large as 120m and filled the gap between classical ismic reflection profiles and the shallow geophysical techniques.Like all geo-physical surveys,the quality of the data is strongly de-pendent on the geologic environment and on the contrast of the physical properties between the juxtapod for-mations.The combined u of various geophysical tech-niques is thus recommended for fault mapping,particu-larly for a preliminary investigation when the geological context is poorly defined.
INTRODUCTION
去世英语This paper describes the application of various geophysical prospecting techniques to locate and im
age Quaternary fault zones as part of a paleoismological project (Meghraoui et al.,2000).Paleoismology aims to determine the Late Pleistocene and Holocene history of near-surface faulting often associated with large earthquakes.This usually requires the excavation of shallow trenches across the trace of the suspected active fault.Active normal faults expod at the surface are usually
Published on Geophysics Online July 11,2000.Manuscript received by the Editor March 22,1999;revid manuscript received June 15,2000.∗
Liege University,LGIH,Bat B19,4000Liege,Belgium.E-mail:ddemanet@ulg.ac.be.
‡Formerly Liege University,LGIH,Bat B19,4000Liege,Belgium.Prently LIRIGM,Universite Joph Fourier-Grenoble 1,BP 53,F 38041Grenoble Cedex 9,France.E-mail:djongmans@ulg.ac.be.∗∗
Royal Obrvatory of Brusls,av.Circulaire 3,1180Brusls,Belgium.§CNR-CS,Geologia Tecnica,via Eudossiana 18,00184Rome,Italy.c 2001Society of Exploration Geophysicists.All rights rerved.
expresd in the topography as fault escarpments.However,in intraplate areas characterized by relatively low rates of tec-tonic deformation,the geomorphic expression of an active fault may be very
subtle as a result of the complex interplay among tectonic,depositional,and erosional process or intensive agricultural exploitation.However,nondestructive geophysi-cal prospecting techniques may be applied to map the near-surface fault trace with great accuracy.In the last few years,a large number of high-resolution ismic reflection surveys have been conducted (e.g.,Williams et al.,1995;Palmer et al.,1997;
78
Imaging Active Faults79
Van Arsdale et al.,1998)to provide information on Quater-nary fault geometry and timing.For very shallow investigation, ground-penetrating radar(GPR),which can bridge the gap be-tween high-resolution ismic surveys and trenching,has been applied by Cai et al.(1996)in the San Francisco Bay region. At the border of Nevada and California,Shields et al.(1998) have ud veral geophysical techniques(ismic reflection, magnetics,and electromagnetics)to locate the extension of the Parhump Valley fault zone.This paper prents the results of a geophysical campaign performed in the Bree area(Roer Graben,northeast Belgium)as a reconnaissance tool prior to trenching,which included refraction ismic records,electro-magnetic(EM)and electrical profiling,elect
rical tomography, ground penetrating radar(GPR),and high-resolution ismic reflection profiles.The foremost aim of this investigation was to determine the exact position of an active fault to precily locate a subquent trench.A cond objective was to image the fault zone at shallow depths,therby allowing a direct com-parison with trench data and hence a confident extrapolation of direct obrvations to greater depths.
strip
GEOLOGICAL SETTING AND TECTONIC ACTIVITY The Roer Graben,which cross three countries(Belgium, The Netherlands,and Germany),is bounded by two north–northwest,south–southeast-trending Quaternary normal fault systems(Figure1).The eastern boundary is defined by the Peel boundary fault,where the5.4-M W1992Roermond earthquake occurred(Camelbeeck and van Eck,1994);while the western boundary is defined by the Feldbiss fault zone,which is partially located in Belgium.Evidence of tectonic activity in the Roer Graben is given by(1)the strong subsidence during the last 150000years(Geluk et al.,1994),(2)the Quaternary faults and their associated morphology along theflanks of the graben, (3)the0.8–2-mm/yr vertical rate of deformation obtained by the comparison of levelings during the last100years(Van den Berg et al.,1994;M¨alzer et al.,1983),and(4)the prent-day ismic activity(Camelbeeck and van Eck,1994).
For the Feldbiss fault zone,tectonic activity is mainly indi-cated at depth by ismic profiles that show
more than600m of offt in Neogene deposits(Demyttenaere and Laga,1988)and about150m at the ba of the Pliocene(De Batist and Versteeg, 1999).By considering the offt of the main terrace of the Mass River determined by Paulisn et al.(1985),Camelbeeck and Meghraoui(1998)obtain0.08±0.04mm/year for the average Late Pleistocene vertical deformation along the Feldbiss fault. Near the town of Bree(Figures1and2)and along the Feldbiss fault,a prominent northwest–southeast-trending fault scarp parates the Campine plateau to the west from the Roer Valley Graben to the east(Paulisn,1973).The geomorphic expression of the scarp consists of a10-km-long escarpment that has15–20m of vertical topographic relief(Figure2).The Belgian Geological Survey acquired150reflection ismic lines in the region with a dozen crossing the scarp(Demyttenaere, 1989).On different ctions,the scarp coincides at the surface with the surface projection of the Feldbiss fault zone and can therefore be considered the morphological expression of the fault’s recent activity.
The Bree fault scarp corresponds to the northeastern border of the Campine Plateau(Figure2),which is covered by terrace gravels deposited by the Mass River(Zutendaal gravels)during the Cromerian(between770000and350000years BP)and which overly sands of Upper Miocene age(Diest Formation)(Paulisn et al.,1985).In the downthrown block (Roer Graben),the Zutendaal g
ravels have been eroded by the Rhine and Maas Rivers,which afterward deposited the Bocholt sands(Paulisn,1983).The formations constitute the bament on which the Maas formed its different terraces at the end of the Middle Pleistocene and during the Late Pleistocene.The terraces are the typical landscape of the region.The region was later covered with aeolian sands during the Saalian and Weichlian glacial ages,which were mixed with the other near-slope deposits in the vicinity of the fault scarp.Afinal pha of deposition created the Holocene alluvium in the center of the Maas Valley.The lithology logs of two boreholes(Van der Sluys,1997)drilled on each side of the scarp are given in Figure3.On the Campine plateau(hole H1),the Zutendaal gravels directly overlie the Upper Miocene sands of the Diest Formation,which were encountered at 11m depth.In the Roer Graben(hole H2),the thickness of the Middle Pleistocene river terraces reaches40m,while the top of the Diest Formation was found at233m depth,
below
F IG.1.Quaternary faults and ismic activity in the lower Rhine embayment.The Bree fault scarp is located along the Feldbiss fault southeast of the town of Bree.
80Demanet et al.
a succession of sand and clay layers from Lower Pliocene to Upper Pliocene.The depth difference in stratigraphic horizons between the two boreholes gives strong evidence of tectonic activity along the Feldbiss fault zone.
Multiple scarplets are superpod on the overall fault scarp,and the frontal fault trace consists of an en echelon geom-etry that suggests a component of left-lateral slip.The fault dips 70◦northeast and offts young deposits (mainly late Weichlian aeolian cover sands and local alluvial terraces).Leveling profiles across the frontal fault scarp yield a vertical displacement ranging from 0.5to 3m.A three-year detailed paleoismic investigation (1996–1998)shows that this frontal scarp corresponds to the latest coismic (occurring during an earthquake)surface ruptures along this gment of the Feld-biss fault.The studies (Camelbeeck and Meghraoui,1996,1998)suggest that the most recent large earthquake occurred along the fault scarp between 610and 890A.D.and produced a vertical coismic displacement of 0.5to 1.0m,with a mini-mum moment magnitude estimated as 6.3M W .Paleoismic in-formation combining the trench and geomorphic obrvations suggests the occurrence of two surface-faulting earthquakes during the last 20000years.A third dates between 28000and 42000years BP .
DATA ACQUISITION
等等翻译
Figure 2shows the location of the four sites where geophysical profiles were performed perpendicular to the fault strike.At the sites trenches were later excavated for a paleo-ismic study (Meghraoui et al.,2000).Six geophysical methods were applied across the scarp:(1)electrical profiling,(2)EM profiling,(3)electrical tomography,(4)GPR,(5)
ismic
F I
micheG .2.Location and geological map showing the frontal escarpment of the Feldbiss fault near Bree and the studied sites (labeled 1to 4).The ismic line (SL)is located between sites 1and 2,at a right angle to the scarp.H1and H2are the boreholes described in Figure 3.Contours indicate
topography.
F I
G .3.Stratigraphic logs of boreholes H1and H2along a schematic southwest–northeast cross-ction (after Van der Sluys,1997).
Imaging Active Faults81
refraction tests,and(6)high-resolution ismic reflection pro-files(Telford et al.,1990;Reynolds,1997).Geophysical tests were performed along the axis of each planned trench except for the ismic reflection profile,which was carried out between sites1and2(Figure2).As afirst step,the variation of the ap-parent ground resistivity along the scarp was measured with electrical and/or EM profiling.EM surveying was conducted with two parate coils connected by a reference c
able moved along the profile at discrete intervals with a constant coil spac-ing(Reynolds,1997).The instrument provides a direct reading of the apparent resistivity of the ground.In this study,the mea-surement spacing was5m and the intercoil paration was 10m.With horizontal coils,the maximum contribution to the condary magneticfield theoretically aris from a depth of around4m.
In electrical profiling,a Schlumberger configuration with cur-rent electrodes spaced12m apart(50m for site4)was moved perpendicular to the profile,providing measurements of the apparent resistivity of the ground as a function of distance.An electrical tomography survey was performed using the Lund imaging system(Dahlin,1996)with a Wenner configuration and an electrode spacing of1or2m.The data were procesd with the algorithm propod by Loke and Barker(1996)to ob-tain a resistivity ction.According to the profile length,the investigation depth was between5and15m.GPR profiles were also performed at three sites with a120-MHz transmitting an-tenna and at site3with a50-MHz antenna.A static correction was made with a mean velocity of80to90mm/ns determined from scattered events.The penetration depth strongly depends on the ground resistivity(ranging between50and500ohm-m in the Bree area)and was limited to a few meters.The GPR vertical resolution was smaller than0.5m with the120-MHz an-tenna ud.At two sites,ismic refraction profiles,44and70m long,were carried out with a geophone spacing of1m and
three tofive shots.The source was a hammer,and twenty-four10-Hz geophones were connected to a16-bit ismograph.Finally, one ismic reflection line was run in a northeast–southwest direction perpendicular to the fault scarp(Figure2).The pro-file extends150m with a4-m source interval.A gun provided the source,stackedfive times for each source location.The op-timum window(Hunter et al.,1984)was determined from30to 56m from a walkaway test.Data were recorded with a16-bit ismograph from40-Hz geophones.The stacked data have a maximum of six-fold subsurface coverage.Processing was performed using SU software(Cohen and Stockwell,1998), and the quence included static corrections,F-Kfiltering, NMO corrections,prestack band-passfiltering,CDP stack and poststack band-passfiltering.
RESULTS AND INTERPRETATION
The results of geophysical tests parallel to trenches T1to T4 are prented in Figure4and Figures6to8as well as a simpli-fied geological description of each trench.The ismic reflec-tion profile is shown in Figure5.
Site1
Thefirst site is located near a stream that cuts a small uplifted alluvial terrace.The trench,which is onl
y2m deep,reveals late Weichlian cover sands,the upper part of which has been reworked by the small river(Figure4a).Disruption of(1)twosorento
gravel horizons within the cover sands and(2)the bleached
Holocene soil at the top indicates the near-surface prence
of a normal fault dipping to the northeast and cloly aligning
with the frontal escarpment.An overlying soil bed just below
the plough zone does not appear to be affected.
Electrical profiling data clearly delineate the fault at a dis-
tance between50and65m(Figure4a)by a sharp increa of
the apparent resistivity values,from70ohm-m in the south-
west block to more than250ohm-m in the northeast block
(Figure4b).An accurate location(within a few meters)of
the fault is,however,impossible to asss.The electrical to-
mography ction(Figure4c)shows a strong lateral resistivity
variation around50m with a contact dipping to the northeast.
In the southwest block,a2-m-thick resistive layer overlies a
conductive formation,while the northeast block consists only
of the resistive layer.Here,the fault juxtaposing different soil
layers can be located at the surface with an accuracy<2m.
A cond strong lateral resistivity variation at20m could be
interpreted as a fault dipping to the southwest.However,this
was shown neither on the ismic profile nor in the trench,and
the anomaly probably results from a dimentary variation.
A70-m-long ismic refraction profile was performed across
the scarp.The time–distance graph inferred from the refracted
wave analysis for the direct shot(Figure4b)shows an unusual
decrea of the apparent velocity from1690to720m/s in the
subsurface.This crossover point is located around50m and
外交学
fits perfectly with the position of the fault.The interpretation
of the ismic data(Figure4a)with the generalized recipro-
cal method(Palmer,1981)shows that the conductive underly-
ing layer is characterized by a relatively high ismic velocity
(V p=1400m/s).In the southwest part of the ction,this hori-zon is covered by a thin layer with a velocity of470m/s,which
dramatically increas in depth across the fault to reach4m in
the hanging wall.The limit between the two ismic horizons地黄牛
家规英语作文could correspond to the depth of the water table,which was
less than2m in the footwall.Both geophysical methods clearly
indicate the prence of a fault below the topographic scarp,
juxtaposing two blocks with different resistivity and ismic
velocity values.The corresponding GPR ction is prented
in Figure4d,where thefirst30ns corresponding to the direct
wave have been muted.The maximum penetration depth is
about4m,corresponding to a two-way traveltime of100ns.
In the southwest part,the ction reveals two main horizontal
reflectors(R1and R2),which are clearlyflexured and cut by
two fault branches.The main one(F1)is located at about50m
along the profile,whereas the cond fault branch F2prob-
ably does not extend to within the reach of the trench.The
shallower reflector(R1)is located at1.6m depth(40ns)and
correlates with the lower gravel horizon expod at the bottom
of trench1.The northeast part of the ction is characterized by
a wedge shape with a southwest-dipping strong reflector(R3)
at its ba.The ba of the wedge is located at3.2m depth.The
different layers inside the wedge appear to beflexured in the
vicinity of the fault.
Seismic line SL(Figure5),150m long,trends southwest–
northeast and cross the frontal escarpment(F)at a right
开国税发票angle.In the Roer graben(northeastern block),the ismic
ction reveals veral well-defined reflections down to0.2s.
The ismic horizons are cut at105m by a fault(F)who
82Demanet et al.
F IG.4.Site1.(a)Schematic stratigraphic cross-ction and ismic velocity model.(b)Electrical profiling(EP)and ismic
traveltime curves(SP).(c)Electrical tomography.(d)Radar ction(120MHz).
