procedure什么意思Application of field geophysics in geomorphology:Advances and
limitations exemplified by ca studies
Lothar Schrott a,⁎,Oliver Sass b
a
Department of Geography and Geology,University of Salzburg,Hellbrunnerstr.34,A-5020Salzburg,Austria b
Department of Geography,University of Augsburg,Universitätsstr.10,D-86135Augsburg,Germany
Received 30December 2005;accepted 27December 2006
Available online 10May 2007
Abstract
凝聚的意思During the last decade,the u of geophysical techniques has become popular in many geomorphological studies.However,the correct handling of geophysical instruments and the subqu
ent processing of the data they yield are difficult tasks.Furthermore,the description and interpretation of geomorphological ttings to which they apply can significantly influence the data gathering and subquent modelling procedure (e.g.achieving a maximum depth of 30m requires a certain profile length and geophone spacing or a particular frequency of antenna).For more than three decades geophysical techniques have been successfully applied,for example,in permafrost studies.However,in many cas complex or more heterogeneous subsurface structures could not be adequately interpreted due to limited computer facilities and time consuming calculations.As a result of recent technical improvements,geophysical techniques have been applied to a wider spectrum of geomorphological and geological ttings.This paper aims to prent some examples of geomorphological studies that demonstrate the powerful integration of geophysical techniques and highlight some of the limitations of the techniques.A focus has been given to the three most frequently ud techniques in geomorphology to date,namely ground-penetrating radar,ismic refraction and DC resistivity.Promising applications are reported for a broad range of landforms and environments,such as talus slopes,block fields,landslides,complex valley fill deposits,karst and loess covered landforms.A qualitative asssment highlights suitable landforms and environments.The techniques can help to answer yet unsolved questions in geomorphological rearch regarding for example diment thickness and internal structures.However,bad on ca studies it can be shown
coat是什么意思中文that the u of a single geophysical technique or a single interpretation tool is not recommended for many geomorphological surface and subsurface conditions as this may lead to significant errors in interpretation.Becau of changing physical properties of the subsurface material (e.g.diment,water content)in many cas only a combination of two or sometimes even three geophysical methods gives sufficient insight to avoid rious misinterpretation.A “good practice guide ”has been framed that provides recommendations to enable the successful application of three important geophysical methods in geomorphology and to help urs avoid making rious mistakes.©2007Elvier B.V .All rights rerved.
Keywords:Ground-penetrating radar;DC resistivity;Seismic refraction;Landforms;Internal structure
1.Introduction
Recently,the u of geophysical techniques has become increasingly important in many geomorpholog-ical studies.Without the application of geophysics our knowledge about subsurface structures,especially over
Available online at
Geomorphology 93(2008)55–
73
/locate/geomorph
⁎Corresponding author.
E-mail address:lothar.schrott@sbg.ac.at (L.Schrott),oliver.sass@geo.uni-augsburg.de (O.Sass).
0169-555X/$-e front matter ©2007Elvier B.V .All rights rerved.doi:10.ph.2006.12.024
larger areas,remains extremely limited.During the last decade the u of geophysical techniques has become a new and exciting tool for many geomorphologists.One reason for the increasing interest in geophysical field methods is certainly related to technical innovations as incread computer power and the availability of light-weight equipment allows for relatively ur-friendly, efficient and non-destructive data gathering.However,the correct handling of the geophysical instruments and subquent data processing are still difficult tasks and the methods often require advanced mathematical treatment for interpretation.It should be pointed out that without clo collaboration between geomorphologists and geo-physicists the accurate and effective u of geophysical techniques and their geophysical and geomorphological interpretation are often very limited.In addition,the correct description and interpretation of geomorpholog-ical ttings and thus,the choice of adequate and meaningful field sites are not an easy task.
In most studies and textbooks,interdisciplinary aspects combining geophysics and geomorphology are poorly addresd.Many textbooks,in their description of geophysical surveying applications,focus on the explo-ration for fossil fuels and mineral deposits,underground water supplies,engineering site
and archaeological Reynolds,1997;Kearey et al.,2002). Although veral potential applications for geophysical methods exist,many of them have not yet been fully integrated into geomorphological rearch.The most common geophysical applications are currently focusing on permafrost mapping,diment thickness determination of talus slopes,block fields,alluvial fans and,increas-ingly,on the depth and internal structures of landslides (Hecht,2000;Tavkhelid et al.,2000;Hauck,2001; Hoffmann and Schrott,2002;Hauck and V onder Mühll, 2003;Israil and Pachauri,2003;Kneil and Hauck, 2003;Schrott et al.,2003;Bichler et al.,2004;Sass et al., 2007-this issue).Other landforms such as karst and colluvia are comparatively rarely investigated(Hecht,2003). Currently,the most common geophysical methods in geomorphological rearch are ground-penetrating radar, DC resistivity,and ismic refraction(Gilbert,1999).Thus, the paper focus on typical applications of the methods.
Each geophysical technique is bad on the interpre-tation of contrasts in specific physical properties of the dielectric constant,electrical conductiv-ity,density).The type of physical property to which a particular geophysical method responds determines and limits the range of applications.As non-geophysicists cannot generally be aware of all limitations and pitfalls, there is a need to develop a t of the most suitable recipes combining geophysical methods.The methods
can then be adjusted to particular environmental conditions and landforms.Studies that compare the application of various interpretation tools and discuss the combined/composite applications of geophysical field methods for a particular landform are still rare(Schrott et al.,2000;Schwamborn et al.,2002;Otto and Sass,2006;Sass,2006a,b).
flynThus,the objectives of the prent paper are:
(i)to show and asss the advances and limitations of
different geophysical methods with the focus on一课一练答案
applied geomorphological rearch;
(ii)to demonstrate the application of the methods in different natural environments and for distinctive
landform types,and
(iii)to illustrate the advantages of combined and composite techniques leading to a t of recom-
mendations for the application of field geophysics
in geomorphology.
As the paper focus on the potential application of three geophysical methods in different environments, the site descriptions have been reduced to a necessary minimum.
2.Ground-penetrating radar(GPR)
2.1.Principle and geomorphic context
Ground-penetrating radar is a technique that us high-frequency electromagnetic waves to acquire information on subsurface composition.The electromagnetic pul is emitted from a transmitter antenna and propagates through the subsurface at a velocity determined by the dielectric properties of the subsurface materials.The pul is reflected by inhomogeneities and layer boundaries and is received by a cond antenna after a measured travel time.In order to calculate depth,wide-angle reflection and refraction(W ARR)or common mid-point(CMP)mea-surements have to be performed.The signal travel time measurements are made with a stepwi increa in the distance between the two antennas.From the distance/ travel time diagram,the propagation velocity of the radar waves in the subsurface can be derived.tsf
The common mode of GPR data collection is fixed-offt reflection profiling(Jol and Bristow,2003).In this step-like procedure,the antennas are moved along a profile line and the measurement is repeated at discrete intervals resulting in a2-D image of the subsurface.The possible working frequencies can range from10MHz to 1GHz depending upon the aim of the investigation. Higher frequencies allow higher spatial resolution of the ground information,but lead to a lower penetration depth.
56L.Schrott,O.Sass/Geomorphology93(2008)55–73
This has important implications in geomorphological applications.Knowledge about the geomorphological context(expected maximum depth and grain-size com-position of a diment body)is esntial for choosing the appropriate frequencies.A comprehensive“good practice guide”for the application of GPR in diments is provided by Jol and Bristow(2003).
The maximum depth of investigation depends mainly upon the dielectric constant(ε)and the electrical conductivity(σ)of the subsurface.A higher ground water and/or clay content(highε,highσ)leads to a stronger attenuation and,therefore,a markedly reduced penetration depth.However,very pure groundwaters that have a low lacial meltwater, bogs)are characterized by relatively
low levels of GPR signal loss.Again,geomorphological experti about possible water and clay layers can help to avoid disappointing measurements.On dry and electrically high-resistive debris,a penetration of between30and 60m can be Smith and Jol,1995).Sandy diments are also favourable for GPR measurements at depths of between15and30m.Due to the strong attenuation in materials that have a high electrical conductivity the penetration depth in wet,silty and/or clayey diments diminishes rapidly to,for example,less than5m in silt(Doolittle and Collins,1995);in clayey soil the application of GPR may be altogether impossible (e Table1).This is,however,only a very rough guide, becau the penetration depth depends upon the device and antenna frequency ud.The vertical resolution of GPR data is a function of frequency and propagation velocity.With higher velocities,resolution decreas and vice versa.As a rule of thumb,in the medium velocity range(0.1m/ns)the resolution is approximately1m using25MHz antennae,0.25m using100MHz and 2.5cm using1GHz.
特洛伊战争Table1
Comparison of common field geophysical methods in geomorphology;examples of applications and some technical considerations and advice Geophysical method Geomorphological application Technical considerations and practical advice
Ground-penetrating radar(GPR)▶Delineation of the boundaries of massive ice in
rock glaciers,moraines and other periglacial
phenomena
▶Small penetration depth in ca of conductive near-surface layers
▶Determining the thickness of a permafrost layer▶Less successful in,for example,wooded terrain with many surface
reflectors and in electrically conductive lay-rich)▶Active layer thickness▶Experience in data processing and interpretation is needed
▶Glacial ice thickness▶Correlate radar data with geomorphic/geologic control and site
obrvations such as exposures or borehole logs
▶Delineation of aquifers
▶Fracture mapping within massive bedrock
▶Mapping of internal structures in diment
留学出国申请storage alus slopes,rock glaciers)
2-D DC resistivity sounding,2-D DC resistivity profiling ▶Determining diment thickness,internal
differentiation and groundwater
internationallove▶Obtaining good electrical contact between electrodes and ground is
esntial
▶Depth and extension of landslide bodies▶Blocky and dry material is very problematic due to poor electrode
coupling
▶Detection of massive ice or permafrost in rock
glaciers,moraines and other periglacial phenomena
▶Experience in data inversion is needed for data processing.A priori
information influences(improves)significantly your iterative
modelling
▶Ice thickness▶Differentiation between ice,air and special rock types can
sometimes be difficult
▶Determining the altitudinal permafrost limit▶Correlate resistivity data with geomorphic/geologic control and site
obrvations such as exposures or borehole logs
▶Moisture distribution in rock walls
▶Seasonal variation in fluid content
Seismic refraction(2-D sounding, tomography)▶Sediment thickness(talus,alluvial fans,etc.)▶Number of geophones should be at least12,with shots at every
other receiver location
▶Detection of massive ice in rock glaciers,
moraines and other periglacial phenomena
▶Sledge hammer as source is sufficient for most shallow
applications(b30m)
▶Differentiation between ice,air and special rock
types
▶Experience in data processing and inversion is needed for data
interpretation
▶Mapping active layer thickness▶Correlate ismic data with geomorphic/geologic control and site
obrvations such as exposures or borehole logs
57 L.Schrott,O.Sass/Geomorphology93(2008)55–73
2.2.Advantages and disadvantages of GPR
Even when low-frequency25and50MHz antennas are ud,GPR still provides a better spatial resolution than standard geophysical techniques.The survey speed even in rough terrain is relatively high and veral hundred metres per day are possible.Steep and rocky slopes limit the progress of the survey,becau low-frequency antennas have large geometrical dimensions and are difficult to handle.Recently developed,so-called,Rough Terrain Antennas(RTAs)may facilitate data gathering,however, conventional antennas are still necessary for CMP measurements or small object detection.
The available antenna frequencies allow for a broad variety of possible applications.However,the extremely variable penetration depth requires careful asssment of the subsurface parameters in the study area in order to minimize the risk of error.The electrical conductivity of the soil provides a rough indicator of potential target depth.From the authors'experience,the u of GPR is not promising if the soil resistivity is lower than ca.50–100Ωm.
The application of GPR is subject to further restric-tions.As the reflectivity at a layer boundary is determined by the contrast in the dielectrical properties of the subsurface units,no distinct reflection i
s found when this contrast is low.Small-scale spatial differences in water content and/or grain-size composition may yield stronger reflections than the target of the investigation (e.g.the bedrock surface).This problem may be overcome by using the radar facies of the diments for interpreta-tion.Different diment units and bedrock yield typical reflection patterns that can be derived from,for example, reference profiles.The reconnaissance of the patterns significantly facilitates the interpretation of the radargram.
A portion of the energy transmitted by an unshielded antenna is emitted into the air and may be reflected by features above the surface.The air wave reflections cannot always be unequivocally distinguished from ground information and may verely affect the data quality.This makes the application of GPR particularly difficult in wooded terrain where,depending on the frequency ud,each tree may act as a single reflector, leading to very noisy or altogether uless data.Although the effect of air wave reflections may be reduced using sophisticated filter Van der Kruk and Slob,2004),measuring in forested areas is not advisable. The u of shielded antennas is only possible for higher frequencies(N100MHz).Taking into consideration the major restrictions arising from“clayey or silty subsur-face”and“wooded terrain”it is clear that GPR shows its potential particularly in arctic or alpine areas above the tree-line and where there is limited
soil development. However,shallow subsurface investigations are also possible in fluvial deposits and even in peat,when the electrical conductivity of the groundwater is low.
2.3.Examples of application
There is a broad range of successful applications of GPR in geomorphological studies.The include the detection of buried structures,asssment of internal diment structures and estimation of depth to bedrock. Various types of diments have been investigated for geomorphological purpos(Bristow et al.,2000).
The internal structures of floodplain deposits and deltaic diments have been visualized by,for example, Leclerc and Hickin(1997),Jol(1996)and Büker et al. (1996).Buried fluvial channels have been detected by Roberts et al.(1997)and Loope et al.(2004).The most frequently ud antenna frequency for comparative studies is100MHz.The penetration depth is dependant upon clay and water content,but usually ranges from10to20m.
Loo diments in arctic and alpine areas have also been the subject of GPR measurements.Lønne and Lauritn(1996)and Overgaard and Jakobn(2001) have investigated internal deformation structures of push-moraines and Berthling et al.(2000)clearly detected internal s
tructures as well as the bedrock ba of rock glaciers.The working groups ud50and100MHz antennas and achieved a penetration depth of up to30m. Sass and Wollny(2001)and Sass(2006a)achieved a penetration depth of up to50m on talus slopes using 25MHz antennas.They found surface-parallel structures in the debris body and evidence for moraine material at the ba of the talus.Studies of diment structures have also frequently been carried out in bogs.Völkel et al.(2001) investigated the subsurface structure of buried periglacial slope deposits,Holden et al.(2002)determined the position and depth of subsurface piping.
caissonThe application of GPR on landslides has repeatedly been tested but with limited success.Wollny(1999) investigated the near-surface moisture distribution at10 landslide areas and gained valuable information from only three sites.Bruno and Mariller(2000)and Wetzel et al. (2006)ud GPR for detecting the vertical extension of landslides but did not reach the slip surface even with the u of low-frequency antennas.However,internal slide structures such as rotational features can be mapped when the slide surface is comparatively Sass et al.(this issue)in displaced limestone blocks).Bichler et al.(2004) obtained very good results,distinguishing ven different facies of loo diments.Performing GPR measurements at landslide sites is only worthwhile when comparatively
58L.Schrott,O.Sass/Geomorphology93(2008)55–73
coar and dry deposits(debris,displaced blocks) superimpo the silty or clayey material of the slip surface.However,detailed asssment of the near-surface propagation velocity allows a rather accurate estimate of the soil moisture content(Topp et al.,1980)which is of interest for landslide investigation and many more geomorphological questions.
Another possible field of application is the investiga-tion of permafrost features.The active layer thickness has been determined,for example,by Arcone et al.(1998)and Hinkel et al.(2001).Moorman et al.(2003)provided instructive pictures of typical reflection patterns in frozen and unfrozen ground.Although the prence or non-prence of permafrost is more difficult to establish with GPR than electrical resistivity techniques,GPR is superior in detecting spatially confined structures such as ice wedges(Hinkel et al.,2001).The thickness and the internal structure of glacier ice(unfrozen water content, cavities)have also been the target of many GPR Moorman and Michel,2000).
The investigation of quasi point-shaped or linear buried structures in high resolution is the aim of many studies in the relatively new field of geo-archaeology that is cloly related to geomorphology(
Baker et al., 1997;Fuchs and Zöller,2006).Leckebusch(2003)has provided a detailed description of the GPR method for archaeological purpos with numerous examples.The working frequencies for the applications are usually rather high(=200MHz);the target depth is usually between1and5m.2.3.1.Ca study:localization of buried structures
The aim of this GPR application was to locate a Roman road buried under1.5to3m of peat and fluvial diments in the Murnauer Moos,Upper Bavaria(Sass et al.,2004).This example primarily highlights the potential for archaeological studies(Fuchs and Hruska, 1996).However,from a geomorphological perspective the Roman road can be ud as a time marker for the evolution of the overlaying peat and interbedded clay-rich diments.The road had been examined at an archaeological excavation nearby.Thus,the structure of the road(logs lying on a gravel layer)was known.The objective of the measurements was the quick localiza-tion in the surroundings of the excavation.
As a result of the high water content of the peat,the propagation velocity derived from WARR measurements was very low(0.45m/ns).However,becau of the low mineralization of the bog water the penetration depth using200MHz antennae was up to5m.The road was quickly and clearly located in a number of cross profiles (Fig.1).Thus,the measurements provided valuable information on the straight-line structure.The cross-profile prented(Fig.1a)shows the radar reflection of the Roman ro
ad at a profile distance of between3and7m. The shallow depression in the middle of the road(as obrved at the excavation site),caud by the weight of the vehicles,can be clearly recognized.The longitudinal profile(Fig.1b)illustrates the linear structure of the road. The main reflection is caud by the artificial gravel layer which obviously shows a distinct dielectrical contrast
to Fig.1.Investigation of a buried Roman road using200MHz GPR profiles.Filters applied:DC-shift removal,bandpass frequency filter and runtime-dependant gain function.Above:cross profile,below:longitudinal profile.The road is recognizable from typical reflection patterns(e text)(Sass et al.,2004).
59
L.Schrott,O.Sass/Geomorphology93(2008)55–73
