Shallow Geophysical Methods Research Articles

Application of SPAC method and electromagnetic wave CT in karst detection of Wuhan Metro Line 8

In recent years, karst construction projects in the built-up area of Wuhan (capital of Hubei Province, China) are increasing, and the karst geological disasters have aroused social concerns. The actual engineering projects usually use shallow geophysical exploration methods to explore karst. This paper uses Spatial Auto-Correlation Method (SPAC) and electromagnetic Computerized Tomography (CT) to detect karst in urban built-up areas. Depending on the different physical properties of rock and soil, the SPAC method can better reveal the interface between soil and rock strata and the interface between soil layers. The electromagnetic CT method can identify strata according to the apparent absorption coefficient, which can better reveal the interface between soil and rock, the interface between the more intact and weathered rock. The SPAC method is mainly qualitative to measure the low-speed area, namely, the wrong geological body i.e., karst cave, but also can detect the fracture zone or filling mode of karst cave, and at the same time, cannot use exploration holes or logging observation. The electromagnetic CT method can accurately detect the location and scale of the karst caves and has a higher accuracy detecting karst bands. In addition, exploration holes or well logging observations are also expected to be conducted, and their detection effect is greatly affected by lithology.

Modeling GPR Wave Propagation in Complex Underground Structures Using Conformal ADI-FDTD Algorithm

Ground Penetrating Radar (GPR) is a shallow geophysical method for detecting and locating subsurface targets. The GPR image echo characteristics of complex underground structures can be obtained by carrying out GPR forward modeling research. The traditional finite-difference time-domain (FDTD) method has low efficiency and accuracy. The alternating direction implicit FDTD (ADI-FDTD) algorithm surmounts the stability limitations of the traditional FDTD method, making it possible to select a larger time step for higher computational efficiency. For circular underground structures, a pseudowave produced by the ladder approximation method can be corrected using the surface conformal technique. This paper proposes a high-efficiency and high-accuracy GPR forward modeling method that combines the ADI-FDTD algorithm and surface conformal technology. The performance of the conformal ADI-FDTD algorithm is verified by a simple two-layer model. Based on the proposed algorithm, the GPR image features of three complex underground structure models are obtained. Finally, a field experiment is used to support the accuracy and usefulness of the conformal ADI-FDTD algorithm. The numerical simulation results and experimental results show that the conformal ADI-FDTD algorithm reduces the pseudodiffraction wave caused by the ladder approximation method and can significantly improve the computing efficiency for complex underground structure models.

The study of applicability of the very shallow geophysical exploration method for the strike-slip fault (part 2; S-wave shallow seismic reflection survey and two-dimensional electrical resistivity survey): the case study of the Go-mura fault zone and Yamada fault zone

The study of applicability of the very shallow geophysical exploration method for the strike-slip fault (part 2; S-wave shallow seismic reflection survey and two-dimensional electrical resistivity survey): the case study of the Go-mura fault zone and Yamada fault zone

Application of magnetometry, georadar (GPR) and geoelectrical methods in archaeo-geophysical investigation of a Napoleonic battlefield with fortification at Pressburg (Bratislava, Slovakia)

This article presents the successful application of several geophysical methods in archaeological prospection with integration of LiDAR data and examination of historical documents. The locality of interest is situated in an urban area of Bratislava city on the Danube River where no previous archaeological research has been undertaken. It comprises the oldest public park in Central Europe which is surrounded by a fortification system dating to the Napoleonic siege of Pressburg (Bratislava) in 1809. Urban areas are often challenging for conducting archaeo-geophysical investigation due to limited access and existing obstacles of modern origin.For the purposes of this investigation shallow geophysical survey methods were applied including magnetometry, ground penetrating radar (GPR), 2D electrical resistivity tomography (ERT) and electrical resistance survey. Magnetometry was used to obtain a general view of the survey areas and to help to target the following surveys. ERT and GPR were used to explore detected features in more detail and obtain information about their shape and depth. The results of the survey show the importance of the combined approach using multiple geophysical methods while also highlighting their benefits and limitations.The survey was successful in investigating the basic elements of the fortification system which includes a rampart, a ditch and a bastion. The results have revealed information about the current state of the ramparts as well as their original construction. Also, the previously unrecorded remains of a bastion were identified along with previous phases of the fortification. The results obtained reveal the archaeological potential of this historical locality and help to better understand its extension, which is essential for its future protection and development.

Numerical Simulation Research on Response Characteristics of Grouting Defects of Ground Penetrating Radar for Detection of Grouting Quality behind Tunnel Wall

The effect of grouting behind tunnel wall directly affects the surrounding ground settlement and the stability of tunnel structure, so the grouting quality detection is very necessary. As an efficient and convenient shallow geophysical exploration method, ground-penetrating radar can meet the high-resolution and non-destructive requirements of grouting quality detection behind the tunnel wall, so it is widely used in engineering in recent years. Most of the existing studies have obvious regional pertinence and special geological conditions, and there are few universal studies on the characteristics of the ground penetrating radar reflection image of the grouting defect behind the tunnel wall. In view of this, this paper uses the finite difference time domain method to simulate several grouting defects behind the wall, such as voids, water-bearing anomaly, cracks, and other grouting defects. The simulation results show that the reflection image of the direct wave is characterized by a white band with strong amplitude; the interface between primary support and second lining, primary support, and surrounding rock is also banded; the circular cavity and water anomaly characteristics are all hyperbolic, the difference is that the phase of the lower part of the radar image of the cavity anomaly is 0, and there are only hyperbolic tails on both sides, and the water-bearing anomaly also has obvious hyperbolic characteristics at each interface; the reflected wave characteristics of the rectangular crack are striped and watery and the reflected wave characteristic of rectangular cracks is striped, and the abnormal range of water-bearing cracks on the radar image is larger than that of air. The research results can provide an effective theoretical reference for the engineering application of ground penetrating radar detection of grouting defects behind the tunnel wall.

Detecting active faults in intramountain basins using electrical resistivity tomography: A focus on Kashmir Basin, NW Himalaya

Kashmir basin in the NW Himalaya is surrounded by the main Himalayan boundary faults, has very well documented historical earthquakes and a good instrumental earthquake record. However, the causative faults of these earthquakes except the 8 October 2005 Kashmir earthquake (M7.6) are not known. One of many historical earthquakes that have struck and caused damage and destruction in the Kashmir basin is the 30 May 1885 Kashmir earthquake (~M6.3). The extensive damage due to this earthquake was reported in the NW part of the basin and as usual the causative fault is not known and mapped. As the earthquake related geomorphic features are not preserved due to the high erosion rates in the Kashmir Himalaya, we mapped certain active fault strands using high resolution digital elevation models (DEM) and the Google Earth imagery, later complemented by the field investigation in the NW Kashmir. The Electrical Resistivity Tomography (ERT) was carried out at certain identified sites in the macroseismic epicentral area of the 1885 Kashmir earthquake. The results show a local active normal fault which was named as the NW Kashmir fault. The ERT results were confirmed by excavating a trench and an already existing road cut at the ERT sites. The results show that ERT is a very useful shallow geophysical method to detect faults in the Karewas. Karewas are the Plio-Pleistocene and Holocene (reworked by rivers), fluvio-lacustrine, soft and unconsolidated, sand-clay-conglomerate sediments, deposited as distinct table-land geomorphic features in the Kashmir basin and are significantly water saturated.

Shallow geophysical methods for recognition of holocene sedimentary sequences in the southern coastal plain of the Río de la Plata (Argentina)

New information about the distribution, geometry, structure, extension and facies variations of the uppermost stratigraphic sequences (particularly the Holocene) of the southern coastal plain of the Río de la Plata was obtained by using geophysical methods. This is the first study performed in the region to investigate the shallow stratigraphic characteristics through continuous geophysical surveys. Methods used were georadar (GPR), electrical resistivity tomography (ERT) and seismic surface waves (SSW), complemented with short drillings. Two transects perpendicular to the coastline were selected on the basis of the previous geological knowledge of the region and accessibility. GPR was used in order to obtain a dense sampling of the near subsoil, whereas ERT and SSW were used in parts of the transects for resolving ambiguous trajectories of the detected GPR horizons. The geophysical results were jointly interpreted, and validated and complemented by the information from the drillings. Two major geophysical horizons were identified; the lower one is located between 5 and 2 m below surface and progressively shallows inland, which represents the late Pleistocene-Holocene boundary; the upper one, located at variable depths of 2 m–0.70 m below surface, represents the base of the Holocene beach ridges system in the region. They delimit three units, the lower one corresponding to the top of the Pampean deposits, the middle one representing the basal Holocene sandy layer, and the upper one corresponding to the beach ridges systems and related inter-ridge, tidal deposits. In this way, the applied methodology allowed to determine the extension of the Holocene sedimentary sequences, the relation between the Holocene sequences and its substratum, and the spatial distribution and geometrical characteristics of partially buried beach ridges and associated inter-ridges deposits. The applied methodology and the obtained geophysical-response patterns can be useful for future investigations of other regions with similar ground characteristics.

Geoelectrical analysis for evaluating the aquifer hydraulic characteristics in Ain El-Soukhna area, West Gulf of Suez, Egypt

ABSTRACT Ain El-Soukhna area is an important industrial area for building and construction, so we used the subsurface shallow geophysical method, as represented by geoelectric resistivity measurements (Vertical Electric Sounding (VES)). These are processed and interpreted in the forms of a geologic model composed of five geoelectric layers of varying resistivities, lithologies, depthsand thicknesses. The thickness and true resistivity maps of the different layers and 2D geoelectric cross sections are generated for the represented layers to delineate their spatial distribution. The geoelectric Dar-Zarrouk parameters (Transverse Resistance and Longitudinal Conductance) were used to evaluate the electric anisotropy and aquifer hydraulic characteristics; such as hydraulic conductivity and transmissivity within the aquifer, as well as the groundwater quality (salinity of the aquifer) are estimated the true resistivity values acquired on the surface through the geoelectric resistivity survey.

Determination of Grave Locations with High Resolution Ground-penetrating Radar (GPR) in War Cemeteries: Sample Study of Biga (Çanakkale) War Graves

The most important turning point in Turkish history of the Çanakkale War, known in English as the Dardanelles Campaign, the Battle of Gallipoli, or the Battle of Çanakkale, involves many omissions in terms of historical records due to the situation facing the country at the time and the threats from both internal and external enemies. Though there is very valuable information like notes and maps left by officers in the Ottoman army (Col. Şevki Paşa etc.), due to the intense severity of the war much important information has not been transferred directly to the present day. Most probably, one of the areas with greatest omissions is the location of war grave sites (martyr’s cemeteries). The main reason for this is considered to be the excessive number of fallen soldiers or martyrs and the extreme loss of life in a very short period. In spite of these negative aspects, the locations of many war grave sites have been determined by detailed research by historians and the use of maps drawn during wartime. However, in these cemetery areas, approximate locations were defined for war graves and in line with this, areas were fenced off as war cemeteries. Using the same logic, an area close to the location used as Biga War hospital during the Çanakkale War was renovated and opened to visitors as Biga War Cemetery with gravestones in certain locations. The high-resolution, non-destructive shallow geophysical method of ground-penetrating radar (GPR) has long been used with the aim of identifying structural elements that are buried (graves, tunnels, archeological remains, etc.). In line with this, its use to research war grave sites in areas known as war cemeteries will illuminate the past and contribute to reorganizing war cemeteries according to war grave locations and providing the necessary importance and respect that war cemeteries deserve.

The Effectiveness of Shallow Surface Geophysical Methods in Shear Wave Velocity Derivation

The Effectiveness of Shallow Surface Geophysical Methods in Shear Wave Velocity Derivation

Imaging and locating paleo-channels using geophysical data from meandering system of the Mun River, Khorat Plateau, Northeastern Thailand

AbstractThe Khorat Plateau from northeast Thailand, the upstream part of the Mun River flows through clastic sedimentary rocks. A massive amount of sand was transported. We aimed to understand the evolution of fluvial system and to discuss the advantages of two shallow geophysical methods for describing subsurface morphology of modern and paleo-channels. We applied Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) to characterize the lateral, vertical morphological and sedimentary structures of paleo-channels, floodplain and recent point bars. Both methods were interpreted together with on-sites boreholes to describe the physical properties of subsurface sediments. As a result, we concluded that four radar reflection patterns including reflection free, shingled, inclined and hummocky reflections were appropriated to apply as criteria to characterize lateral accretion, the meandering rivers with channel-filled sequence and floodplain were detected from ERT profiles. The changes in resistivity correspond well with differences in particle size and show relationship with ERT lithological classes. Clay, silt, sand, loam and bedrock were classified by the resistivity data. Geometry of paleo-channel embayment and lithological differences can be detected by ERT, whereas GPR provides detail subsurface facies for describing point bar sand deposit better than ERT.

Criteria and geological setting for the generic geothermal underground research laboratory, GEOLAB

High flow rate injection and related hydromechanical interaction are the most important factors in reservoir development of Enhanced Geothermal Systems (EGS). GeoLaB, a new generic geothermal underground research laboratory (URL), is proposed for controlled high flow rate experiments (CHFE) to address limited comprehension of coupled processes connected to EGS reservoir flow conditions. As analogue for typical EGS development, CHFE require specific hydromechanical conditions including a connected fracture network in crystalline basement rock, sufficient hydraulic fracture transmissivities, a strike-slip to normal faulting tectonic regime, controllable hydraulic boundary conditions, and hydrothermal alteration fracture fillings that improve conditions for hydromechanical interaction. With the aim to identify most appropriate areas for future site selection, four criteria have been established based on the EGS reference site of Soultz. Two URLs in crystalline basement worldwide approximate the requirements of a new generic GeoLaB and may be used for accompanying experimentation. Besides favourable geological, hydraulic, and stress conditions, the vicinity to long-term EGS production favours the southern Black Forest as potential region for GeoLaB. Therefore, an exemplary site assessment has been carried out at “Wilhelminenstollen” in the southern Black Forest (Germany). New remote sensing, hydrochemical, and geophysical analyses as well as reactivation potential, and stress modelling were added to complement existing geological and hydrogeological information. At this site, reactivation potential analysis reveals two local maxima prone for shear reactivation as strike-slip faults. The highest lineament density is observed for the N110°E strike direction that is associated with both slip and dilation tendency maxima. Clay minerals occur in fractures and the matrix. Local, partly water-bearing fractures, when partly filled with ore minerals, were connected to veins in the tunnel using shallow geophysical methods. Hydrochemical data reveal infiltration of the tunnel water from at least 500 m above the tunnel. The results suggest a crystalline basement with a fracture network that is regionally connected and water-conducting. Hydraulic conductivity in the southern Black Forest granite is estimated to amount to about 4.5·10−8 m s−1 at 500 m depth. The hydraulic boundary conditions exclude unknown drainage. Analyses of the influence of topography on orientation and magnitude of the maximum stress indicate a minimum overburden of about 500 m for regional reactivation to be valid. In conclusion, the southern Black Forest and in particular “Wilhelminenstollen” offers favourable condition for CHFE. Final decision on the GeoLaB site is to be drawn from forthcoming exploration wells.

Using shallow geophysical methods to characterise the monumental building at the Segeda I site (Spain)

The Segeda I site is located in northeastern Spain and holds the remains of an important city mentioned in Roman sources on the conquest of the Iberian Peninsula, particularly the Celtiberian war, from 153 to 133 BC. Recent excavations at this site have revealed a peculiar structure outside the perimeter wall that can be related more to defensive or ritual functions than to production or domestic use. Several questions have arisen regarding the functionality of this unusual building. If it had been built for ritual purposes, a comparison with other sacred buildings would suggest the presence of foundation deposits or an inhumation urn. If it were a singular building, it would probably have a different constructive pattern and special foundations compared with the other constructions on the site.In order to investigate these two aspects, non-destructive geophysical methods have been applied to survey the buried inner parts of the building (architectonic target), and investigate the terrain under and around it (geologic target). A 2D-magnetic gradient survey was planned over the structure to locate possible foundation elements. 3D-laser scanner topography and 3D-ground penetrating radar (GPR) have been combined to ascertain the internal arrangement of ashlars and blocks. Finally, 2D-electrical resistivity tomography (ERT) profiles were performed to investigate the geological materials underneath the structure. The geophysical models obtained reveal the inner construction pattern and the siting of the structure.

Green Waste and Archaeological Geophysics

AbstractEnvironmental concerns, supported by regulatory frameworks, have encouraged the conversion of organic and biological waste into fertilizers and soil conditioners (so‐called green waste) that are being increasingly used on arable fields. Recent work has shown that the level of ferrous contaminants within this waste can have a detrimental impact on shallow geophysical prospection methods that use the principles of magnetism. This paper highlights the negative impact of this new agricultural practice on the historic environment and calls for tighter regulation of green waste. Copyright © 2015 John Wiley & Sons, Ltd.

Volcanic stratigraphy of the Quaternary La Garrotxa Volcanic Field (north‐east Iberian Peninsula)

ABSTRACTThe monogenetic Quaternary La Garrotxa Volcanic Field forms part of the Catalan Volcanic Zone (north‐east Iberian Peninsula), one of the alkaline volcanic provinces of the European rift system. It harbours more than 50 basaltic monogenetic cones that range in age from the Middle Pleistocene to the Early Holocene and include cinder and scoria cones, lava flows, tuff rings and maars. This study is the result of extensive fieldwork, including the study of ephemeral outcrops and the stratigraphic logging of new water wells and geotechnical drill holes, also taking into account existing information gathered by recent geophysical studies that have applied shallow geophysical methods to establish the substrate geology of this volcanic field. We have obtained a comprehensive volcanic stratigraphy of the area that identifies the products of each single eruption, their relative stratigraphy and their surface area. This volcanic stratigraphy constitutes an essential tool for understanding the evolution of this volcanic field and for establishing a correct volcanic hazard assessment for the area, but it also provides a precise reference for the Quaternary tephrochronology of the lake sediments in neighbouring areas.

Three-dimensional geological modelling of anthropogenic deposits at small urban sites: A case study from Sheepcote Valley, Brighton, UK

Improvements in computing speed and capacity and the increasing collection and digitisation of geological data now allow geoscientists to produce meaningful 3D spatial models of the shallow subsurface in many large urban areas, to predict ground conditions and reduce risk and uncertainty in urban planning. It is not yet clear how useful this 3D modelling approach is at smaller urban scales, where poorly characterised anthropogenic deposits (artificial/made ground and fill) form the dominant subsurface material and where the availability of borehole and other geological data is less comprehensive. This is important as it is these smaller urban sites, with complex site history, which frequently form the focus of urban regeneration and redevelopment schemes. This paper examines the extent to which the 3D modelling approach previously utilised at large urban scales can be extended to smaller less well-characterised urban sites, using a historic landfill site in Sheepcote Valley, Brighton, UK as a case study. Two 3D models were generated and compared using GSI3D™ software, one using borehole data only, one combining borehole data with local geological maps and results from a desk study (involving collation of available site data, including ground contour plans). These models clearly delimit the overall subsurface geology at the site, and allow visualisation and modelling of the anthropogenic deposits present. Shallow geophysical data collected from the site partially validate the 3D modelled data, and can improve GSI3D™ outputs where boundaries of anthropogenic deposits may not be clearly defined by surface, contour or borehole data. Attribution of geotechnical and geochemical properties to the 3D model is problematic without intrusive investigations and sampling. However, combining available borehole data, shallow geophysical methods and site histories may allow attribution of generic fill properties, and consequent reduction of urban development risk and uncertainty.

Site classification and estimation of surface level seismic hazard using geophysical data and probabilistic approach

This paper presents the site classification of Bangalore Mahanagar Palike (BMP) area using geophysical data and the evaluation of spectral acceleration at ground level using probabilistic approach. Site classification has been carried out using experimental data from the shallow geophysical method of Multichannel Analysis of Surface wave (MASW). One-dimensional (1-D) MASW survey has been carried out at 58 locations and respective velocity profiles are obtained. The average shear wave velocity for 30 m depth (Vs 30) has been calculated and is used for the site classification of the BMP area as per NEHRP (National Earthquake Hazards Reduction Program). Based on the Vs 30 values major part of the BMP area can be classified as “site class D”, and “site class C”. A smaller portion of the study area, in and around Lalbagh Park, is classified as “site class B”. Further, probabilistic seismic hazard analysis has been carried out to map the seismic hazard in terms spectral acceleration (S a) at rock and the ground level considering the site classes and six seismogenic sources identified. The mean annual rate of exceedance and cumulative probability hazard curve for S a have been generated. The quantified hazard values in terms of spectral acceleration for short period and long period are mapped for rock, site class C and D with 10% probability of exceedance in 50 years on a grid size of 0.5 km. In addition to this, the Uniform Hazard Response Spectrum (UHRS) at surface level has been developed for the 5% damping and 10% probability of exceedance in 50 years for rock, site class C and D. These spectral acceleration and uniform hazard spectrums can be used to assess the design force for important structures and also to develop the design spectrum.

Comparing different shallow geophysical methods in a tidal estuary, Verdronken Land van Saeftinge, Western Scheldt, the Netherlands

AbstractIn order to validate existing models of sedimentation in active sedimentary environments, detailed stratigraphic information is indispensable. Near-surface geophysical methods provide a means to acquire high-resolution images of the stratigraphic succession in the shallow subsurface. Land-based and marine methods have been tested in the Verdronken Land van Saeftinge. This intertidal flat area is cut by numerous tidal gullies, and high tidal amplitudes enable the application of different techniques at various water levels. Data acquisition focused on the upper 10 – 20 m of the active sediment bodies. Applied techniques include high-resolution seismic acquisition, geo-electrical methods (DC resistivity), electromagnetic techniques, CPT, and manual drilling. In general the acoustic methods allowed more reliable and detailed interpretation of the sedimentary structures than the electric/electromagnetic methods. The latter suffered from the effect of tidal action and salt-water intrusion, and their application on land proved very strenuous. CPT and shallow cores provided valuable ground-truth information. The results clearly indicate that no single technique can provide all the answers. Only an integrated use of (complementary) methods will allow getting a better grip on the sedimentary architecture and preservation potential in active estuarine sedimentary environments.

Pseudo-3D imaging of a low-slip-rate, active normal fault using shallow geophysical methods: The Geleen fault in the Belgian Maas River valley

In a detailed site survey for paleoseismic trenching, we applied shallow geophysical prospecting techniques, including ground-penetrating radar (GPR), electric resistivity tomography (ERT), and resistivity mapping to identify, locate, and visualize in 3D the Geleen fault, an active normal fault bordering the Roer Valley graben in northeast Belgium. Because of a low slip rate, the geomorphic expression of this fault is very faint in the relatively young deposits of the Maas River valley. ERT profiles show the fault as a broad, near-vertical anomaly characterized by sharp lateral resistivity contrasts, with an associated vertical offset of sediment layers. We observed offsets of [Formula: see text] and [Formula: see text] for the base and top, respectively, of a middle-late Pleistocene fluvial gravel layer. Shallow ERT and GPR profiles indicate that younger sediments are also affected by faulting, but the amount and sense of offset appear to be conflicting: ERT profiles show a near-surface, high-resistivity layer with an apparent reverse offset, and GPR profiles portray the fault as a sharp and laterally consistent disruption of reflectors, often without a clearly identifiable offset. Resistivity maps at different depths map the fault as a narrow, linear, lateral resistivity gradient matching the anomalies observed with other techniques. This method proved to be efficient in determining the precise position and orientation of dip-slip faults, and could potentially be very useful for the identification of lateral changes in fault geometry, such as splays and step-overs. Subsequent trenching confirmed the presence of a normal fault at the location predicted by the geophysical survey. Correlation with the sediments exposed on the trench walls demonstrated that, close to the surface, resistivity and dielectric permittivity contrasts mostly occur in a postdepositional soil, which developed differently on either side of the fault. This explains why shallow geophysical variations do not reflect the true fault offset.

Identification and removal of above‐ground spurious signals in GPR archaeological prospecting

AbstractGround‐penetrating radar (GPR) is a shallow geophysical method increasingly used in archaeological prospecting to detect buried remains and to map the stratigraphy of the uppermost earth layers embodying the archaeological features. The GPR sections, however, can be contaminated by spurious correlated signals caused by above‐ground obstacles (buildings, trees, fences, power lines) placed either in‐line or off‐line with respect to the profile (surface scattering). In the case study presented this problem is analysed for a 35 MHz GPR survey carried out for stratigraphical purposes in the archaeological site of the Roman Ships near Pisa (Italy). The investigation inside the archaeological excavation, when the level was about 5 m below the ground surface, produced a severe surface scattering problem caused by the iron sheet‐piling protecting the excavation walls and bordering some of the partly excavated boats. An attempt to interpret the profiles was carried out in a zone where the relatively simple geometry of the metallic enclosure allowed understanding of the possible origin of spurious events, also thanks to the high density of profiles acquired along two orthogonal directions. Migration at the air velocity and geometrical considerations helped the identification of hyperbolic and slightly slanted features as surface scattering phenomena from different sides of the iron sheet‐piling. A simple but original subtraction procedure was successful for the attenuation of some of the spurious reflections. After this partial removal, other spurious signals could be recognized more easily as well as two weak subhorizontal reflections of probable stratigraphical meaning. Subsequently, filtering procedures based on f‐k and Radon transform methods were tried to further reduce the spurious signals, thus enhancing the visibility of the interesting reflections. Copyright © 2005 John Wiley & Sons, Ltd.