Helicopter-borne Electromagnetic Data Research Articles

A comparison of 2D inversion of RMT and 1D weighted joint inversion of ground-based and helicopter-borne electromagnetic data

A Pleistocene buried valley, the Cuxhaven Valley, located in Northern Germany, is extensively studied by three different electromagnetic methods. In order to enhance the resolution of the very shallow subsurface structures, the radiomagnetotelluric (RMT) measurements were conducted on the buried valley in addition to the helicopter-borne electromagnetic (HEM) and transient electromagnetic (TEM) data, which were available from the study area along the same profile. First, the RMT soundings are interpreted using the conventional 1D and 2D inversion algorithms. The RMT interpreted models inferred two-layer structure of the subsurface up to a depth of 30 m, which comprises a resistive layer at the top. Available lithological data is used for correlating the resistivity values derived from the 2D inversion of the RMT data with the subsurface formation. Subsequently, a 1D weighted joint inversion algorithm is utilized to interpret these three data sets jointly. The 2D interpreted models of RMT soundings are compared with the 1D weighted joint interpreted models of RMT, HEM, and TEM soundings.

Multi-Line 1D Inversion of Frequency-Domain Helicopter-Borne Electromagnetic Data with Weighted 3D Smoothness Regularization: A Case Study from Northern Iran

An efficient pseudo-3D Occam’s inversion scheme is proposed here to stabilize the traditional 1D inversion for the frequency-domain helicopter-borne electromagnetic (FHEM) data. In this scheme, multiple flight lines are inverted simultaneously for layered 1D models minimizing a common objective function with lateral, vertical, and cross-line in the model regularization function. Applying the lateral, vertical, and cross-line weighting factors into the regularization matrix yields a more stable solution and produces geologically more realistic results. In addition, we investigate how the errors of height measurements obscure the FHEM response and affect recovered resistivity models. In this inversion, attempt is made to recover a correct altitude that deals with distortions caused by the presence of measurement height errors in the reconstructed resistivity models. The comparison among 1D, pseudo-2D, and pseudo-3D Occam’s inversions is made through the analysis of data from two different 3D synthetic models and one field dataset acquired from the north of Iran. The results indicate that pseudo-3D Occam’s inversion provides fewer inversion artifacts, better model recognition, and smoother and more continuous models, while, reduces the effects of data noise in an effective manner.

Development of a novel approach for recovering SIP effects from 1-D inversion of HEM data: Case study from the Alut area, northwest of Iran

We have developed a novel approach for recovering the induced polarization (IP) effects from one-dimensional (1-D) inversion of helicopter-borne electromagnetic data (HEM). To achieve this aim, we incorporate both electromagnetic and spectral induced polarization effects within the HEM data by taking the frequency dependence behaviour of resistivity/conductivity of polarizable formation into account. Furthermore, we have included the EM sensor altitude, magnetic permeability and dielectric permittivity of subsurface layers in order to conduct the full solution of forward operator. In this regard, we have calculated the magnetic permeability from the inversion of aeromagnetic data and incorporated the sensor altitude and dielectric permittivity as additional model parameters to be recovered using the inversion of the HEM data. We have carried out both forward and inverse modelling of the HEM data by implementing fast Hankel transform and hybrid algorithm including very fast simulated annealing (VFSA) and Marquardt-Levenberg (ML) strategies, respectively. This approach enables us to recover model parameters comprising of relaxation parameters of Cole-Cole model, dielectric permittivity and thickness of layered half space model and sensor altitude above the earth surface using 1-D inverse modelling of HEM data.We have tested our approach on both synthetic and field DIGHEM data. Implementing the forward solution indicates that incorporation of the IP effects may shift the frequency domain electromagnetic (FDEM) response so that it yields the sign reversal signature in the in-phase component at low frequencies. Moreover, the results of the inverse modelling of the HEM data show that the IP effect could be recovered using our approach effectively. It provides fast and cost-effective quantitative tool for recovering the IP effect from HEM data for the sake of more precise characterization of buried polarizable formations in the wide area. In addition, incorporation the IP effect along with magnetic permeability and sensor altitude provides a significant degree of improvement in the inversion and interpretation of negative responses occurred in the in-phase component at low frequencies.

Characterization of a regional coastal zone aquifer using an interdisciplinary approach – an example from Weser-Elbe region, Lower Saxony, Germany

In this study, interdisciplinary approaches are considered to characterize the coastal zone aquifer of the Elbe-Weser region in the North of Lower Saxony, Germany. Geological, hydrogeological, geochemical and geophysical information have been considered to analyze the current status of the aquifers. All the information collectively states that the salinity distribution in the subsurface is heterogeneous both horizontally and vertically. Early age flooding also contributed to this heterogeneity. No general classification of groundwater quality (according to some piper diagrams) could be identified. Helicopter-borne electro-magnetic data clearly show the presence of freshwater reserves below the sea near the west coast. Groundwater recharge largely happens in the moraine ridges (west side of the area) where both the surface elevation and the groundwater level are high. Consequently, submarine groundwater discharge occurs from the same place. All these information will facilitate to develop the planned density driven groundwater flow and transport model for the study area.

Sea-ice thickness from field measurements in the northwestern Barents Sea

The Barents Sea is one of the fastest changing regions of the Arctic, and has experienced the strongest decline in winter-time sea-ice area in the Arctic, at -23 +/- 4% per decade. Sea-ice thickness in the Barents Sea is not well studied. We present two previously unpublished helicopter-borne electromagnetic (HEM) ice thickness measurements from the northwestern Barents Sea acquired in March 2003 and 2014. The HEM data are compared to ice thickness calculated from ice draft measured by ULS deployed between 1994 and 1996. These data show that ice thickness varies greatly from year to year; influenced by the thermodynamic and dynamic processes that govern local formation vs long-range advection. In a year with a large inflow of sea-ice from the Arctic Basin, the Barents Sea ice cover is dominated by thick multiyear ice; as was the case in 2003 and 1995. In a year with an ice cover that was mainly grown in situ, the ice will be thin and mechanically unstable; as was the case in 2014. The HEM data allow us to explore the spatial and temporal variability in ice thickness. In 2003 the dominant ice class was more than 2 years old; and modal sea-ice thickness varied regionally from 0.6 to 1.4 m, with the thinner ice being either first-year ice, or multiyear ice which had come into contact with warm Atlantic water. In 2014 the ice cover was predominantly locally grown ice less than 1 month old (regional modes of 0.5–0.8 m). These two situations represent two extremes of a range of possible ice thickness distributions that can present very different conditions for shipping traffic; or have a different impact on heat transport from ocean to atmosphere.

Geophysical and geotechnical studies of geology and sediment properties at a quick-clay landslide site at Esp, Trondheim, Norway

Increased urbanisation and development in previous marginal areas have led to the need to protect the population, the environment and infrastructure from hazards such as quick-clay landslides. In this paper the opportunity which was presented following the gathering of a large amount of data in the area of the quick-clay landslide at Esp, Trondheim, Norway in 2012 is explored. The data is of particular interest given the availability of Frequency domain Helicopter-borne ElectroMagnetic data (FHEM) and extensive LiDAR imaging as well as a full range of in situ geophysical and geotechnical soundings and samples. The ground conditions in the area are shown to be complex and it is clear that a multi-disciplinary type approach combining geophysical, geomorphological and geotechnical investigations is necessary in order to understand the hazards posed by potential quick-clay landslides. The investigation shows that very good overview mapping of the area can be obtained from FHEM and that the data obtained from local electrical resistivity tomography (ERT) and resistivity cone (RCPTU) are consistent with each other and with the FHEM data. Use of ERT immediately after the landslide was particularly useful in delineating further risk areas and as an aid for assessment of evacuation of the local population. LiDAR data is particularly useful in understanding the landslide “trigger” mechanisms, the depth of the landslide, the form of the run-out and the nature of the material in the run-out. Geotechnical data collected in the area is characteristic of that of soft sensitive Norwegian marine clays and the various geotechnical and geophysical parameters correlate well with one another and also follow well-established patterns. The data shows that no single investigative method is available to accurately define quick-clay layers but that a number of complimentary in situ and laboratory techniques are necessary for that purpose.

A cut-&-paste strategy for the 3-D inversion of helicopter-borne electromagnetic data — II. Combining regional 1-D and local 3-D inversion

As a standard procedure, multi-frequency helicopter-borne electromagnetic (HEM) data are inverted to conductivity–depth models using 1-D inversion methods, which may, however, fail in areas of strong lateral conductivity contrasts (so-called induction anomalies). Such areas require more realistic multi-dimensional modelling. Since the full 3-D inversion of an entire HEM data set is still extremely time consuming, our idea is to combine fast 1-D and accurate but numerically expensive 3-D inversion of HEM data in such a way that the full 3-D inversion is only carried out for those parts of a HEM survey which are affected by induction anomalies. For all other parts, a 1-D inversion method is sufficient. We present a newly developed algorithm for identification, selection, and extraction of induction anomalies in HEM data sets and show how the 3-D inversion model of the anomalous area is re-integrated into the quasi-1-D background. Our proposed method is demonstrated to work properly on a synthetic and a field HEM data set from the Cuxhaven tunnel valley in Germany. We show that our 1-D/3-D approach yields better results compared to 1-D inversions in areas where 3-D effects occur.

A cut-&-paste strategy for the 3-D inversion of helicopter-borne electromagnetic data — I. 3-D inversion using the explicit Jacobian and a tensor-based formulation

We present an inversion concept for helicopter-borne frequency-domain electromagnetic (HEM) data capable of reconstructing 3-D conductivity structures in the subsurface. Standard interpretation procedures often involve laterally constrained stitched 1-D inversion techniques to create pseudo-3-D models that are largely representative for smoothly varying conductivity distributions in the subsurface. Pronounced lateral conductivity changes may, however, produce significant artifacts that can lead to serious misinterpretation. Still, 3-D inversions of entire survey data sets are numerically very expensive. Our approach is therefore based on a cut-&-paste strategy whereupon the full 3-D inversion needs to be applied only to those parts of the survey where the 1-D inversion actually fails. The introduced 3-D Gauss–Newton inversion scheme exploits information given by a state-of-the-art (laterally constrained) 1-D inversion. For a typical HEM measurement, an explicit representation of the Jacobian matrix is inevitable which is caused by the unique transmitter–receiver relation. We introduce tensor quantities which facilitate the matrix assembly of the forward operator as well as the efficient calculation of the Jacobian. The finite difference forward operator incorporates the displacement currents because they may seriously affect the electromagnetic response at frequencies above 100. Finally, we deliver the proof of concept for the inversion using a synthetic data set with a noise level of up to 5%.

Towards an improved geological interpretation of airborne electromagnetic data: a case study from the Cuxhaven tunnel valley and its Neogene host sediments (northwest Germany)

AbstractAirborne electromagnetics (AEM) is an effective technique for geophysical investigations of the shallow subsurface and has successfully been applied in various geological settings to analyse the depositional architecture of sedimentary systems for groundwater and environmental purposes. However, interpretation of AEM data is often restricted to 1D inversion results imaged on resistivity maps and vertical resistivity sections. The integration of geophysical data based on AEM surveys with geological data is often missing and this deficiency can lead to uncertainties in the interpretation process. The aim of this study is to provide an improved methodology for the interpretation of AEM data and the construction of more realistic 3D geological subsurface models. This is achieved by the development of an integrated workflow and 3D modelling approaches based on combining different geophysical and geological data sets (frequency-domain helicopter-borne electromagnetic data (HFEM), time-domain helicopter-borne electromagnetic data (HTEM), three 2D reflection seismic sections and 488 borehole logs). We used 1D inversion results gained from both HFEM and HTEM surveys and applied a 3D resistivity gridding procedure based on geostatistical analyses and interpolation techniques to create continuous 3D resistivity grids. Subsequently, geological interpretations have been performed by combining with, and validation against, borehole and reflection seismic data. To verify the modelling results and to identify uncertainties of AEM inversions and interpretation, we compared the apparent resistivity values of the constructed 3D geological subsurface models with those of AEM field measurements. Our methodology is applied to a test site near Cuxhaven, northwest Germany, where Neogene sediments are incised by a Pleistocene tunnel valley. The Neogene succession is subdivided by four unconformities and consists of fine-grained shelf to marginal marine deposits. At the end of the Miocene an incised valley was formed and filled with Pliocene delta deposits, probably indicating a palaeo-course of the River Weser or Elbe. The Middle Pleistocene (Elsterian) tunnel valley is up to 350 m deep, 0.8–2 km wide, and incised into the Neogene succession. The unconsolidated fill of the Late Miocene to Pliocene incised valley probably formed a preferred pathway for the Pleistocene meltwater flows, favouring the incision. Based on the 3D AEM resistivity model the tunnel-valley fills could be imaged in high detail. They consist of a complex sedimentary succession with alternating fine- and coarse-grained Elsterian meltwater deposits, overlain by glaciolacustrine (Lauenburg Clay Complex) and marine Holsteinian interglacial deposits. The applied approaches and results show a reliable methodology, especially for future investigations of similar geological settings. The 3D resistivity models clearly allow a distinction to be made between different lithologies and enables the detection of major bounding surfaces and architectural elements.

Calculation of One-dimensional Forward Modelling of Helicopter- borne Electromagnetic Data and a Sensitivity Matrix Using Fast Hankel Transforms

The helicopter-borne electromagnetic (HEM) frequency-domain exploration method is an airborne electromagnetic (AEM) technique that is widely used for vast and rough areas for resistivity imaging. The vast amount of digitized data flowing from the HEM method requires an efficient and accurate inversion algorithm. Generally, the inverse modelling of HEM data in the first step requires a precise and efficient technique provided by a forward modelling algorithm. The exact calculation of the sensitivity matrix or Jacobian is also of the utmost importance. As such, the main objective of this study is to design an efficient algorithm for the forward modelling of HEM frequency-domain data for the configuration of horizontal coplanar (HCP) coils using fast Hankel transforms (FHTs). An attempt is also made to use an analytical approach to derive the required equations for the Jacobian matrix. To achieve these goals, an elaborated algorithm for the simultaneous calculation of the forward computation and sensitivity matrix is provided. Finally, using two synthetic models, the accuracy of the calculations of the proposed algorithm is verified. A comparison indicates that the obtained results of forward modelling are highly consistent with those reported in Simon et al. (2009) for a four-layer model. Furthermore, the comparison of the results for the sensitivity matrix for a two-layer model with those obtained from software is being used by the BGR Centre in Germany, showing that the proposed algorithm enjoys a high degree of accuracy in calculating this matrix.

Numerical modelling of climate change impacts on freshwater lenses on the North Sea Island of Borkum using hydrological and geophysical methods

Abstract. A numerical, density dependent groundwater model is set up for the North Sea Island of Borkum to estimate climate change impacts on coastal aquifers and especially the situation of barrier islands in the Wadden Sea. The database includes information from boreholes, a seismic survey, a helicopter-borne electromagnetic (HEM) survey, monitoring of the freshwater-saltwater boundary by vertical electrode chains in two boreholes, measurements of groundwater table, pumping and slug tests, as well as water samples. Based on a statistical analysis of borehole columns, seismic sections and HEM, a hydrogeological model is set up. The groundwater model is developed using the finite-element programme FEFLOW. The density dependent groundwater model is calibrated on the basis of hydraulic, hydrological and geophysical data, in particular spatial HEM and local monitoring data. Verification runs with the calibrated model show good agreement between measured and computed hydraulic heads. A good agreement is also obtained between measured and computed density or total dissolved solids data for both the entire freshwater lens on a large scale and in the area of the well fields on a small scale. For simulating future changes in this coastal groundwater system until the end of the current century, we use the climate scenario A2, specified by the Intergovernmental Panel on Climate Change and, in particular, the data for the German North Sea coast. Simulation runs show proceeding salinisation with time beneath the well fields of the two waterworks Waterdelle and Ostland. The modelling study shows that the spreading of well fields is an appropriate protection measure against excessive salinisation of the water supply until the end of the current century.

Accurate 1D forward and inverse modeling of high-frequency helicopter-borne electromagnetic data

Frequency-domain helicopter-borne electromagnetic (HEM) data are commonly interpreted using quasistatic approximations in forward and inverse solutions. At high-frequencies ([Formula: see text]), the accuracy of this approach is often insufficient. Implementation of a full solution of the forward problem that includes displacement currents in both subsurface and air, however, may cause singularity problems during numerical evaluation of the secondary field integral using Hankel or Laplace transforms, particularly at high frequencies. These effects can be reduced by a wavenumber shift during numerical evaluation, a transformation of the integral by partial integration, or a combination of both. Based on these corrections, the resulting forward responses obtained with various fast Hankel transforms or numerical Laplace transforms are nearly identical. The inversion of HEM data also requires some modification. For a half-space inversion, the parameter sets, such as look-up tables, linear or polynomial approximations have to be modified for frequencies above some 10 kHz. The quasistatic formulas used in multi-layer inversion procedures have to be replaced by the accurate formulas. Fortunately, singularity problems are restricted to the forward calculation as the derivatives of the Jacobian matrix are not affected. Accurate high-frequency forward and inverse modeling enables a successful usage of helicopter-borne electromagnetics for near-surface investigations and expands the range of applications in environmental and geotechnical issues.

Utilizing airborne electromagnetic data to map groundwater salinity and salt store at Chowilla, SA

The Chowilla floodplain in South Australia is part of the Bookmark Biosphere Reserve with wetlands supporting a wide range of plants and animals, and has provided recreational opportunities to the general public (MDBC, 2006). However, the threats from saline groundwater and salt accumulation at the surface, combined with the overall low rainfall and the lack of natural flooding events in the recent past, have had a negative consequence for vegetation health, especially the river red gums. Information on the spatial patterns of groundwater salinity and the near surface (top 15 m) salt stores in the floodplain is integral to our understanding of floodplain hydrological processes that may be active in the Lower Murray Basin of Australia. This information can be used to help develop appropriate remedial activities for the management of ecologically important wetlands and in particular assist in the maintenance of a healthy vegetation community. A helicopter-borne electromagnetic (HEM) survey was conducted across the floodplain in conjunction with a validation drilling and borehole geophysical logging program. Based on the laboratory results from cored sediments and their pore fluids, we report on the salinity and salt store models calculated from the inverted HEM data.

Levelling of helicopter-borne frequency-domain electromagnetic data

For about three decades helicopter-borne electromagnetic (HEM) measurements have been used to reveal the resistivity distribution of the upper one hundred metres of the earth's subsurface. HEM systems record secondary fields, which are 3–6 orders of magnitude smaller than the transmitted primary fields. As both the primary fields and the secondary fields are present at the receivers, well-designed bucking coils are often used to reduce the primary fields at the receivers to a minimum. Remaining parts of the primary fields, the zero levels, are generally corrected by subtracting field values recorded at high altitudes (standard zero levelling) or estimated from resistivities of neighbouring lines or from resistivity maps (advanced zero levelling). These zero-levelling techniques enable the correction for long-term, quasi-linear instrumental drift. Short-term variations caused by temperature changes due to altitude variations, however, cannot be completely corrected by this procedure resulting in stripe patterns on thematic maps. Statistical methods and/or 2-D filter techniques called statistical levelling (tie-line levelling) and empirical levelling (microlevelling), respectively, used to correct stripe patterns in airborne geophysical data sets are, in general, not directly applicable to HEM data. Because HEM data levelling faces the problem that the parameter affected by zero-level errors, the secondary field, differs from the parameter generally levelled, the apparent resistivity. Furthermore, the dependency of the secondary field on both the resistivity of the subsurface and the sensor altitude is strongly nonlinear. A reasonable compromise is to microlevel both half-space parameters: apparent resistivity and apparent depth, followed by a recalculation of the secondary field components based on the half-space parameters levelled. Advantages and disadvantages of the diverse levelling techniques are discussed using a HEM data set obtained in a hilly region along the Saale River between the cities of Saalfeld and Jena in central Germany. It turns out from a comparison of apparent resistivity and apparent depth maps derived from levelled HEM data that manually advanced zero levelling of major level errors and automatic microlevelling of remaining minor level errors yield the best results.

Repeatability study of helicopter-borne electromagnetic data

Helicopter-borne electromagnetic (EM) responses depend very much upon the altitude and plan-view flight path, especially when the resistivity of the terrain’s materials varies laterally and/or vertically. Spatially consistent flight paths are required for repeatability analysis of the EM data. Caution should be used in examining the repeatability of the EM data because poor repeatability could result from spatially inconsistent flight paths. However, the apparent resistivity converted from the EM responses is virtually independent of the sensor altitude and directly reflects variations in the resistivity. Therefore, more meaningful repeatability analyses are achieved if the apparent resistivity is used instead of the EM response itself. We have analyzed 32 flights over a control line by using the EM amplitude, the phase, and the apparent resistivity. Our results show that the crosscorrelation for all 496 paired combinations of flights is better for the apparent resistivity than for the EM amplitude or phase. The apparent-resistivity data have average correlation coefficients from 0.89 to 0.94 as the frequency increases, whereas the amplitude and the phase data have average correlation coefficients from 0.78 to 0.85 without obvious frequency dependency.

Robust 1D inversion and analysis of helicopter electromagnetic (HEM) data

Ground-based electrical and electromagnetic methods are used systematically for quantitative hydrogeologic investigations in Denmark. In recent years, a desire for faster and more cost-efficient methods has led to growing interest in the possibility of using airborne systems, and in 2001 a number of test flights were performed using a frequency-domain, helicopter-borne electromagnetic (HEM) system. We perform a theoretical examination of the resolution capabilities of the applied system. Quantitative model parameter analyses show that the system only weakly resolves conductive, near-surface layers but can resolve layer boundary to a depth of more than 100 m. Modeling experiments also show that the effect of altimeter errors on the inversion results is serious. We suggest a new interpretation scheme for HEM data founded solely on full nonlinear 1D inversion and providing layered-earth models supported by data misfit parameters and a quantitative model-parameter analysis. The backbone of the scheme is the removal of cultural coupling effects followed by a multilayer inversion that in turn provides reliable starting models for a subsequent few-layer inversion. A new procedure for correlation in the model space ensures model sections with slow lateral variations in resistivity, normally assumed in sedimentary environments. A field example from a Danish survey demonstrates that the interpretation scheme can produce satisfactory results within the limitations of the system.

Rapid three-dimensional inversion of multi-transmitter electromagnetic data using the spectral Lanczos decomposition method

In this paper, we develop a new method of three-dimensional (3D) inversion of multi-transmitter electromagnetic data. We apply the spectral Lanczos decomposition method (SLDM) in the framework of the localized quasi-linear inversion introduced by Zhdanov and Tartaras (2002 Geophys. J. Int. 148 506–19). The SLDM makes it possible to find the regularized solution of the ill-posed inverse problem for all values of the regularization parameter α at once. As an illustration, we apply this technique for interpretation of the helicopter-borne electromagnetic (HEM) data over inhomogeneous geoelectrical structures, typical for mining exploration. This technique helps to accelerate HEM data inversion and provides a stable and focused image of the geoelectrical target. The new method and the corresponding computer code have been tested on synthetic data. The case history includes interpretation of HEM data collected by INCO Exploration in the Voisey's Bay area of Canada.

Topographic modelling and correction in frequency-domain airborne electromagnetics

Many helicopter-borne electromagnetic (HEM) surveys have been carried out in mountainous areas. However, the effects of topography on the HEM response have not been reported in the literature for a long time. We simulate the response to topography using a staggered-grid finite-difference method. Modelling shows that a hill produces a high-resistivity anomaly over its top and a low-resistivity anomaly over its foot (when the magnetic-field response is transformed into the apparent resistivity) and that topographic effects increase with increasing frequency. In order to reduce those effects, a simple correction procedure is presented and tested on synthetic data. Results indicate that the corrected data do not reproduce the effects of the actual resistivity structure accurately enough to permit the quantitative interpretation assuming a flat-earth model. The reason for this is that the geometrical relationship between the coil system and the subsurface structure changes. The most rigorous and accurate approach to interpreting HEM data with topographic effects is to incorporate a forward-solution scheme capable of modelling topography into inversions. A 3-D inversion method is successfully tested on synthetic data.F2

Topographic effects in frequency-domain helicopter-borne electromagnetics

Many helicopter-borne electromagnetic (HEM) surveys have been carried out in mountainous areas. However, there have been few studies of the effects of topography on the HEM response reported in the literature. We simulate the response to topography using a staggered-grid finite-difference method. Modelling shows that a hill produces a high-resistivity anomaly over its top, and a low-resistivity anomaly over its foot, when the magnetic field response is transformed into the apparent resistivity. In order to reduce these effects, a simple correction procedure is presented and tested on synthetic data. Results indicate that the corrected data do not reproduce the effects of the actual resistivity structure accurately enough to permit quantitative interpretation assuming a flat-earth model. The reason for this is that the geometrical relationship between the coil system and the subsurface structure changes. The most rigorous and accurate approach to interpreting HEM data with topographic effects is to incorporate a forward-solution scheme, capable of modelling topography, into inversions. A 3D inversion method is successfully tested on synthetic data.

Landslide surveys in Tertiary soft rock areas using HEM

This paper describes case histories for landslide surveys in Tertiary soft rock areas in Japan using a Helicopter-borne Electromagnetic (HEM) system operating at five frequencies between 220 Hz and 137,500 Hz.The usual method for conducting landslide surveys consists of airphoto interpretation, field investigation, and test borings. The ground dc resistivity method is seldom used for landslide surveys, because of the intermixture of topographic effects, the lack of uniformity of data and the difficulty of obtaining closely spaced survey lines along the slopes. On the other hand, HEM data are densely sampled, are very uniform over a survey area and are not as likely to suffer from topographic effects since the HEM “footprint” is relatively small compared with terrain changes. These factors make HEM measurements ideal for detecting the small resistivity changes caused by the landslides.HEM results show that the landslide locations coincide well with anomalously low resistivity zones in HEM apparent resistivity profiles and maps. This correlation occurs because the sliding surfaces of Tertiary type landslides are always accompanied by montmorillonite clay, which lowers the resistivity of the rocks. Furthermore, landslide locations very often correspond to an upheaval or raised pattern of low-resistivity zones in imaged AEM-resistivity sections. Shallow low-resistivity zones such as sliding surfaces also lower the apparent resistivity values at lower frequencies.The HEM method is effective in detecting and delineating landslides associated with cohesive soil, collapsed soil, or weathered zones in areas of Tertiary soft rocks.