Helicopter-borne Frequency-domain Electromagnetic Data Research Articles

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.

3D interpretation of helicopter-borne frequency-domain electromagnetic (HEM) data from Rams\xe5 Basin and adjacent areas at And\xf8ya, Norway

The Ramså Basin is situated on the strandflat on the northeastern side of Andøya between Ramså and Skarstein. It is the only known Mesozoic sedimentary basin onshore Norway and therefore represents an easily accessible analogue to study the setting and development of the offshore Mesozoic basins along the Norwegian continental shelf. Basement rocks (100–10,000 Ωm) and sedimentary rocks (5–1000 Ωm) have distinct ranges of the electrical resistivity. Resistivity of the subsurface can be investigated by electric and electromagnetic (EM) methods to delineate the extent of the sedimentary basin. This paper presents the interpretation of helicopter-borne frequency-domain electromagnetic (HEM) data and their correlation with data obtained from borehole logging and two-dimensional (2D) electrical resistivity tomography (ERT). Resistivity data obtained from these methods show a good agreement. Spatially constrained inversion (SCI) of HEM presents a three-dimensional (3D) subsurface resistivity image of a larger area in comparison to what could have been drawn from few ERT lines and borehole logging. HEM interpretation together with other ERT and borehole resistivity data indicates the extent of Ramså Basin beyond the presently mapped boundary. This extension is also supported by interpretation of earlier collected gravity, magnetic and seismic data. The Ramså sedimentary basin shows a wide range of electrical resistivity between ca. 10 and 600 Ωm at different depths. Two highly conductive areas are newly identified by HEM data interpretation. In one of these areas, outcropping graphite and sulfide minerals were found by field observations.

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.

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.

Geological structure imaging from airborne electromagnetic and magnetic data, a case study in Kalat-e-Reshm area, Iran

In this paper, a set of airborne magnetic data and helicopter-borne frequency domain electromagnetic data are used in order to retrieve a joint model of magnetic susceptibility and electrical conductivity. The study area called Kalat-e-Reshm located in Semnan province of Iran consists of an arc-shaped porphyry andesite covered by sedimentary units which has high potential of mineral occurrences especially porphyry copper. Based upon Occam approach assuming a multilayered earth model with fixed thicknesses, a conductivity–depth plot from airborne electromagnetic data is generated to image the desired porphyry type geological structure. The 3D visualization of the 1D conductivity models along all flight lines provided a resistive geological unit which corresponds to the desired porphyry andesite. A multi-constraint method is also applied on the airborne magnetic data to recover a 3D distribution of magnetic susceptibility model which reveals extension of an arc-shaped porphyry andesite at depth. The high susceptibility, low conductivity porphyry andesite shows a thicker structure than what is shown on the geological map and extends to depth.

Approximate resistivity and susceptibility mapping from airborne electromagnetic and magnetic data, a case study for a geologically plausible porphyry copper unit in Iran

This paper describes the application of approximate methods to invert airborne magnetic data as well as helicopter-borne frequency domain electromagnetic data in order to retrieve a joint model of magnetic susceptibility and electrical resistivity. The study area located in Semnan province of Iran consists of an arc-shaped porphyry andesite covered by sedimentary units which may have potential of mineral occurrences, especially porphyry copper. Based on previous studies, which assume a homogenous half-space earth model, two approximate methods involving the Siemon and the Mundry approaches are used in this study to generate a resistivity-depth image of underground geologically plausible porphyry unit derived from airborne electromagnetic data. The 3D visualization of the 1D inverted resistivity models along all flight lines provides a resistive geological unit which corresponds to the desired porphyry andesite. To reduce uncertainty arising from single geophysical model, i.e., the resistivity model acquired from the frequency domain electromagnetic data, a fast implementable approach for 3D inversion of magnetic data called the Lanczos bidiagonalization method is also applied to the large scale airborne magnetic data in order to construct a 3D distribution model of magnetic susceptibility, by which the obtained model consequently confirms the extension of an arc-shaped porphyry andesite at depth. The susceptible-resistive porphyry andesite model provided by integrated geophysical data indicates a thicker structure than what is shown on the geological map while extends down at depth. As a result, considering simultaneous interpretation of airborne magnetic and frequency domain electromagnetic data certainly yield lower uncertainty in the modeling of andesite unit as a potential source of copper occurrences.

Inverse modeling of HEM data using a new inversion algorithm

Helicopter-borne frequency-domain electromagnetic (HEM) surveys are used extensively for mineral and groundwater exploration and a number of environmental investigations. To have a meaningful interpretation of the measured multi- frequency HEM data, in addition to the resistivity maps which are provided in each frequency or for some particular depth levels, it is a necessity to have a suitable modeling technique to produce resistivity cross-section along some specific profiles. This paper aims to: (1) develop a new inversion method to handle HEM data; (2) compare its results with the well known Amplitude, Niblett-Bostick (NB), and Siemon inversion methods. The basic formulation of this new inversion routine was provided based on the Zonge spatial filtering procedure to cure static shift effect on the magnetotelluric (MT) apparent resistivity curves. When the relevant formulas and the required algorithm for the inverse modeling of HEM data were provided, they were then coded in Matlab software environment. This new inversion program, named as SUTHEM, was used to invert some sets of one and two dimensional (1D and 2D) model synthetic data which were contaminated by random noise. It was also applied to invert one set of real field data acquired in the NW part of Iran by the DIGHEM system. The obtained results of this method and their comparison with those of the aforementioned methods indicate that SUTHEM is able to produce the results like those produced by the commercial Siemon routine. In addition, the new inversion method is superior to the Amplitude and the NB methods particularly in inversion of the noisy data.

Laterally constrained inversion of helicopter-borne frequency-domain electromagnetic data

Helicopter-borne frequency-domain electromagnetic (HEM) surveys are used for fast high-resolution, three-dimensional resistivity mapping. Standard interpretation tools are often based on layered earth inversion procedures which, in general, explain the HEM data sufficiently. As a HEM system is moved while measuring, noise on the data is a common problem. Generally, noisy data will be smoothed prior to inversion using appropriate low-pass filters and consequently information may be lost. For the first time the laterally constrained inversion (LCI) technique has been applied to HEM data combined with the automatic generation of dynamic starting models. The latter is important because it takes the penetration depth of the electromagnetic fields, which can heavily vary in survey areas with different geological settings, into account. The LCI technique, which has been applied to diverse airborne and ground geophysical data sets, has proven to be able to improve the HEM inversion results of layered earth structures. Although single-site 1-D inversion is generally faster and — in case of strong lateral resistivity variations — more flexible, LCI produces resistivity — depth sections which are nearly identical to those derived from noise-free data. The LCI results are compared with standard single-site Marquardt–Levenberg inversion procedures on the basis of synthetic data as well as field data. The model chosen for the generation of synthetic data represents a layered earth structure having an inhomogeneous top layer in order to study the influence of shallow resistivity variations on the resolution of deep horizontal conductors in one-dimensional inversion results. The field data example comprises a wide resistivity range in a sedimentary as well as hard-rock environment. If a sufficient resistivity contrast between air and subsurface exists, the LCI technique is also very useful in correcting for incorrect system altitude measurements by using the altitude as a constrained inversion parameter.