Step-on versus step-off signals in time-domain controlled source electromagnetic methods using a grounded electric dipole

The time‐domain controlled source electromagnetic method is a geophysical prospecting tool applied to image the subsurface resistivity distribution on land and in the marine environment. In its most general setup, a square‐wave current is fed into a grounded horizontal electric dipole, and several electric and magnetic field receivers at defined offsets to the imposed current measure the electromagnetic response of the Earth. In the marine environment, the application often uses only inline electric field receivers that, for a 50% duty‐cycle current waveform, include both step‐on and step‐off signals. Here, forward and inverse 1D modelling is used to demonstrate limited sensitivity towards shallow resistive layers in the step‐off electric field when transmitter and receivers are surrounded by conductive seawater. This observation is explained by a masking effect of the direct current signal that flows through the seawater and primarily affects step‐off data. During a step‐off measurement, this direct current is orders of magnitude larger than the inductive response at early and intermediate times, limiting the step‐off sensitivity towards shallow resistive layers in the seafloor. Step‐on data measure the resistive layer at times preceding the arrival of the direct current signal leading to higher sensitivity compared to step‐off data. Such dichotomous behaviour between step‐on and step‐off data is less obvious in onshore experiments due to the lack of a strong overlying conductive zone and corresponding masking effect from direct current flow. Supported by synthetic 1D inversion studies, we conclude that time‐domain controlled source electromagnetic measurements on land should apply both step‐on and step‐off data in a combined inversion approach to maximise signal‐to‐noise ratios and utilise the sensitivity characteristics of each signal. In an isotropic marine environment, step‐off electric fields have inferior sensitivity towards shallow resistive layers compared to step‐on data, resulting in an increase of non‐uniqueness when interpreting step‐off data in a single or combined inversion

2-D Joint Inversion of Semi-Airborne CSEM and LOTEM Data in Eastern Thuringia, Germany

Various electromagnetic (EM) techniques have been developed for exploring natural resources. The novel frequency-domain semi-airborne controlled source electromagnetic (semi-AEM) method takes advantages of both ground and airborne techniques. It combines ground-based high-power electrical dipole sources with large scale and spatially densely covered magnetic fields measured via airborne receivers. The method can survey the subsurface down to approximately 1000 m and is particularly sensitive towards conductive bodies (e.g. mineralized bodies) in a more resistive host environment. However, the signal-to-noise ratio of semi-AEM is lower than that of ground-based methods such as long-offset transient electromagnetics (LOTEM), mainly due to the limited stacking time and motion induced noise. As a result, the semi-AEM often has reduced depth of investigation in comparison to LOTEM. One solution to overcome these flaws is to analyse and interpret semi-AEM data together with information from other EM methods using a joint inversion. Since our study shows that LOTEM and semi-AEM data have complementary subsurface resolution capabilities, we present a 2-D joint inversion algorithm to simultaneously interpret frequency-domain semi-AEM data and transient electric fields using extended dipole sources. The algorithm has been applied to the field data acquired in a former mining area in eastern Thuringia, Germany. The 2-D joint inversion combines the complementary information and provides a meaningful 2-D resistivity model. Nevertheless, obvious discrepancies appear between the individual and joint inversion results. Consequent synthetic modelling studies illustrate that the discrepancies occur because of i) differences in lateral and depth resolution between the semi-AEM and LOTEM data caused by different measuring configurations, ii) different measured EM components, and iii) differences in the error weighting of the individual datasets. Additionally, our synthetic study suggests that more flexible land-based configurations with sparse receiver locations are possible in combination with semi-AEM without a significant loss of target resolution, which is promising for accelerating data acquisition and for survey planning and logistics, particularly when measuring in inaccessible areas

2D DCR/TDIP Two-step Inversion: Detectability of ore deposits and Depth of Investigation

The joint application of the Direct Current Resistivity (DCR) and Time-domain Induced Polarization (TDIP) methods is a helpful tool to investigate ore deposits with sulfidic content. The combined interpretation yields a resistivity and chargeability model of the subsurface which is superior to a pure resistivity model, especially when dealing with disseminated deposits.We present a newly developed smoothness constrained 2D inversion algorithm for DCR and TDIP data applying a two-step approach. It contains a Finite Element forward calculation on an unstructured triangular mesh which amongst others simplifies the incorporation of topography.The new algorithm is tested regarding the quality of the chargeability resolution with respect to the resistivity model. Furthermore, the resolution of the lower boundary of a highly chargeable model block is investigated and compared to the depth of investigation which is determined by the coverage matrix.Finally, 2D inversion results of field data acquired on a sulfidic copper ore deposit in Turkey are shown using the newly developed algorithm and a feasible interpretation is given

2D inversion of DCR and Time Domain IP data using a Finite Element approach on an unstructured mesh: an example from ore exploration

Ore deposits often appear as disseminated sulfidic materials. Exploring these deposits with the Direct Current Resistivity (DCR) method alone is often challenging because the resistivity signatures caused by disseminated material is hard to detect. On the other hand, the Time-domain Induced Polarization (TDIP) method is qualified to detect areas with disseminated sulfidic ores due to large electrode polarization effects which result in large chargeability anomalies. By employing both methods we gain information about both, the resistivity and the chargeability distribution of the subsurface.Here, we present the newly developed 2D smoothness constraint inversion algorithm for DCR and TDIP data. The implemented forward algorithm uses a Finite Element approach with an unstructured mesh. The model parameters resistivity and chargeability are connected by a conductivity perturbation approach. It is shown, that the resolution of chargeability anomalies is not strongly effected by the corresponding resistivity anomaly.As a case study, the 2D inversion results of DCR/TDIP and RMT data obtained during a survey on a sulfidic copper ore deposit in Turkey are presented

On the exploration of a marine aquifer offshore Israel by long-offset transient electromagnetics

The existence of aquifers extending from land beneath the sea floor up to a distance of several kilometres has been observed and examined all over the world. The coastal aquifer of Israel is a heavily used groundwater reservoir which has to be constantly monitored to ensure the drinking water supply. Former land-based electromagnetic measurements show that it is, in several places, blocked to seawater intrusion and is consequently a candidate for submarine extension. Multicomponent long-offset transient electromagnetic measurements were carried out offshore on the coast of Israel. We deployed a 400-m-long grounded dipole as transmitter and several electric and magnetic receivers on the sea floor up to a distance of 4.8 km from the coast. Altogether, we deployed 8 transmitter positions and received data sets at 14 receiver stations onshore and offshore, with offsets of mostly 400 and 800 m. In this paper, we present the survey and 1D Occam and Marquardt inversions of the offshore horizontal electric components in the broadside and inline configuration. In addition, the vertical magnetic component in the broadside position is also considered. Only single inversions, both single offset and single component, were used to detect the aquifer under sea sediments. We prove the submarine existence of the Israeli coastal aquifer up to a distance to the coast of approximately 3.2 to 3.6 km using all measured long-offset transient electromagnetic components. In addition, we present modelling studies with synthetic data derived from a subsurface model adjusted to our survey area with very shallow water from 10 to 50 m. As part of the planning before the survey, a parameter study of the expected subsurface, the examination of the airwave phenomenon and the justification for our 1D inversion strategy are shown. More detailed eigen parameter analyses are added to explain the measured data

Application of the Newly Developed 2D DCR/TDIP Inversion Algorithm: Inversion Studiesand Analysis of Field Data Acquired on a Copper Ore Deposit

We present inversion studies obtained with a newly developed 2D inversion algorithm for DirectCurrent Resistivity (DCR) and Time-domain induced polarization (TDIP) data. The algorithm usesthe finite element approach on unstructured meshs which simplifies the incorporation oftopography. The model parameters resistivity and chargeability are connected by a perturbationapproach and the inversion is conducted in two steps.By inversion studies with synthetic data, it is shown that chargeability structures can be reproducedby the algorithm in presence or absence of a corresponding resistivity anomaly. A further studycompares the effects of different smoothness constraint matrices that approximate first orderderivatives of parameter transitions of neighboring mesh elements. It is shown that it is preferable touse a constraint that considers the shape and size of the elements of unstructured meshs overconstraints that weight all element boundaries equally. Furthermore, a method is presented thatapproximates a depth of investigation by deriving a threshold of sensitivity coverage from the ruleof thumb.As a case study, inversion results of field data acquired on a copper ore deposit located in Turkeyare presented. The ore deposit is elongated beneath a valley structure and has sulfide content. Dueto its disseminated form, resistivity anomalies induced by this deposit are hard to detect. However,chargeability inversion results show anomalies below 10 m depth with values of >100 mV/V. Thisclearly indicates the presence of ore with sulfide content

Radiomagnetotelluric and Direct Current Resistivity measurements for the characterization of conducting soils

International audienceThe application of geophysical techniques to hydrogeological questions has grown in importance due to the increasing concern for environmental issues. To improve the hydrogeological image of the subsurface we carried out electric and electromagnetic measurements using Radiomagnetotellurics and Direct Current Resistivity in East-Frisia, North-West Germany. The aim of our experiments was to compare the two methods and improve the hydrogeological image of the subsurface. This allows us to assess the threat groundwater exploitation poses to wetland habitats (biotopes). Pumping groundwater from deep aquifers can drain water from near surface aquifers. Hence it is vital to know ground water confining units. Prior to our measurements the hydrogeological image of those confining units was based on interpolation between boreholes. Some single Direct Current sounding measurements were also carried out and were used with the information from the boreholes to generate a hydrogeological model of the area. By combining the results from our measurement profiles with the hydrogeological model we were able to revise the hydrogeological model. By using the geophysical modeling results we could define regions where the hydrogeological model is inadequate due to errors in interpolation. We apply both Direct Current Resistivity and Radiomagnetotellurics and perform a practical comparison of the two methods. To ensure the comparability of the resulting resistivity models we examine the depth of exploration of the two methods and demonstrate the application of the depth of exploration index to Radiomagnetotelluric data. Both methods detect conducting structures, which correlate with low permeable clay structures in the hydrogeological models

Integrated Interpretation of Magnetic and ERT Data to Characterize a Landfill in the North-West of Cologne, Germany

Electrical resistivity tomography (ERT) and ground magnetic surveys were applied to characterize an old uncontrolled landfill in a former exploited sand and gravel quarry in an area to the north-west of the city of Cologne, Germany. The total magnetic field and its vertical gradient were recorded using a proton precession magnetometer to cover an area of about 43,250 m2. The magnetic data were transferred to the frequency domain and then reduced to the north magnetic pole. The amplitude of the analytical signal was calculated to define the magnetic materials within and outside the landfill. Eight ERT profiles were constructed based on the results of the magnetic survey using different electrode arrays (Wenner, dipole–dipole, and Schlumberger). In order to increase both data coverage and sensitivity and to decrease uncertainty, a non-conventional mixed array was used. The subsurface resistivity distributions were imaged using the robust (L1-norm) inversion method. The resultant inverted subsurface true resistivity data were presented in the form of 2D cross sections and 3D fence diagram. These non-invasive geophysical tools helped us to portray the covering soil, the spatial limits of the landfill, and the depth of the waste body. We also successfully detected low resistivity zones at deeper depths than expected, which probably be associated with migration pathways of the leachate plumes. The findings of the present study provide valuable information for decision makers with regards to environmental monitoring and assessment.Volkswagen Foundation http://dx.doi.org/10.13039/501100001663Universität zu Köln (1017

Archaeogeophysical exploration in Neuss‐Norf, Germany using electrical resistivity tomography and magnetic data

A combination of electrical resistivity tomography (ERT) and magnetic gradiometry was selected to examine a hypothesis concerning the presence of remains of one of the oldest archaeological churches in the Rhineland, located in Neuss‐Norf, Germany. The gradiometer survey was carried out to measure the vertical gradient of the magnetic field using a proton precession magnetometer along several profiles. The magnetic data were reduced to the magnetic pole; then analytic signal and power spectrum techniques were applied. The ERT survey was based on the magnetic results, and both Wenner and dipole–dipole configurations were employed to collect the apparent resistivity data along 12 ERT profiles. The field ERT data from these two arrays were merged into one dataset to form a non‐conventional mixed array. The robust (blocky) inversion technique was applied to the resistivity data in order to derive the two‐dimensional resistivity distribution of the subsurface. Despite the noisy surroundings, the magnetic survey was able to give an indication of potential walls of the ancient church in addition to several subsurface magnetic sources. Moreover, highly resistivity anomalies were observed within the first 1–2 m of the subsurface soil and were interpreted as possible remains of man‐made structures. This depth range was also confirmed by the spectral analysis of the magnetic data. A strong consistency between the two methods was observed in some locations of the site. In addition, the ERT measurements confirm and complement most of the magnetic results. We successfully detected anomalous zones that could be associated with the walls of at least one ancient church building in addition to several possible archaeological structures in the survey area