Airborne Time-domain Electromagnetic Research Articles

Quasi-3D modeling of airborne TEM data by spatially constrained inversion

We present a new methodology, spatially constrained inversion (SCI), that produces quasi-3D conductivity modeling of electromagnetic (EM) data using a 1D forward solution. Spatial constraints are set between the model parameters of nearest neighboring soundings. Data sets, models, and spatial constraints are inverted as one system. The constraints are built using Delaunay triangulation, which ensures automatic adaptation to data density variations. Model parameter information migrates horizontally through spatial constraints, increasing the resolution of layers that would be poorly resolved locally. SCI produces laterally smooth results with sharp layer boundaries that respect the 3D geological variations of sedimentary settings. SCI also suppresses the elongated artifacts commonly seen in interpretation results of profile-oriented data sets. In this study, SCI is applied to airborne time-domain EM data, but it can also be implemented with other ground-based or airborne data types.

Practical 3D EM inversion – the P223F software suite

Over the past 27 years, the AMIRA P223 project series has produced an extensive body of EM modelling and inversion programs used by the minerals, environmental and defence industries for planning and interpreting EM surveys, and for the development of new EM exploration instruments. This project series is now into its final overtime. Historically, the software generated by these projects has been available only to the project sponsors and their designated contractors. All programs are now commercially available through EMIT?s Maxwell graphical user interface. From 2010, the Fortran 90 source code for all programs will be open source. The purpose of this paper is to make the wider EM modelling community aware of the capabilities offered by this extensive, commercially-available software suite. The models for both modelling and inversion include a general 3D full domain finite elements (Loki class), 3D compact finite-elements (Samaya class), 2.5D full-domain finite-elements (Arjuna class), multiple 3D plates embedded in a multi-layered host (Leroi class) and a 1D layered earth (Airbeo and Beowulf). The programs can be used for any frequency or time-domain airborne, ground or downhole EM system. Sources can include multi-vertex closed loops, grounded wires, magnetic dipoles and plane waves. Receiver types can include multi-vertex loops, grounded wires, and magnetic and electric dipoles. Survey types include the variety of airborne configurations, fixed sources with independent surface or downhole receiver lines, moving sources with multiple fixed-offset receivers and magnetotellurics. Inversion for all model classes is based on an iterative, damped SVD method which concentrates on those parameters that most affect the data whilst ignoring those that are irrelevant.

Spatially Constrained Inversion for Quasi 3-D Modelling of AEM Data

The spatially constrained inversion (SCI) produces quasi-3D modelling of geoelectrical and EM data of varying spatial density, using a 1D forward solution. Information migrate horizontally through spatial constraints and allow resolution of layers that would be locally poorly resolved. The constraints are built using the Delaunay triangulation, which ensures automatic adaptation to data density variations. It also allows creation of subsets suitable for parallel computing. In this study the SCI was applied to airborne time domain EM soundings (SkyTEM data from the Island of Lolland, Denmark), but it can be implemented also with all other data types. The field case study proves that the SCI produces laterally c onsistent results that respect the 3D geological variations of the sedimentary settings. The resolution of near surface tills and water bearing sand and gravel layers is greatly enhanced and so is the boarder of the saline groundwater intrusion beneath the island. The SCI also suppresses the elongated artifacts commonly seen in profile oriented data sets.

Bathymetry and sediment depth investigation in Broken Bay using a prototype AEM time domain system (SeaTEM)

The use of airborne electromagnetic (AEM) methods for measuring water depth and estimating sediment thickness has been demonstrated using commercial AEM equipment that is not optimized for marine surveying. A new prototype helicopter time domain AEM system, SeaTEM(0), is under development for bathymetric surveying. The first sea trial of the SeaTEM(0) system took place over Broken Bay, NSW, in shallow water up to ~ 30 m in depth. The SeaTEM(0) system was untested and the Broken Bay survey identified instrumentation problems that will be addressed in future surveys. Broken Bay was chosen because the separate paleodrainage systems for the Hawkesbury River, Brisbane Waters and Pittwater which join in Broken Bay give rise to paleo-valleys infilled with unconsolidated sediments, ranging in thickness between 0 m (bedrock outcrop) and ~ 200 m. Sediment thickness and water depth is predicted from stitched 1D inversion of data based on a simplified two-layer model that represents seawater and sediment overlying a resistive half-space basement (bedrock). The resulting bathymetric profiles show agreement typically to within ~ ±1 m with known water depths in areas less than 20 m deep. The inverted depth profile of the second (sediment) layer is noisy; however, the profiles reveal coarse topographic features of paleochannels to depth limits of ~ 80 m below sea level in 30 m water depth.

Towards remote sensing of sediment thickness and depth to bedrock in shallow seawater using airborne TEM

Following a successful bathymetric mapping demonstration in a previous study, the potential of airborne EM for seafloor characterisation has been investigated. The sediment thickness inferred from 1D inversion of helicopter-borne time-domain electromagnetic (TEM) data has been compared with estimates based on marine seismic studies. Generally, the two estimates of sediment thickness, and hence depth to resistive bedrock, were in reasonable agreement when the seawater was ~20 m deep and the sediment was less than ~40 m thick. Inversion of noisy synthetic data showed that recovered models closely resemble the true models, even when the starting model is dissimilar to the true model, in keeping with the uniqueness theorem for EM soundings. The standard deviations associated with shallow seawater depths inferred from noisy synthetic data are about ± 5% of depth, comparable with the errors of approximately ± 1 m arising during inversion of real data. The corresponding uncertainty in depth-to-bedrock estimates, based on synthetic data inversion, is of order of ± 10%. The mean inverted depths of both seawater and sediment inferred from noisy synthetic data are accurate to ~1 m, illustrating the improvement in accuracy resulting from stacking. It is concluded that a carefully calibrated airborne TEM system has potential for surveying sediment thickness and bedrock topography, and for characterising seafloor resistivity in shallow coastal waters.

A holistic approach to inversion of time-domain airborne EM

A “holistic” method for simultaneously estimating conductivity and calibration models from 1-D inversion of time-domain airborne electromagnetic (AEM) data is proposed. The work extends the concept of holistic inversion that been successfully applied to frequency-domain AEM data. The entire multi-component airborne dataset and available independent conductivity and interface-depth data are simultaneously inverted. A spline-based conductivity model covering the complete survey area is estimated. Unmonitored elements of the system geometry are included as unknown parameters of the calibration model and are solved for in the inversion.Conventional 1-D inversion methods invert each airborne sample in isolation from other samples. However, by simultaneously considering all of the available information together in a holistic inversion formulation, we are able to exploit the inter-component and spatial coherency characteristics of the airborne data. The formulation ensures that the conductivity and calibration models are optimal with respect to the airborne data and prior information.

Airborne EM surveys over the Barrow Creek Prospect, NT

Three time-domain airborne EM surveys were recently flown over a known mineral deposit at Barrow Creek, NT. The Ni/Cu/Ag deposit is located in an area characterised by variable conductive overburden and is considered a difficult exploration target.Data from the three airborne systems and a moving loop ground EM survey were processed and analysed to compare the relative abilities of the systems to locate the target.It was found that the fixed-wing systems (TEMPEST and GEOTEM) produced clear responses to the known mineralisation that were consistent with the model developed from the ground EM responses. The EM responses measured with the helicopter system (HoistEM) did not indicate the target.

2.5D inversion of airborne electromagnetic data

2.5D electromagnetic (EM) modelling computes the response of a 3D source from an arbitrary 2D geoelectrical model. As such, it is practical for airborne EM (AEM) data to be inverted using 2.5D modelling provided that the geoelectrical cross-section is relatively constant along a strike length that exceeds the AEM system footprint. The program ArjunAir is introduced for modelling and inversion based on a 2D finite-element method that enables the accurate simulation of 3D source excitation for full domain models inclusive of topography, non-conforming boundaries, and very high resistivity contrasts. Inversion is based on an iterative Gauss-Newton method that is solved using the damped eigenparameter algorithm. Examples are presented for synthetic and practical frequency and time-domain AEM surveys for which inversion run-times are on the order of hours.

Practical 3D AEM inversion using 2.5D modelling

2.5D electromagnetic (EM) modelling computes the response due to a 3D source from an arbitrary 2D geoelectrical model which has an infinite strike length. As such, airborne EM (AEM) data can be inverted using 2.5D modelling provided that the geoelectrical cross-section is relatively constant along a strike length that exceeds the AEM system footprint. The program ArjunAir is introduced for modelling and inversion based on a 2D finite-element method that enables the accurate simulation of 3D source excitation for full domain models inclusive of topography, non-conforming boundaries and very high resistivity contrasts. Inversion is based on an iterative Gauss-Newton method that is solved using the damped eigenparameter algorithm. Examples are presented for synthetic frequency and time-domain AEM surveys for which inversion run-times are in the order of hours.

Electrical and EM methods, 1980–2005

The last two decades saw major advances in data collection, processing, and interpretation of electrical and EM data. Lower transmitter frequencies for airborne time-domain EM systems have made possible surveys in areas where conductive cover previously screened basement conductors. As with every other branch of technology, the evolving speed of the silicon chip and of streaming data to hard disk has revolutionized data collection and noise reduction processing. Major advances have been made on increasing the signal-to-noise ratios in ground EM data acquisition systems. Full-waveform recording and the use of multiple receivers are becoming common for ground EM techniques. Previously intractable 2D and 3D data inversions are now slowly becoming available. Finally, controlled-source EM techniques are now being used to detect and characterize hydrocarbon-bearing reservoirs in deepwater areas.

LocalizedS‐inversion of time‐domain electromagnetic data

Interpretation of time‐domain electromagnetic (TDEM) data over inhomogeneous geological structures is a challenging problem of geophysical exploration. The most widely used approach of interpreting TDEM data is based on the smooth 1‐D layered resistivity inversion. We have developed an effective technique of fast TDEM inversion based on thin‐sheet conductance approximation that we call S‐inversion. In this paper we extend the S‐inversion technique, approximating the conductivity cross‐section by adding a local inhomogeneous disk with an excess conductance ΔS to the horizontal conductive thin sheet used in S‐inversion. Localized S‐inversion determines the distribution of this excess conductance as a function of a depth and a horizontal coordinate. This new method takes into account the limited horizontal extent of the inhomogeneities, localizing inversion. The numerical modeling results and inversion of practical TDEM data demonstrate that the method resolves local geological targets better than traditional 1‐D inversion and original S‐inversion. The method can be applied to interpret both ground and airborne TDEM data sets.

Airborne TEM surveying with a SQUID magnetometer sensor

Traditionally airborne time-domain electromagnetic (AEM) survey systems use induction coils as the sensor (receiver). We have replaced the induction coil in a transient electromagnetic (TEM) system with a liquid-nitrogen cooled superconducting quantum interference device (SQUID) magnetometer sensor. Using this prototype system, we aimed to improve performance in detecting conductive mineralization, particularly where the conductive mineralization of interest is covered by a conductive regolith. We successfully demonstrated one- and three-component SQUID sensors in airborne TEM surveying, and achieved performance comparable to the induction-coil systems. Implementation of the SQUID system required development of devices capable of operating in magnetically unshielded environments with low noise, high slew rate, and wide bandwidth. Operation of the SQUID sensor in the highly dynamic environment of a towed bird was also necessary, and this implies a high dynamic range and high level of noise associated with the motion in Earth's magnetic field. The high dynamic range of the SQUID response was handled by a combination of resetting the SQUID flux locked loop, reducing the bandwidth, and providing high-gain feedback in parallel with the flux locked loop. A digital stacking filter was used to eliminate low-frequency noise associated with sensor motion. Isolation of the sensor from motion at the TEM signal frequencies required development of a sophisticated suspension system. The SQUID systems were tested over two known conductive targets, and their performance compared with the induction-coil TEM system. A comparative performance measure is developed to take the different sensitivities of the SQUID magnetometer and induction-coil receivers into account. This measure indicates that the SQUID system has superior performance for responses over earth structures with decay time constants greater than ∼6 ms when compared with the induction-coil signals. We also estimate the performance in comparison with integrated outputs of the induction-coil system and show that, at the demonstrated levels of SQUID performance, it is expected to have poorer performance by a factor of two or more. This disadvantage will be reduced for lower frequency, wider channel width TEM configurations or by improvements in the SQUID devices.

AeroTEM characteristics and field results

After half a century of rapid AEM (airborne EM) development and application, the 1980s were a “decade of uncertainty” (Fountain 1998) in which improvements focused mainly on increased bandwidth, multiple coil systems, and other advantages attendant on improved electronics and signal processing. An exception was the University of California Berkeley UNICOIL cryogenic helicopter system which adopted a single coil as both transmitter and receiver. Morrison et al. (1998) showed that this array maximized the ratio of target-to-host response in conductive environments. UNICOIL development was suspended in the early 1990s, but the same principle was used by AeroQuest in the AeroTEM transient (time domain) AEM system, which places the receiving coil centrally within the transmitting loop, thus achieving the same coupling with ground conductors simultaneously in both coils. This configuration has been shown (e.g. Buselli, 1977) to have significant advantages over the loop-loop method in ground TEM (transient EM) applications, particularly in conditions of high ground conductivity. Among the advantages are maximization of target-to-background response, simpler and sharper anomalies, enhancement of discrete conductors, and insensitivity to conductor strike direction. In addition, it has been shown by numerous authors (e.g., Smith and West, 1989) that the central loop configuration is optimally configured to excite a unique, negative response from bodies of modest polarizability. This feature is, in itself, a breakthrough of major importance in airborne prospecting because it is another step along the road to AIP (airborne IP). AeroQuest began development of AeroTEM in 1996. A key feature is the rigid mounting of the receiving coil, centrally within the transmitting loop. Other features include a triangular current pulse of 1150 μs, a base frequency of 150 Hz, a transmitting loop diameter of 5 m, and orthogonal receiving coils (Z and X). Three hundred records are recorded digitally per second. Each record holds …

Experience with SQUID Magnetometers in Airborne TEM Surveying

High temperature superconducting quantum interference device (SQUID) magnetometers have been developed in a collaborative project between BHP and CSIRO specifically for application in airborne time domain electromagnetic (TEM) surveying. The objective of this development was to improve the performance of the system in detection of conductors with longer decay time constants, particularly in the presence of a conductive overburden. The sensors were incorporated into a specially designed receiver system and successfully tested as receivers for the GEOTEM system with the assistance of Fugro Airborne Surveys. Their performance was shown to be comparable with, but not superior to current induction coil TEM systems. The development of the receiver system required solutions to a range of problems for the sensor devices and for the receiver system. The principal obstacle for the receiver was in overcoming the high dynamic environment of the towed receiver bird and the consequent high level of noise associated with motion of the sensor in the Earth’s magnetic field. The high dynamic range of the magnetometer response, which arises from this motion, was addressed by a combination of modification of the sensor fluxlocked loop and periodic resetting of the sensor offset. A digital stacking filter was used to eliminate low-frequency noise associated with motion and a specially designed suspension system was developed to isolate the sensor from higher-frequency motions of the towed bird.

Improving conductivity models using on-time EM data

Modelling of data from half-sine time-domain airborne EM systems is traditionally limited to the period following the transmitter turn-off, the off-time data. The joint use of on-time and off-time data widens the resolvable range of conductivities. Using a half-sine waveform with a base frequency of 25 Hz and a pulse width of 4 ms, conductivities and conductances as low as 0.5 mS/m and 0.02 S, respectively, become resolvable. Also, with the constraint of on-time data, automated modelling becomes more stable as noise at late time is less likely to be confused with signal.Conductivity-depth sections of QUESTEM-450 (Lane et ah, 1998) data from Forrestania, Western Australia were modelled using a layered-earth inversion (LEI) and a current imaging technique. The known mineralisation, including massive sulphides and a banded iron formation, is indicated by the off-time data. For both methods, additional use of on-time data led to a model improvement.

Black Range and Vernon Hill: A comparison of two airborne EM surveys in south east Australia

In south east Australia during the first half of 1996, Rio Tinto Exploration Pty. Limited commissioned QUESTEM airborne time domain electromagnetic (AEM) surveys at Black Range in western Victoria and Vernon Hill in northern South Australia. The aim of these surveys was direct detection of conductivity anomalies: base metals in Cambrian greenstones at Black Range and diamonds in the Precambrian Yoolperlunna Inlier at Vernon Hill. Neither was successful in their primary objective, and the data from each survey are used here to investigate the value of AEM in geological mapping.At Black Range, lacustrine deposits along the drainage features which cut through many of the major geological units. This conductive surficial material dominates the AEM survey data. Away from the streams, it is still possible to discriminate the major rock types of the area but not successfully map the boundaries between them.In contrast, conductive surficial material at Vernon Hill was less extensive and the QUESTEM data appears to be providing a considerable amount of geological information. Although very little geological mapping has been done in the area, the close relationship between magnetic and conductive domains within the survey area and the evidence for structural boundaries in the topography which coincide with boundaries of the same conductive domains is very compelling.The data show that, in areas where there is limited obscuration by surficial conductors, it is possible to use airborne electromagnetics to map geology. In such circumstances the AEM information is complementary to that provided by magnetics so that information from one enhances the interpretation of the other and vice versa. In this way, the use of both data sets provides more than twice as much information.

Streamed data - A source of insight and improvement for time domain airborne EM

The capacity to record streamed data, a time series of raw data from the transmitter current monitor and the electromagnetic sensors, was introduced to the QUESTEM 450 airborne EM system by the partners of the Airborne EM Systems Program at the CRC AMET during 1997. This has provided the opportunity to replay, analyse and reprocess data from any survey flown with the system. Much has been learnt about the various elements that contribute to the recorded response, and improvements have been made to the way in which the ground response is isolated from other unwanted influences.QUESTEM 450 is a time domain airborne EM system. A waveform with half-sine current pulses of alternating polarity is fed into a large horizontal transmitter loop slung around an aircraft. A three-component receiver coil set is towed in a bird behind the aircraft. Signals are sent from the bird to a receiver unit in the aircraft, where they are processed and recorded. Streamed data represents the most fundamental level of recorded data.The strongest contribution to the overall response is a combination of the voltage induced in the receiver coils from changes in the magnetic field associated with currents flowing in the transmitter loop and signals related to secondary currents induced in the metal frame of the aircraft. These two signals are never separated, and together make up the response termed the “primary”. The amplitude and shape of the primary varies as the transmitter to receiver loop geometry changes. Streamed data can reveal the effects of these changes in great detail.Another element of the response is related to the movement of the coil in the Earth’s magnetic field and is termed “coil motion noise”. This voltage will be induced under two circumstances; when the coil rotates with respect to the Earth’s field, and when the strength of the field varies at a rate within the pass band of the system. Computer-intensive processing methods applied post-flight to streamed data achieve higher levels of suppression of this form of noise than do methods traditionally applied onboard in real time.“Sferic” signals are the result of lightning strokes. Streamed data reveal that only the high amplitude, high frequency components of these signals are significant in the context of the present QUESTEM 450 system. A study of the character of these signals in streamed data resulted in an improvement to the method of suppression applied in real time on-board the aircraft.Powerlines radiate strong, time-varying magnetic fields which are visible in streamed data over very broad areas. The principal means of suppression for this element of the measured signal is a combination of careful choice of base frequency and stacking of the response. In the vicinity of a powerline, the amplitude of the response and/or the spatial variation in the field can significantly impair the ability to extract the ground response from the total response.

Case Study: The Evolution of Airborne Time Domain Electromagnetic Applications for Geologic Mapping; a Noranda Perspective

The advances in airborne time domain EM over the last seven years have facilitated its evolution into an effective mapping system from what has been ostensibly considered primarily a massive sulphide targeting tool. During this period Noranda undertook a series of programs which provided insights into the technical issues and interpretation challenges that the use of airborne time domain electromagnetics (TDEM) technology presents to exploration geologists and geophysicists. Following a model study and test flying program over a sediment hosted copper prospect on Victoria Island in the Canadian Arctic, a series of covered terrain porphyry copper exploration mapping programs were undertaken in southwest Arizona and Sonora Mexico between 1993 and 1997 using both the QUESTEM and GEOTEM commercial airborne TEM systems. The results of this work were encouraging but underscored the need for ground validation programs and robust geologic models to properly interpret the data. A need for lower noise and better calibrated EM systems was also recognized.The thrust of Airborne TDEM technology development has been to improve conductor (massive sulphide) detection to greater depth in ever increasing complex geologic environments. This application is often referred to as bump finding in reference to discrete anomaly amplitude signatures superimposed along ostensibly flat background, profiled channel data. However, as processing software improved (i.e., conductivity-depth inversion/transformation algorithms, CDI/CDT) and integrated exploration models were applied, it became clear that this technology could be applied as a broad geologic mapping tool. This required a shift in how the data was viewed; instead of “anomaly picking’ which normally utilised around 1% of the data, the previously ignored background data which contained useful geologic information became the focus of analysis. As airborne TDEM instrumentation and interpretation software improved, airborne TDEM has become a much more effective tool for geologic mapping.GEOTEMDEEP surveying in 1997 over Noranda’s San Jose, Arizona, and adjoining Sol porphyry properties reliably identified known subsurface bedrock features ranging in depth of 150m to 200m. Diamond drilling results suggest that airborne TDEM mapping locally defined the bedrock interface to depths in excess of 300 m. Evidence also suggests that lithologic discrimination under cover is locally possible.Survey results over the Poston Butte, Arizona, porphyry copper deposit show how airborne TDEM technology can be used as a direct targeting tool. The signature is unfortunately not unique, and similar to inliers of conductive cover. For this reason ground validation programs are essential components of any exploration program.While airborne TDEM technology has evolved to the point where pseudo 3-D mapping (spatial analysis of 1-dimensional CDTs) capabilities are possible, more reliable and robust inversion software, improvements in system noise levels and stability, plus a better understanding of the geologic models are still needed to provide reliable interpretation.

OVERVIEWS: Airborne geophysics in Western Australia

Aeromagnetics became recognised as a valuable aid to exploration in Western Australia during the nickel boom of the 1960s. The now commonplace term aeromagnetics evolved during the 1980s, mainly to meet demands of a resurgent gold exploration industry which required a greater understanding of geological structure and rock type than was possible with conventional field mapping. High-resolution aeromagnetic surveys are typically flown at a line spacing of 200 m, a terrain clearance of 60 m and with a magnetometer sampling interval of 7 m. Imaging of the data provides a greater range of options for highlighting subtle structural features than conventional contours. Image maps may be interpreted using skills of photogeology and satellite-image interpretation and have been a major factor in bridging the communication gap between geophysicists and geologists. The effectiveness of radiometric data in Western Australia, collected simultaneously on virtually all high-resolution aeromagnetic surveys, has been constrained by lack of outcrop and transported soils. In appropriate areas the method is successful at mapping different types of felsic igneous, sedimentary and metasedimentary rocks, and also in identifying hydrothermal alteration zones displaying potassic enrichment. Problems of high surface conductivity have, in the past, limited the application of airborne time-domain electromagnetic surveys. The new generation of digital systems (e.g., QUESTEM) has improved depths of penetration through the real-time removal of unwanted noise. Airborne time-domain electromagnetics is no longer only an anomaly detector, but a three-dimensional regional conductivity mapping tool which, when integrated with magnetic data, enables the targeting of bedrock conductors to be controlled by interpreted geology.

Airborne Geophysics in Western Australia

Aeromagnetics became recognised as a valuable aid to exploration in Western Australia during the 'nickel boom' of the 1960s. The now commonplace term 'high-resolution aeromagnetics evolved during the 1980s, mainly to meet demands of a resurgent gold exploration industry which required a greater understanding of geological structure and rock type than was possible with conventional field mapping. High-resolution aeromagnetic surveys are typically flown at a line spacing of 200 m, a terrain clearance of 60 m and with a magnetometer sampling interval of 7 m. Imaging of the data provides a greater range of options for highlighting subtle structural features than conventional contours. Image maps may be interpreted using skills of photogeology and satellite-image interpretation and have been a major factor in bridging the communication gap between geophysicists and geologists. The effectiveness of radiometric data in Western Australia, collected simultaneously on virtually all high-resolution aeromagnetic surveys, has been constrained by lack of outcrop and transported soils. In appropriate areas the method is successful at mapping different types of felsic igneous, sedimentary and metasedimentary rocks, and also in identifying hydrothermal alteration zones displaying potassic enrichment. Problems of high surface conductivity have, in the past, limited the application of airborne time-domain electromagnetic surveys. The new generation of digital systems (e.g., QUESTEM) has improved depths of penetration through the real-time removal of unwanted noise. Airborne time-domain electromagnetics is no longer only an anomaly detector, but a three-dimensional regional conductivity mapping tool which, when integrated with magnetic data, enables the targeting of bedrock conductors to be controlled by interpreted geology.