Exploration Geophysics - Volume 47, Issue 4, 2016

There have been multiple occurrences in the literature in the past several years of what has been referred to as the induced polarisation (IP) effect in airborne time domain electromagnetic (TDEM) data. This phenomenon is known to be responsible for incorrect inversion modelling of electrical resistivity, lower interpreted depth of investigation (DOI) and lost information about chargeability of the subsurface and other valuable parameters. Historically, there have been many suggestions to account for the IP effect using the Cole-Cole model. It has been previously demonstrated that the Cole-Cole model can be effective in modelling synthetic TDEM transients. In the current paper we show the possibility of extracting IP information from airborne TDEM data using this same concept, including inverse modelling of chargeability from TDEM data collected by VTEM, with field examples from Canada (Mt Milligan deposit) and Russia (Amakinskaya kimberlite pipe).

There has been evidence of the IP effect in TDEM data. This phenomenon is known to be responsible for incorrect inversion modelling of electrical resistivity, lower DOI and lost information about chargeability. In the current paper, we show the possibility of extracting IP information from VTEM data using Cole-Cole modelling.

In environments of suitable conductivity contrast, airborne electromagnetic surveys can map variations in the depth to bedrock for mine infrastructure planning. A survey in northern Finland illustrates the success of this approach for guiding the placement of a mine crusher and related infrastructure.

Airborne electromagnetic (AEM) surveys with near-surface vertical resolution provide rapid and comprehensive coverage of a mine site ahead of infrastructure planning. In environments of sufficient electrical conductivity contrast, the data will map variations in the depth to bedrock, providing guidance for expected excavation depths for solid building foundations, or mine pre-strip volumes. Continuous coverage overcomes the severe areal limitation of relying only on drilling and test pits. An AEM survey in northern Finland illustrates the success of this approach for guiding the placement of a mine crusher and related infrastructure. The cost of the EM data collection and interpretation is insignificant in comparison to the US$300 million capital cost of the mine infrastructure. This environment of shallow glacial cover challenges the limits of AEM resolution, yet analysis of subsequently collected three-dimensional (3D) surface seismic data and actual pre-strip excavation depths reinforces the predictive, but qualitative, mapping capability of the AEM. It also highlights the need to tune the modelling via petrophysics for the specific goal of the investigation, and exposes the limitations of visual drill core logging.

CGG deployed HELITEM, a helicopter-borne time domain electromagnetic (TDEM) system over the Lalor Deposit and other proximal deep conductors of Hudbay Minerals in northern Canada. The Lalor Deposit is ~570 m deep and is a difficult target for airborne EM systems. The system was configured with a dipole moment of up to 1.9 MAm² at base frequencies of 15 Hz and 30 Hz. The results of this survey have been used to characterise the TDEM response from deep volcanogenic massive sulphide (VMS) deposits within the region. HELITEM is the only airborne controlled source TDEM system to have detected the Lalor Deposit.

CGG deployed HELITEM, a helicopter-borne time domain electromagnetic (TDEM) system over the ~570 m deep Lalor Deposit in Canada. The results have been used to characterise the TDEM response from deep volcanogenic massive sulphide deposits within the region. HELITEM is the only airborne controlled source TDEM system to have detected the Lalor Deposit.

If the horizontal and vertical derivatives of the total field magnetic anomalies of thin dykes and contacts (and the magnetic anomaly from a thin dyke) are put into an amplitude-phase form, then this can aid in their interpretation. First, it allows derivatives of fractional order to be calculated analytically; second, estimates of the source dip can be made; and finally, the relationships between local wavenumber and source-distance semi-automatic interpretation methods are clarified. The approach is demonstrated on synthetic data and aeromagnetic data from South Africa.

If the horizontal and vertical derivatives of the total field magnetic anomalies of thin dykes and contacts are put into an amplitude-phase form, then this can aid in their interpretation. In addition, the relationships between local wavenumber and source-distance semi-automatic interpretation methods are clarified.

A new noise reduction method that iteratively projects data to a lower height and upward continuing the data back to the survey height is described. This method can significantly improve the signal-to-noise ratio of noisy gravity gradient data, and has been successfully applied to both synthetic and real data.

Airborne gravity gradient (AGG) measurements offer an increased resolution and accuracy compared to terrestrial measurements. But interpretation and processing of AGG data are often challenging as levelling errors and survey noise affect the data, and these effects are not easily recognised in the gradient components. We adopted the classic method of upward continuation in the noise reduction using the noise level estimates by the AGG system. By iteratively projecting the survey data to a lower level and upward continuing the data back to the survey height, parts of the high-frequency signal are suppressed. The filter, which is defined by this approach, is directly dependent on the noise level of the AGG data, the maximum number of iterations and the iterative step. We demonstrate the method by applying it to both synthetic data and real AGG data over Karasjok, Norway, and compare the results to the directional filtering method. The results show that the iterative filter can effectively reduce high-frequency noise in the data.

Gamma-ray logging tools are applied worldwide. At various locations, calibration facilities are used to calibrate these gamma-ray logging systems. Several attempts have been made to cross-correlate well known calibration pits, but this cross-correlation does not include calibration facilities in Europe or private company calibration facilities.

Our aim is to set-up a framework that gives the possibility to interlink all calibration facilities worldwide by using ‘tools of opportunity’ – tools that have been calibrated in different calibration facilities, whether this usage was on a coordinated basis or by coincidence.

To compare the measurement of different tools, it is important to understand the behaviour of the tools in the different calibration pits. Borehole properties, such as diameter, fluid, casing and probe diameter strongly influence the outcome of gamma-ray borehole logging. Logs need to be properly calibrated and compensated for these borehole properties in order to obtain in-situ grades or to do cross-hole correlation. Some tool providers provide tool-specific correction curves for this purpose. Others rely on reference measurements against sources of known radionuclide concentration and geometry.

In this article, we present an attempt to set-up a framework for transferring ‘local’ calibrations to be applied ‘globally’. This framework includes corrections for any geometry and detector size to give absolute concentrations of radionuclides from borehole measurements. This model is used to compare measurements in the calibration pits of Grand Junction, located in the USA; Adelaide (previously known as AMDEL), located in Adelaide Australia; and Stonehenge, located at Medusa Explorations BV in the Netherlands.

Gamma-ray tools are used heavily in borehole logging, both in oil and gas, as well as in mining applications. For proper functioning, these tools need to be calibrated against a source of known activity and geometry. In this paper, we present a holistic approach to tool calibration, aiming to interlink several calibration facilities around the world, allowing quantitative interpretation of data.

A method to determine the depth to two superimposed sources from a self-potential anomaly profile has been developed. The method uses a relationship between the depths to the two superimposed structures determined from a combination of observations at symmetric points with respect to the coordinate of the sources’ centre.

In this paper, we develop a method to determine the depth to two superimposed sources from a self-potential anomaly profile. The method is based on finding a relationship between the depths to the two superimposed structures from a combination of observations at symmetric points with respect to the coordinate of the sources’ centre. Formulae have been derived for two classes of geometric sources: spheres and cylinders. Equations are also formulated to estimate the other model parameters, including the polarisation angle and electric dipole moment of both structures. The proposed method is tested both on synthetic data with and without random noise, as well as real self-potential data from a set of field data collected in Turkey. In all cases, the model parameters obtained are in good agreement with the actual ones.

We investigated the range of sensitivity of five tools for measuring the conductivity or resistivity in drillholes and on drill core. Each tool has a limited range of sensitivity, so one or more tools should be used to cover the range of values expected on a particular project.

A study has been undertaken to acquire conductivity data using the EM39 low-induction-number conductivity tool. Measurements were taken in three holes in the Sudbury, Ontario, area: at Victoria in the south-west part of the Sudbury structure; at Levack, in the north range; and at the Lady Violet deposit near Copper Cliff. These data were compared with pre-existing data acquired using four other tools and measurements taken on core extracted from the holes. The four tools are the DGI galvanic downhole resistivity tool, the IFG downhole conductivity tool, and the handheld KT-10 and GDD meters. The comparison shows that each tool has a finite range of sensitivity. The resistivity tool used by DGI Geoscience is sensitive to conductivities primarily in the range 0.01 to 100 mS/m; the EM39 tool is sensitive to conductivities in the range of ~30 mS/m to 3000 mS/m and the IFG tool to conductivities greater than 30 mS/m. In the sub-ranges where the ranges of two instruments overlap, one might expect a good correlation between the measurements derived from the two tools. However, this is not always the case, as the instruments can have a different volume of sensitivity: the EM39 has a coil separation of 50 cm and will see material greater than 20 cm away from the hole; whereas the IFG conductivity tool seems to have a smaller spatial scale of sensitivity due to its 10 cm coil size. The handheld instruments used to log the conductivity of the core are sensitive to more conductive material (greater than ~1 S/m). The scale of the sensors of these handheld instruments is a few cm, so they are focussing on a very local estimate. The spatial characteristics of the handheld instruments are similar to the IFG tool, so there is a reasonable linear correlation between the conductivities derived from these three different instruments. However, the slopes are not unity; for example, the GDD instrument gives values three times greater than the KT-10. When selecting tools for measuring the resistivity and conductivity, ensure that the values that you expect to measure fall within the range of sensitivity of the instrument and that the features sought are comparable in size to the volume of sensitivity.

We present a hybrid inversion scheme for airborne TEM data that introduces approximation only in the calculation of partial derivatives. The objective function is evaluated with a full nonlinear one-dimensional (1D) forward model.

Airborne transient electromagnetic (TEM) methods target a range of applications that all rely on analysis of extremely large datasets, but with widely varying requirements with regard to accuracy and computing time. Certain applications have larger intrinsic tolerances with regard to modelling inaccuracy, and there can be varying degrees of tolerance throughout different phases of interpretation. It is thus desirable to be able to tune a custom balance between accuracy and compute time when modelling of airborne datasets. This balance, however, is not necessarily easy to obtain in practice. Typically, a significant reduction in computational time can only be obtained by moving to a much simpler physical description of the system, e.g. by employing a simpler forward model. This will often lead to a significant loss of accuracy, without an indication of computational precision.

We demonstrate a tuneable method for significantly speeding up inversion of airborne TEM data with little to no loss of modelling accuracy. Our approach introduces an approximation only in the calculation of the partial derivatives used for minimising the objective function, rather than in the evaluation of the objective function itself. This methodological difference is important, as it introduces no further approximation in the physical description of the system, but only in the process of iteratively guiding the inversion algorithm towards the solution. By means of a synthetic study, we demonstrate how our new hybrid approach provides inversion speed-up factors ranging from ~3 to 7, depending on the degree of approximation. We conclude that the results are near identical in both model and data space. A field case confirms the conclusions from the synthetic examples: that there is very little difference between the full nonlinear solution and the hybrid versions, whereas an inversion with approximate derivatives and an approximate forward mapping differs significantly from the other results.

Seismic inverse problems aim to infer the properties of subsurface geology, such as elastic and petrophysical properties. Existing seismic inversion methods for the joint estimation of these properties are mainly based either on Gassmann theory for prestack seismic data processed with stochastic optimisation techniques or on the Wyllie formula for poststack seismic data processed by deterministic optimisation techniques. The purpose of this study is to develop a strategy for the joint estimation of elastic and petrophysical properties from prestack seismic data based on Gassmann equations with deterministic optimisation techniques. Given poor-quality prestack seismic data, two regularisation parameters are introduced to control the trade-off between fidelity to the data and the smoothness of the solution. An appropriate linearised system of equations for the joint model update is derived from Newton’s method, which fits seismic data, the description of the rock physics medium and prior information, simultaneously. We show the preliminary results obtained with the proposed framework for synthetic and real data examples.

We present a strategy for the joint estimation of elastic and petrophysical properties from prestack seismic data based on Gassmann equations with deterministic optimisation techniques. Given poor-quality prestack seismic data, two regularisation parameters are introduced to control the trade-off between fidelity to the data and the smoothness of the solution.

A high-resolution seismic survey was designed to improve the quality of seismic data and study the evolution of inner shelf deposits off the southern coast of Korea. A 0.5 L air gun and 6- or 8-channel streamer cable with a 5 m group interval were used as a seismic source and receiver system, respectively. Data recording was digitally performed at a shot interval of 2 s and a sample interval of 0.1 ms using a personal computer (PC)-based recording system with an analogue to digital (A/D) converter. In the data processing, deconvolution and static corrections were very effective for improving the data resolution. The data resolution and signal-to-noise (S/N) ratio were improved by using multi-channel digital seismic systems as opposed to a single-channel analogue streamer. The results show that a small-scale multi-channel seismic system is an effective way to investigate late Quaternary deposits. Seismic stratigraphic analysis of the high-resolution seismic profiles revealed that inner shelf deposits, up to 30 m thick, consist of three sedimentary units, which comprise transgressive and highstand systems tracts deposited after the last glacial period. The transgressive systems tract includes two units (I and II) separated by a ravinement surface (RS). The lower Unit I, lying below the RS, consists of estuarine sediments left behind by shoreface erosion during transgression, and represents a paralic component. The upper Unit II, including two sub-units (IIa and IIb), lies above the RS and consists of sand produced by shoreface erosion during landward transgression, which corresponds to a marine component. The uppermost Unit III lies above the maximum flooding surface and consists of thick mud clinoforms of the modern Nakdong and Seomjin subaqueous deltas. This unit likely formed during the recent sea level highstand and represents the highstand systems tract.

The results of the present study show that a small-scale multi-channel seismic system is an effective way to improve the quality of seismic records and to document the evolution of shallow sediment deposits. The shelf deposits in the Korea Strait inner shelf consist of three distinct seismic units formed during the late Quaternary.