We present the CHAOS-7 model of the time-dependent near-Earth geomagnetic field between 1999 and 2020 based on magnetic field observations collected by the low-Earth orbit satellites Swarm, CryoSat-2, CHAMP, SAC-C and Orsted, and on annual differences of monthly means of ground observatory measurements. The CHAOS-7 model consists of a time-dependent internal field up to spherical harmonic degree 20, a static internal field which merges to the LCS-1 lithospheric field model above degree 25, a model of the magnetospheric field and its induced counterpart, estimates of Euler angles describing the alignment of satellite vector magnetometers, and magnetometer calibration parameters for CryoSat-2. Only data from dark regions satisfying strict geomagnetic quiet-time criteria (including conditions on IMF 
$$B_z$$

 and 
$$B_y$$

 at all latitudes) were used in the field estimation. Model parameters were estimated using an iteratively reweighted regularized least-squares procedure; regularization of the time-dependent internal field was relaxed at high spherical harmonic degree compared with previous versions of the CHAOS model. We use CHAOS-7 to investigate recent changes in the geomagnetic field, studying the evolution of the South Atlantic weak field anomaly and rapid field changes in the Pacific region since 2014. At Earth’s surface a secondary minimum of the South Atlantic Anomaly is now evident to the south west of Africa. Green’s functions relating the core–mantle boundary radial field to the surface intensity show this feature is connected with the movement and evolution of a reversed flux feature under South Africa. The continuing growth in size and weakening of the main anomaly is linked to the westward motion and gathering of reversed flux under South America. In the Pacific region at Earth’s surface between 2015 and 2018 a sign change has occurred in the second time derivative (acceleration) of the radial component of the field. This acceleration change took the form of a localized, east–west oriented, dipole. It was clearly recorded on ground, for example at the magnetic observatory at Honolulu, and was seen in Swarm observations over an extended region in the central and western Pacific. Downward continuing to the core–mantle boundary, we find this event originated in field acceleration changes at low latitudes beneath the central and western Pacific in 2017.

/pdf/the-chaos-7-geomagnetic-field-model-and-observed-changes-in-3c4reuhbrh.pdf

The CHAOS-7 geomagnetic field model and observed changes in the South Atlantic Anomaly

The generation of the Earth's continental crust modified the composition of the mantle and provided a stable, buoyant reservoir capable of capturing mantle material and ultimately preserving ore deposits. Within the continental crust, lithospheric architecture and associated cratonic margins are a first-order control on camp-scale mineralization. Here we show that the evolving crustal architecture of the Archaean Yilgarn Craton, Western Australia, played a key role in controlling the localization of camp-scale gold, iron and nickel mineralized systems. The age and source characteristics of Archaean lithosphere are heterogeneous in both space and time and are recorded by the varying Nd isotopic signature of crustal rocks. Spatial and temporal variations in isotopic character document the evolution of an intra-cratonic architecture through time, and in doing so map transient lithospheric discontinuities where gold, nickel and iron mineral systems were concentrated. Komatiite-hosted nickel deposits cluster into camps localized within young, juvenile crust at the isotopic margin with older lithosphere; orogenic gold systems are typically localized along major structures within juvenile crust; and banded iron formation (BIF)- hosted iron deposits are localized at the edge of, and within, older lithospheric blocks. Furthermore, this work shows that crustal evolution plays an important role in the development and localization of favourable sources of nickel, gold and iron by controlling the occurrence of thick BIFs, ultramafic lavas and fertile (juvenile) crust, respectively. Fundamentally, this study demonstrates that the lithospheric architecture of a craton can be effectively imaged by isotopic techniques and used to identify regions prospective for camp-scale mineralization.

Crustal evolution, intra-cratonic architecture and the metallogeny of an Archaean craton

We present results of synthetic time-lapse and real repeatability multi-transient electromagnetic
surveys over the North Sea Harding field. Using Archie’s law to convert
porosity and fluid saturation to resistivity we created 3D isotropic models of the reservoir
resistivity at different stages of production from the initial state in 1996 through
to complete hydrocarbon production by 2016 and, for each stage, we simulated an
east-west transient electromagnetic survey line across Harding. Unconstrained 1D
full-waveform Occam inversions of these synthetic data show that Harding should be
detectable and its lateral extent reasonably well-defined. Resistivity changes caused
by hydrocarbon production from initial pre-production state to production of the oil
rim in 2011 are discernible as are significant changes from 2011–2016 during the
modelled gas blowdown phase.
The 2D repeatability surveys of 2007 and 2008 tied two wells: one on and the other
off the structure. Between the two surveys the segment of the field under investigation
produced 3.9 million barrels of oil – not enough to generate an observable time-lapse
electromagnetic anomaly with a signal-to-noise ratio of 40 dB. Processing of the
2007 and 2008 data included deconvolution for the measured source current and
removal of spatially-correlated noise, which increased the signal-to-noise ratio of the
recovered impulse responses by about 20 dB and resulted in a normalized root-meansquare
difference of 3.9% between the data sets. 1D full-waveform Occam inversions
of the real data showed that Harding was detectable and its lateral extent was also
reasonably well-defined.
The results indicate that the multi-transient electromagnetic method is suitable for
exploration, appraisal and monitoring hydrocarbon production.

Multi‐transient electromagnetic repeatability experiment over the North Sea Harding field‡

We provide a tutorial on the marine controlled-source electromagnetic (CSEM) method aimed at geoscientists and explorationists who are new to EM methods This tutorial highlights some of the issues to be considered in planning and executing a CSEM survey, and interpreting the results CSEM methods can add valuable information on the resistivity structure of the subsurface, which complements measurements obtained from seismic or other geophysical methods We also discuss the development of the method and provide an overview of the CSEM acquisition approaches applied today Understanding the sensitivity of the CSEM method to seafloor resistivity structures is key to ensuring a successful survey We illustrate this with simple examples, demonstrating the effect of reservoir and overburden properties and the effect of electrical anisotropy It is also important to understand how well a given resistivity structure can be recovered from realistic survey data We apply an inversion approach to illustrate

Marine controlled-source electromagnetic methods in the hydrocarbon industry: A tutorial on method and practice

We have developed the open-source toolbox custEM (customizable electromagnetic modeling) for the simulation of complex 3D controlled-source electromagnetic (CSEM) problems. It is based on t...

custEM: Customizable finite-element simulation of complex controlled-source electromagnetic data

The Python-code empymod computes the 3D electromagnetic field in a layered earth with vertical transverse isotropy by combining and extending two earlier presented algorithms in this journal. The bottleneck in frequency- and time-domain calculations of electromagnetic responses derived in the wavenumber-frequency domain is the transformations from the wavenumber to the space domain and from the frequency to the time domain, the so-called Hankel and Fourier transforms. Three different Hankel transform methods (quadrature, quadrature-with-extrapolation [QWE], and filters) and four different Fourier transform methods (fast Fourier transform [FFT], FFTLog, QWE, and filters) are included in empymod, which allows us to compare these different methods in terms of speed and precision. The best transform in terms of speed and precision depends on the modeled frequencies. Published digital filters for the Hankel transform are very fast and precise for frequencies in the range of controlled-source electromag...

An open-source full 3D electromagnetic modeler for 1D VTI media in Python: empymod

We developed a methodology to estimate resistivities from seismic velocities. We applied known methods, including rock physics, depth trends, structural information, and uncertainty analysis. The result is the range of background resistivity models that is consistent with the known seismic velocities. We successfully tested the methodology with real data from the North Sea. These 2D or 3D background resistivity models yield a detailed insight into the background resistivity, and they are a powerful tool for feasibility studies. They could also serve as starting models or constraints in (iterative) forward modeling of electromagnetic data for the determination of subsurface resistivities.

/pdf/background-resistivity-model-from-seismic-velocities-45eucdzyoz.pdf

Background resistivity model from seismic velocities

The open-source code fdesign makes it possible to design digital linear filters for the Hankel and Fourier transforms used in potential, diffusive, and wavefield modeling. Digital filters c...

/pdf/a-tool-for-designing-digital-filters-for-the-hankel-and-wphrnotqz2.pdf

A tool for designing digital filters for the Hankel and Fourier transforms in potential, diffusive, and wavefield modeling

Controlled-source electromagnetic (CSEM) surveys are a common geophysical investigationtool in the search for, amongst other, groundwater, hydrocarbons, and minerals. The nu-merical modelling of CSEM data requires the solution of the Maxwell equations. These canbe simplified in the particular case of CSEM, as the frequencies used in surveys are usuallysufficiently low to ignore any displacement currents. A diffusive problem remains, which hasthe resulting system of equations given in the frequency domain byE

/pdf/emg3d-a-multigrid-solver-for-3d-electromagnetic-diffusion-4zeobncqhg.pdf

emg3d: A multigrid solver for 3D electromagnetic diffusion

A method for determining resistivity anisotropy of subsurface rock formations includes imparting a transient electromagnetic field into the subsurface rock formations. Electromagnetic response of the formations is measured at a plurality of offsets from a position of the imparting. For each offset, an arrival time from the imparting is determined of a peak of an impulse response such that the response is related to subsurface horizontal and vertical resistivities. For each offset, a step response of the formations is determined at a time from the imparting selected such that the step response is related substantially only to mean resistivity. The arrival time of the peak of the impulse response and the late time value of the step response are used to determine the resistivity anisotropy.

Dieter Werthmüller

Papers

An open-source full 3D electromagnetic modeler for 1D VTI media in Python: empymod

Background resistivity model from seismic velocities

A tool for designing digital filters for the Hankel and Fourier transforms in potential, diffusive, and wavefield modeling

emg3d: A multigrid solver for 3D electromagnetic diffusion

Method for Determining Resistivity Anistropy From Earth Electromagnetic Transient Step Response and Electromagnetic Transient Peak Impulse Response