Z-axis Tipper Electromagnetic Research Articles

3D joint inversion of airborne ZTEM and ground MT data using the finite element method with unstructured tetrahedral grids

As an airborne electromagnetic method induced by natural sources, the Z-axis tipper electromagnetic (ZTEM) system can primarily recover near-surface shallow structures, due to band-limited frequencies (usually 30–720 Hz) of the airborne survey and high sample rate acquisition along the terrain. In contrast, traditional ground magnetotellurics (MT) allows better recovery of deep structures as the data acquired are typical of large site intervals (usually higher than 1 km) and lower frequencies (usually lower than 400 Hz). High-resolution MT surveys allow for shallow small and deep large anomalies to be adequately interpreted but need large site intervals and broadband frequency range, which are seldom used as they are quite costly and laborious. ZTEM data are tippers that relate local vertical to orthogonal horizontal fields, measured at a reference station on the ground. As the 1D structures produce zero vertical magnetic fields, ZTEM is not sensitive to background resistivity. Thus, in general, ZTEM can only reveal relative resistivities and not real resistivities. A combination of the ZTEM and MT methods can be an effective technique, alleviating the shortcomings of the individual methods. At present, complex underground structures and topography introduce difficulties for data inversion and interpretation, as conventional ZTEM and MT forward modeling are generally used on structured grids with limited accuracy. To effectively recover complex underground structures with topography, we developed a 3D framework for joint MT and ZTEM inversion with unstructured tetrahedral grids. The finite element method is used for the forward problem because of its flexibility with unstructured tetrahedral meshes. The limited-memory quasi-Newton algorithm (L-BFGS) for optimization is used to solve the joint inverse problem, which saves memory and computational time by avoiding the explicit calculation of the Hessian matrix. To validate our joint inversion algorithm, we run numerical experiments on two synthetic models. The first synthetic model uses two conductive anomalous bodies of different sizes and depths. At the same time, a simple quadrangular is used for comparing the inversions with and without topography. In contrast, the second synthetic model represents a realistic topography with two different conductivities and the same depth. Both single-domain and joint inversions of the ZTEM and MT data are carried out for the two synthetic models to demonstrate the complementary advantages of joint inversion, while the second model is also used to test the adaptability of the joint inversion to complex topography. The results demonstrate the effectiveness of the finite element method with unstructured tetrahedral grids and the L-BFGS method for joint MT and ZTEM inversion. In addition, the inversion results prove that joint MT and ZTEM inversion can recover deep structures from the MT data and small near-surface structures from the ZTEM data by alleviating the weaknesses of the individual methods.

3D finite-difference modeling algorithm and anomaly features of ZTEM

The Z-Axis tipper electromagnetic (ZTEM) technique is based on a frequency-domain airborne electromagnetic system that measures the natural magnetic field. A survey area was divided into several blocks by using the Maxwell’s equations, and the magnetic components at the center of each edge of the grid cell are evaluated by applying the staggered-grid finite-difference method. The tipper and its divergence are derived to complete the 3D ZTEM forward modeling algorithm. A synthetic model is then used to compare the responses with those of 2D finite-element forward modeling to verify the accuracy of the algorithm. ZTEM offers high horizontal resolution to both simple and complex distributions of conductivity. This work is the theoretical foundation for the interpretation of ZTEM data and the study of 3D ZTEM inversion.

Helicopter AFMAG (ZTEM) EM and magnetic results over sedimentary exhalative (SEDEX) lead-zinc deposits at Howard’s Pass in Selwyn Basin, Yukon

In 2008, a regional scale 24,675 line-km survey covering a 25,000 km2 area (1 km line spacing) was flown in the Selwyn Basin. The survey footprint straddles east-central Yukon and overlaps into the western North-west Territories. In March 2013, Yukon Geological Survey purchased the survey data and, in November 2013, released the data publicly. The Selwyn Basin area is prospective for sedimentary exhalative (SEDEX)-style Pb–Zn–Ag mineralisation and the z-axis tipper electromagnetic (ZTEM) survey data provide insights into regional structures and plutons in the region. The survey overflew the Howard’s Pass SEDEX deposits at the south-eastern edge of the Selwyn Basin survey area that hosts a ~250 million tonne resource with ~4.5% Zn and ~1.5% Pb. Airborne geophysics has not been extensively used in SEDEX exploration of the Selwyn Basin and the ZTEM survey is one of the few publicly available airborne audio-frequency magnetic (AFMAG) EM-magnetic datasets that offer the opportunity to study the deposit response at Howard’s Pass in close detail.Rock physical properties indicate that the lowest resistivities are associated with the Road River Group that contains the Pb–Zn mineralised horizon at Howard’s Pass, but also include graphitic shales in the same formation. Major NW–SE to ESE and minor NNW–SSE linear conductive trends correlate with known regional geologic, structural and inferred mineral trends that were previously not visible in magnetic results. At the deposit scale, a thin NW–SE trending conductive lineament extends along the > 37-km-long ‘Zinc Corridor’ horizon at Howard’s Pass, but must include both the Pb–Zn sulphide mineralisation deposit horizon as well as the surrounding graphitic black shales. 2D and 3D ZTEM inversions reveal zones of enhanced conductivity along strike and at depth that appear to correlate with the clustering of Pb–Zn deposits, which had not been previously noticed.

Resistivity imaging in a fold and thrust belt using ZTEM and sparse MT data

As part of a CO2 reservoir exploration programme, NEOS acquired and analysed a variety of geophysical data within a ~2900 km2 region of the northern Raton Basin of southern Colorado (Figure 1a). The purpose of the survey was to better define the basin architecture and identify structures that may serve as traps for the CO2 gas. In addition to the Z-axis Tipper Electromagnetic (ZTEM) and magnetotelluric (MT) data discussed here, magnetic data were collected and publicly available ground gravity and satellite remote sensing data were analysed. We also had limited access to proprietary two-dimensional (2D) seismic and well data. Note that the acquisition area contains significant topography with elevations ranging from 1500 m above sea level in the basin to mountain peaks with elevations exceeding 4000 m.

Three-dimensional imaging of a Ag-Au-rich epithermal system in British Columbia, Canada, using airborne z-axis tipper electromagnetic and ground-based magnetotelluric data

We have evaluated results from a study combining airborne z-axis tipper electromagnetic (ZTEM) and ground-based magnetotelluric (MT) data to image an epithermal system in British Columbia. The spatially coincident use of these two methods allowed for a direct comparison of both data sets in the overlapping frequency band and showed that both measurements were consistent. Inversion of just the ZTEM data suffered from the lack of electric field amplitude information, which could be provided by the MT data. Three-dimensional inversion modeling of the two individual data sets was performed. Models of electrical resistivity derived from both data sets were consistent and could be correlated with the geological and structural setting of the mineralization. Gold is associated with disseminated pyrite and marcasite in quartz-sericite-altered felsic volcanic rocks and intrusions, especially near the contact with mafic volcanic rocks and a late diorite intrusion. The quartz-sericite alteration yields a conductivity anomaly, relative to the more resistive mafic country rocks. Although ZTEM and MT do not possess the resolution of the geologic model derived from borehole data, our model agrees well with a regional assessment of the deposit.

ZTEM and VTEM airborne EM and magnetic results over the Lalor copper‐gold volcanogenic massive sulfide deposit region, near Snow Lake, Manitoba

The 14.4-Mt Lalor copper-zinc-gold deposit situated in the Flin Flon Greenstone Belt of north-central Manitoba is large ([Formula: see text]) but deeply buried ([Formula: see text]) and was discovered using deep penetrating ground fixed-loop time-domain electromagnetic (EM). Results from three different airborne EM surveys over a five-year span permit comparisons of the system responses over the Lalor volcanogenic massive sulfide deposit region, assisted using 1D-2D-3D inversions. In late March, 2007, soon after its drilling discovery, Hudson Bay Exploration contracted Geotech to fly helicopter versatile time-domain electromagnetic (VTEM) surveys over the region west of Snow Lake, Manitoba. The deposit was not detected due to its large depth of burial ([Formula: see text]) and the lack of magnetic response. In 2009, Geotech carried out a helicopter z‐axis tipper electromagnetic (ZTEM) passive EM test survey over the Lalor deposit, which was successful and led to a larger survey over the area. In 2012, a second VTEM survey test was flown, using the more deeply penetrating [Formula: see text] system. Although it had sufficient penetration to detect the orebody, on-site mine-site development appeared to overprint the deeper [Formula: see text] deposit response. However, a deep ([Formula: see text]) conductive anomaly from a stringer sulfide occurrence, known as South Bull’s Eye, which lies south of Lalor, was well resolved in the ZTEM and [Formula: see text] surveys.

2D and 3D separate and joint inversion of airborne ZTEM and ground AMT data: Synthetic model studies

Abstract The ZTEM (Z-axis Tipper Electromagnetic) method measures naturally occurring audio-frequency magnetic fields and obtains the tipper function that defines the relationship among the three components of the magnetic field. Since the anomalous tipper responses are caused by the presence of lateral resistivity variations, the ZTEM survey is most suited for detecting and delineating conductive bodies extending to considerable depths, such as graphitic dykes encountered in the exploration of unconformity type uranium deposit. Our simulations shows that inversion of ZTEM data can detect reasonably well multiple conductive dykes placed 1 km apart. One important issue regarding ZTEM inversion is the effect of the initial model, because homogeneous half-space and (1D) layered structures produce no responses. For the 2D model with multiple conductive dykes, the inversion results were useful for locating the dykes even when the initial model was not close to the true background resistivity. For general 3D structures, however, the resolution of the conductive bodies can be reduced considerably depending on the initial model. This is because the tipper magnitudes from 3D conductors are smaller due to boundary charges than the 2D responses. To alleviate this disadvantage of ZTEM surveys, we combined ZTEM and audio-frequency magnetotelluric (AMT) data. Inversion of sparse AMT data was shown to be effective in providing a good initial model for ZTEM inversion. Moreover, simultaneously inverting both data sets led to better results than the sequential approach by enabling to identify structural features that were difficult to resolve from the individual data sets.

Helicopter AFMAG (ZTEM) Survey Results over the Ad Duwayhi intrusion related gold deposit (IRGD) in the Western Arabian Shield, KSA

As part of a larger survey campaign in the western Arabian Shield, helicopter electromagnetic survey tests were performed using the ZTEM (z-axis tipper electromagnetic) passive airborne EM (AFMAG) system over the 30.6Mt Ad Duwayhi gold porphyry deposit, in order to determine its airborne geophysical signatures. Ad Duwayhi is a Precambrian, shear-zone/vein-hosted, gold porphyry deposit that is located approximately 450 km SW of Riyadh and 450 km E/NE of Jeddah, KSA. The deposit is hosted within a late- to post-orogenic, Neoproterozoic age, northwest oriented granite body and a comagmatic, square quartz-porphyry that is possibly a younger phase of the granite. The Ad Duwayhi gold mineralization is characterized by low sulphide and base-metal content that typifies gold porphyries. The survey appears to have been able to characterize the regional geologic and localized resistivity signatures associated with the Ad Duwayhi gold porphyry deposit. It defined the larger, low porosity felsic-rich/quartz-altered intrusive host as a resistivity high, whereas the more fractured/porous vein systems that host the mineralization are defined as more localized, weak, linear resistivity lows. The resistivity high signature appears to lack a well-defined, surrounding resistivity low halo, due to phyllic-propyllitic alteration, that normally characterizes other copper-porphyries surveyed. More importantly, a flanking resistivity and magnetic high feature was mapped 1.5km further west, below a thin layer of conductive overburden, and might also represent another target of interest. The survey results suggest that Ad Duwayhi remains open to the southwest. The combined aeromagnetic and passive AEM signatures (high resistivity and low magnetic susceptibility) for Ad Duwayhi might be distinguishing characteristics for similar porphyry gold deposits regionally. Although 2D inversion modeling supports the interpretation of the ZTEM data in plan, more advanced 3D inversion and ground follow-up are required to further validate these targets of interest.

3D inversion of ZTEM data for uranium exploration

. (5) SUMMARY We present a Gauss-Newton-based 3D inversion method for airborne ZTEM (Z-axis Tipper Electromagnetic) data to define resistivity structure relating to uranium deposits in the Athabasca Basin of Saskatchewan, Canada. The geophysical targets in this region can be represented by conductive plunging dykes in a resistive basement beneath a thick, more resistive overburden. We demonstrate using synthetic examples the effectiveness of the inversion method for detecting and delineating the target dykes and discuss how the inversion results are affected by various factors. It is shown that the dykes can be well imaged to depths more than 2 km even for the data from 200-m receiver height, provided the flight line is oriented perpendicular to the strike, and that the inversion results are relatively robust to the choice of the starting model. It is also shown that topographic effects are not serious for detecting the dykes at depth, because topographic effects are more significant at higher frequencies, while the sensitivity to the dykes increases with decreasing frequencies. One important finding is that if the flight line is oblique to the strike, the dependence of the starting model increases and the overall resolution decreases, compared to the 2D case, due to 3D effects.

ZTEM data inversion and interpretation using the UBC-GIF MTinv3D code: A case history at the Silver Queen project, British Columbia

Z-axis Tipper Electromagnetic (ZTEM) surveys are rapidly becoming an integral part of geophysical exploration. This airborne AFMAG EM system measures the tipper of natural magnetotelluric fields at frequencies typically from 30Hz to 720Hz. The ZTEM system responds primarily to current channelling and operates at lower frequencies than active-source EM systems. As such, it maps bulk conductivity of the ground to lower values and greater depths than active-source airborne EM systems. ZTEM is particularly suited to mapping large regional structures, sulfide vein systems and intrusives that characterize porphyry copper deposits. The 3D resistivity model produced by inversion of ZTEM data using the UBC-GIF MT3Dinv code proves very useful for focussing exploration into the most prospective zones of a project area. The Silver Queen polymetallic vein system is a high grade past producer south of Houston, British Columbia. Current exploration around the old Silver Queen mine by New Nadina Explorations Ltd is conceptually based targeting of a blind, buried bulk tonnage deposit near the old mine and deeper in the mineralized system. Inversion of the ZTEM data and the magnetic data acquired over and surrounding the old mine, has identified a favourable setting close to an interpreted nearby intrusive body and within a large regional structure that flexes around it. Exploration is now focussed in this area, with a deeply penetrating induced polarization, electrical resistivity, and magnetotelluric ground survey completed over the target areas to direct drilling. The ZTEM processing and the inversion results from the ZTEM and magnetic data are presented.

3D inversion of SPECTREM and ZTEM data from the Pebble Cu-Au-Mo porphyry deposit, Alaska

This case study compares 3D inversion results from Spectrem Air?s SPECTREM 2000 fixed-wing timedomain airborne electromagnetic (AEM) system, and Geotech?s Z-axis Tipper Electromagnetic (ZTEM) airborne audio-frequency magnetics (AFMAG) system flown over the Pebble Cu-Au-Mo deposit in Alaska. Within the commonality of their physics, 3D inversions of both SPECTREM and ZTEM recover conductivity models consistent with each other and with the known geology. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West. The 3D interpretation of both surveys has yielded improved the understanding of the geology, alteration and mineralization of the Pebble system. There are distinct practical advantages to the use of both SPECTREM and ZTEM, so we draw no recommendation on one system over the other. We do conclude however, that 3D inversions of AEM and ZTEM surveys add significant value to exploration.