Introduction to AMT

Overview

Natural-source Audio-frequency Magnetotellurics between the earth’s magnetosphere and (AMT) is an electromagnetic survey solar wind. High-frequency sources in technique that uses naturally-occurring the audio range (> 1 Hz) are generated ionospheric currents and lightning by thunderstorms worldwide. storms — passive energy sources — to The AMT and MT geophysical methods electrically map geologic structure to combine measurements made of site- depths of 500 meters or more. specific electric and magnetic fields Natural-source electromagnetic (EM) using grounded dipoles and magnetic signals are generated in the atmosphere field antennas over a wide band of and magnetosphere. The time-varying frequencies. Low frequencies sample electric and magnetic fields induce deep into the earth and high frequencies currents into the earth and oceans, (AMT) correspond to shallow samples. AMT is a passive which produce magnetotelluric (MT) electromagnetic Ground resistivity values are calculated signals, which are measured by AMT from the magnitude and ratios of these imaging method and MT data acquisition systems. components and then mapped using using the earth’s Low-frequency magnetotelluric EM signals Zonge inversion and modeling software. magnetotelluric (< 1 Hz) are generated by the interaction Ground resistivity relates to the geology. field to map Advantages of AMT geologic contacts and structure. • No need for power source or high- • Zonge’s backpack portable system voltage electrodes. allows for use in difficult terrain, • Minimal environmental impact. • Relatively easy field logistics for large-scale regional reconnaissance • Stations can be acquired almost exploration and for detailed surveys of anywhere and can be placed any local geology. Fast data collection. distance apart. AMT Exploration Applications

• Mineral, groundwater and geothermal • Onshore oil and gas exploration exploration where seismic is not feasible or is cost prohibitive • Excellent for imaging moderately deep geologic structure and near- • Mining and water resource surface geology to depths of about management 500 meters in detail WWW.ZONGE.COM

Field Logistics

The Zonge AMT equipment system Scalar AMT is collected by a series of consists of the GDP multichannel data electrode pairs, each measuring Ex, set processor (receiver), ANT/6 magnetic- up along a survey line with one field sensor(s), electric-field cabling, and magnetic-field sensor measuring Hy. non-polarizing electrodes for measuring Tensor AMT acquires additional electric- ground potentials. This system is easily field Ey and magnetic-field Hx readings, transported and multiple systems can be which provide information about synchronized to provide a remote impedance directionality at a particular reference for noise cancellation. location. The Hz component used on lower-frequency MT surveys is not AMT signals are extremely subtle and usually recorded for AMT surveys. measuring them requires highly sensitive magnetometers, very low-noise electric- Scalar AMT field sensors, and careful installation. • Easily modified to accommodate The Zonge ANT/6 magnetic-field sensor terrain or other needs in the field has a noise threshold level below typical • Fast linear collection rates natural-source AMT signal levels. With • Limited in resolving or delineating this system, it is possible to collect the most useful data in the attenuation band multi-dimensional targets • under the most conditions. Images the most geologic contacts when data is collected roughly At each survey station, electrode pairs perpendicular to geologic strike (if are used to measure the electric (E) known) field, and magnetometers are used to measure the magnetic (H) field. The Tensor AMT electrical conductivity of the subsurface • Resolves ambiguity of strike which affects the amplitude, phase, and may not be known at the beginning of directional relationships of the E and H the survey fields on the earth’s surface. The • Provides structural definition and frequency ranges of interest are typically directionality from 4 to 8192 Hz for AMT. • Slower production rates compared to “Ex” and “Ey” are used to designate Scalar array but still far faster than MT measured components of the electric field. “Hx” and “Hy” designate measured components of the magnetic field. Different array types may be used to acquire the most beneficial data for a particular survey area and objectives. Zonge ’s backpack portable AMT acquisition system allow s

for use in difficult terrain and relatively easy field logistics.

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Inversion and Modeling

Both 1D and 2D Smooth-Model inversion difference between the E and H phase software is used to convert measured E components. and H components to resistivity versus Inversion models of AMT Cagniard depth profiles. Smooth-model inversion Resistivity and Impedance Phase data mathematically back-calculates (“inverts”) provide detailed images of the from the measured data to determine a conductivity structure at depth. likely location, size and depth of the source or sources of resistivity changes. The Zonge AMT modeling software allows various parameters to be used to Imaging natural-source AMT data is a create the most geologically reasonable multistage process in which both model. 1D and 2D software will produce resistivity and phase are used in the modeled depth sections for scalar AMT. inversion to image resistivity changes 2D software will produce modeled depth associated with the geology. Cagniard sections for tensor AMT. Resistivity and Impedance Phase data are calculated from the collected electric 2D inversion produces two-dimensional field and magnetic field data. Cagniard shaped features (e.g., edges associated Resistivity is a frequency-dependent, with contacts at depth) and is able to apparent resistivity calculation based on model any dipole orientation, but in the ratio of the E and H magnitude doing so assumes the survey line components of the electromagnetic field. crosses the geologic strike on the Impedance Phase is defined as the perpendicular.

Zonge 2D smooth -model i nversion corrects data for topographic effects. When possible, AMT survey lines should be aligned perpendicular to geologic strike for best results.

Lateral and Vertical Resolution Inversion models show reasonable detail related to size with resolving features at to certain depths. The skin depth (depth depth. Survey resolution is also a where field strength decreases to 37% in function of target size and conductivity a homogeneous earth) is considered the contrasts. Lateral survey resolution is depth of penetration. It is possible for estimated at 15% of the depth of AMT results to image the subsurface at investigation. This is one reason that greater depth in more resistive ground survey design for natural source AMT by decreasing the floor frequency. often differs radically from that used for However there are practical limitations MT surveys.

Final Product

The results of processing and modeling at a constant elevation or depth. Plan natural-source AMT data can be views can help highlight trends between presented in several forms: modeled lines. Fence diagrams show 2D cross cross sections, plan views, fence or 3D sections of the resistivity inversion diagrams. When stations are collected results in a spatially relevant 3D context. Zonge International is along several lines in the same area, Dotted lines in the fence diagram below data can be displayed in plan-view plots represent mapped faults. an employee-owned company providing ground geophysics field services, consulting and customized equipment to geoscientists and engineers worldwide.

The company is known for its expertise in the development and application of broad- band electrical and EM methods. Plan views can help highlight trends Smooth model inversion results . between lines of stations. Fence diagrams in 3D context. Tucson, Arizona, USA 1 520-327-5501 Reno, Nevada 1 775-355-7707 Reference [email protected] Cagniard, L., “Basic Theory of the magnetotelluric method of geophysical prospecting.” Geophysics, 18, pp. 605-635, 1953. Goldstein, M.A. and Strangway, D.W., “Audio-frequency magnetotellurics with a grounded electric dipole source.” Geophysics, 40, pp. 669-683, 1975. Zonge, K.L. and Hughes, L.J., “Controlled source audio-frequency magnetotellurics.” Electromagnetic Methods in Applied Geophysics, Vol. 2, edited by Nabighian, M.N., pp. 713-809. Society of Exploration Geophysicists, 1991. Wight, D. E. and F. X. Bostick, “Cascade decimation, a technique for real time estimation of power spectra.” Proceedings of IEEE Internal Conference on Acoustic, Speech Signal Processing, Denver, Colorado, April, 1980. Vozoff, K., “The Magnetotelluric Method.” Electromagnetic Methods in Applied Geophysics, Vol. 2, edited by Nabighian, M.N., pp. 641-711. Society of Exploration Geophysicists, 1991. For more information, see www.zonge.com/geophysical-methods/

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