Magnetotelluric

 

    

UPSL has recently aquired a state of  the art Phynix V5-2000 24-bit MT system.  Our main focus is the interpretation of Magnetotelluric data and field QC of data acquisition. We also work with other techniques such as AMT, CSAMT, and EM. UPSL's personnel  have supervised field acquisition in various countries  for petroleum, mineral, and geothermal resources.

 

 

Recent applications of Magnetotellurics have produced results superior to those of earlier years by many improvements in the technology, the application and the interpretation of the method. MT continues to be useful in those areas where seismic acquisition is either difficult or expensive, where the surface inhibits effective seismic penetration, or where the use of MT is cost-effective as a predecessor to seismic.

Improvements made to MT within the last few years include: multi-site acquisition with varied station spacings and omni-directional, low frequency antennae; smaller, higher storage field computers, sensor-box modifications for filtering unwanted signal (such as lightning); post-acquisition re-processing of the data via robust schemes; and faster, larger 2-D and 3-D modeling capabilities.

Within the last two years, MT has been used world-wide for a variety of exploration targets. These include the continued use of MT for mapping sub-carbonate plays in Papua New Guinea with on-going drilling. Here, use of seismic is precluded due to thick, high-velocity surface limestone. MT has been used as a precursor to seismic in overthrust regimes in Nevada and Colorado where MT is used to highgrade areas as prospects before seismic programs are acquired. In Greenland, MT has been used for reconnaissance mapping in large basin environments sub-basalt.

On-going system modifications are being made to provide larger channel systems and future marine acquisition of MT.


Exploration programs are being driven into areas of increasing seismic difficulty with associated escalation in cost. Depth imaging, potential field and electrical methods geophysics are playing an increasingly important role to help the explorationist reduce risk in these areas. In so far as electrical methods are concerned, exploration environments can be broadly cast into the following categories: limestone, volcanic, basement thrusts or other dense layer at the surface or in the geological section, severe topography, and environmentally sensitive areas. Now, thanks to new technology, electrical methods, in particular Magnetotelluric (MT), are making a strong comeback as a cost-effective aid to exploration showing an increasingly valuable role augmenting seismic where seismic data quality is poor and difficult to interpret. Hundreds of MT systems are currently in use throughout the world (Christopherson, 1997). However the results of several surveys conducted in highly noisy environments, as in Italy. have shown the severe limitation of the MT method when applied to these extreme. Good news! This paper focuses on the practical application of the current state of the art in acquisition and processing capable of producing reliable and interpretable MT data in these challenging environments.

The problem
The noise sources in difficult MT areas are mainly of two kinds. The first kind is due to the presence of power plant equipment, power lines and other local cultural disturbances. All these local disturbances produce an incoherent noise mainly affecting higher frequencies - usually above 1 Hz, - which can be removed using a remote-reference site only few kilometers away. The second kind of noise is due to the presence of very strong sources of coherent electromagnetic signals such as direct current electric trains and active cathodic protection of pipelines that can render MT data useless at frequencbelow 1 Hz. Although the data may appear very coherent, the apparent resistivity and phase derived from these data look very much like the response due to a local dipole source instead of the plane wave MT source. Once the reference processing has removed the bias noise, the data scatter and poorer point-to-point continuity indicate the poor signal to noise ratio of what remains. The effects of the electromagetic field produced by electric trains and cathodic protection systems are sources of serious artificial disturbance for natural magnetic and electric fields measurements even if the observation sites are situated some ten kilometers away from the noise source. Standard remote reference processing coupled with various sort of robust data adaptive weighting scheme are ineffective when confronted with these sorts of difficulties, in particular, if there are outliers or strong correlated noise signals in most data sections.

The solution

I It is our view, that in order to fully optimize the potential of MT applied to highly noisy environments; a comprehensive overall approach is required. To this end, critical parts of the acquisition and processing flows have been reworked. New acquisition and processing technologies have been introduced specifically tuned to high coherent noise constraints. The following elements have formed the overall frame of the comprehensive approach:

  • Data acquisition techniques based on real data monitoring.

  • Recording of multiple GPS-synchronized sites.

  • Use of multiple very far-fixed remote reference sites tested to be free of regional correlated noise sources.

  • Introduction of an iterative data acquisition strategy.

  • Use of an increased spatial density of MT sites and logistic flexibility.

  • Introduction of 24 bit acquisition and processing technology.

  • Use of an improved multi-source smooth MT transfer function method (Larsen ET al., 1996).

  • Use of a new processing approach based on: selection of non or low-contaminated subsets of data followed by filtering of these subsets using a reweighted least median of squares for the magnetic data and a recurrent neural network approach for the electric data (Zerilli et al., 1997).

 

 

The V5 System 2000 uses a new approach to EM data acquisition based on multiple, rugged, lightweight, independent data acquisition units (MTUs), which are synchronized by time signals broadcast by the GPS (Global Positioning Satellite) system. Use of 24-bit analog-to-digital converters provides wide dynamic range, simplified circuitry, and lower cost.

The V5 System 2000 is offered for the MT (Magnetotelluric, AMT (Audiofrequency MT) techniques. Future enhancements will provide data acquisition capability for other commonly used EM techniques, including Long Period Transient EM and Induced Polarization (IP).

The field configuration and spacing of the MTUs can be varied with complete flexibility, according to the requirements of the survey. No cable links are required between the boxes - an important advantage in areas with rugged topography, lakes, swamps and other access difficulties.

Where access is easier, or if required, the MTUs can be deployed at very close spacing or for continuous profiling.

 


The MTU is provided in 3 configurations. The MTU-2E measures 2 channels of electric field data; the MTU-3H measures 3 channels of magnetic field data; the MTU-5 combines both.

When the V5 System 2000 is used for MT, the electric field is preferably measured at a closer spacing than the magnetic field. Thus, most of the deployed units are usually MTU-2Es, with a smaller number of MTU-3Hs or MTU-5s. The MTU-2E with electric field sensors costs significantly less than the MTU-3H / MTU-5 with magnetic sensors, so that the required sampling density can be achieved at an acceptable cost.

Each MTU is configured with ample solid state memory, onboard processor and diagnostics, onboard GPS antenna and auxiliary precision timing. The solid aluminum enclosures are equipped with rugged, military style connectors.

The lightweight MTUs can be easily carried to remote locations. In the distributed (stand alone) configuration, data is retrieved by ruggedized PCs for transfer to base camp.

An important benefit of the innovative architecture of the V5 System 2000 is the light weight and extreme portability of the MTUs. All the equipment for a single 2-E measurement is easily backpacked by two field workers, reducing labor costs and permitting economical MT and other EM surveys even in remote, rugged, off-road areas.

The MTUs can be deployed without any cable links between the boxes, providing great logistic flexibility.

In reconnaissance of large areas, a multi-pass data acquisition strategy is used. The MTUs are deployed at relatively wide spacing in a first "pass" of data acquisition. If areas of interest are discovered in the first pass the MTUs can be immediately redeployed in those areas at closer spacing in the second pass to provide a higher resolution image. In this way the spatial sampling density is always optimized and cost is always minimized.

New data acquisition techniques provide far more data than previous generation equipment. New multi-site processing techniques provide better interpretation, even in noisy areas.