Seismic Methods in Mineral Exploration

Heather Schijns

Australian Exploration Challenges

Numerous challenges exist to exploration in Australia• Sparse outcrop limits geological mapping

• Limited knowledge of depth to basement impacts ability to constrain potential field inversions

• Deep cover impacts resolution of potential field methods

• Conductive cover limits penetration and/or resolution of electromagnetic methods

• Difficult to interpret robust 3D models of geological structure

Geophysical inversion and interpretation can be improved by including geological structure.

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Gravity for IOCG exploration

Most IOCG’s to date have an associated gravity anomalyEven under significant cover, the hematite in known IOCG’s hastypically resulted in gravity anomalies of >2mGal.

Synthetic modeling shows that IOCG bodies are likely to bedetectable to significant depth when located under a consistentcover sequence.

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Integrating structure with geophysical interpretation

Variations in cover thickness affect detectabilityIf there is a significant variation in cover thickness it may be moredifficult to identify or correctly interpret even a significant densityanomaly in gravity data.Understanding structure can allow us to better vector to highpotential areas and correctly interpret geophysical measurements.

Seismic can assist with these problems, but it is often not realistic toacquire 3D reflection seismic at a large scale or at all in greenfieldsexploration.

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Seismic methods and innovation

How can we assess whether seismic will work for the exploration problem without doing a full 3D seismic survey? And how can we get more out of the seismic we are acquiring?

• Petrophysical measurement

• Passive HVSR

• Microseismic tomography

• 2D reflection seismic

• Seismic attributes

• Advanced processing

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Evaluating physical properties contrasts

Core velocity measurements are useful but notinfallibleLow permeability rocks may undergo permanentmicroscopic damage during the depressurizationthat occurs during core retrieval. This can lowerlaboratory velocities significantly from what wouldbe measured in-situ.

Sonic measurements may be higher than seismicvelocities, especially in lower pressure regimes inthe near surface, due to porosity.

It is difficult to know if these effects are present anddifficult to correct for them. The effects are notnecessarily consistent between different lithologiesor even within the same lithology.

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Gneiss

Meta-serpentinite

Lithology

Passive HVSR

Using Horizontal / Vertical Spectral Ratio (HVSR) to characterize depth to basement• Acquisition of HVSR is relatively inexpensive, fast and low impact

• HVSR can allow estimation of depth to basement provided there is sufficient impedance contrast between the cover and basement

• The method is most sensitive to the first large impedance contrast

• Acquisition can be impacted by sources of surface noise if they are too close by

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Microseismic tomography

Using microseismic tomography to map structure• Passive acquisition is low cost and low

impact

• Required time depends on local naturalseismicity

• Method is sensitive to variations inseismic velocity

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2D seismic reflection

2D seismic can provide context to 3D interpretationAlthough 3D will usually provide a better image, well placed2D data can provide context to interpretation of other 3Dgeophysics and support or disprove geologicalinterpretations.

N

S

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Seismic attributes

Using seismic attributes to assist with interpretation andquantificationQuantitative interpretation requires carefully acquired and carefullyprocessed data, but can yield important clues about lithology.

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NS

Improved acquisition and processing

Acquisition and processing parameters impact final data qualityTesting the impact of different acquisition parameters – from 3C to low frequency– to determine their impact on data quality, in addition to testing of newerprocessing algorithms such as full waveform inversion and pre stack depthmigration can significantly raise the quality of the final reflection data imagery.

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Summary

• Understanding structure can be critical to exploration success

• Low cost seismic methods can assist with structural interpretation

• Seismic reflection methods can benefit from careful attention to acquisition and processing parameters

• Additional information can be extracted from seismic data through the use of seismic attributes

• Petrophysics can be indicative but the best way to determine if seismic works in a given area for a given target is to try it

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