Passive Seismic for mapping and estimating the depth of paleochannels

Transcription

1 Passive Seismic for mapping and estimating the depth of paleochannels Matt Owers ASEG Passive Seismic Workshop Adelaide, August 2016

2 Mapping paleochannels in order to: 1. Find specific mineral deposits (such as U, Au, potash, etc.) in the paleochannels 2. Avoid the deep parts of the paleochannel while exploring for targets under the paleochannel, i.e. avoid collecting pointless geochemical samples and/or carrying out unnecessary drilling

3 Other methods commonly used to map paleochannels: 1. EM probably the most common method 2. Gravity can be ambiguous 3. Seismic reflection or refraction very costly 4. High resolution magnetics only where basement is magnetic 5. IP? 6. Drilling! Each method has advantages and disadvantages

4 Tromino passive seismic has a couple of clear advantages: 1. Fast easily acquire up to 80 readings a day with two-man crew 2. Simple don t need to be a geophysicist to use the instrument 3. Reliable while amplitudes may vary from day-to-day, the peak frequency remains consistent 4. Lightweight can carry them with you on a plane 5. Powered by 2 AA batteries 6. QC and initial processing software comes with the instrument 7. Cheap! Tromino surveying for depth of cover mapping will become a common tool for mineral exploration programs part of every exploration geologist s toolkit!

5 Multiple H/V peaks are possible with peaks occurring at resonant frequencies relating to each subsequent layer We hope to see a high amplitude, low frequency H/V peak corresponding to the paleochannel sediment-basement interface

6 A velocity model can be produced for each field reading, and velocity models can be constrained by taking readings at drillhole locations where the depths to geological horizons are known Modelling is carried out so that the shape of the velocity model roughly matches the measured H/V amplitude vs frequency profile The final result is a depth vs shear wave velocity profile for a particular site, set of transects or a mapped area The calculated depths obtained from the modelling can then be contoured to show the geometry of the paleochannel below surface

7

8

9 o Target was the sediment-bedrock interface beneath a salt lake playa o A strong impedance contrast gave rise to a high-amplitude peak H/V response o Modelled depths tied to existing drilling led to the creation of a depthto-bedrock map, highlighting deep paleochannel axis (thalweg)

10 o Calibration of peak frequency to bedrock depth from drilling o Generates a formula for depth estimation away from drillholes

11 o o Cross-section plots show distinct shape of paleochannel hidden beneath salt lake playa and deposits Final depth map was done at a fraction of the cost and time it would take to complete a drilling or conventional seismic program, and it penetrated conductive salt lake deposits and groundwater and where EM and IP surveying would not be able to penetrate, and gravity is too ambiguous Courtesy of Goldphyre Resources Ltd (ASX: 15/12/2015)

12 o Mapping deep paleochannel axis below a salt lake Applications for alluvial mineral deposits, paleochannel uranium and potash deposits, ground water, lignite deposits, etc. Courtesy of Goldphyre Resources Ltd (ASX: 15/12/2015)

13 o Different depth estimation methods provide similar results o Cross-section plots with frequency response shows sedimentary layering within paleochannel deposits

14 o Then proved with drilling o Mapping deep paleochannel central axis used to locate new (high cost) rotary mud drillholes o Coarse grained reservoir hit at the base of two 167 m deep drillholes

15 o Paleochannel represented in 3D

16 o Cross-section plot show distinct shape of paleochannel o An airborne EM survey was previously flown over the area Courtesy of Energy Metals Limited (ASX: 28/10/2015)

17 o Airborne EM survey poorly defined the shape of the paleochannel

18 o Airborne EM survey also poorly defined the depth of the paleochannel

19 o Small Proof of concept survey for gold producer in the Kambalda area o Tromino survey lines easily mapped paleochannel both on and off the salt lake o EM survey worked quite well but was slow and expensive Survey line off to side of salt lake Survey line over salt lake

20 o Tromino survey acquired in July 2016: 3 survey lines oriented N-NNE over known uranium deposit o U deposit is a verticallydipping sandstone-hosted stratiform body with some late remobilisation o Faults interpreted from high resolution aeromagnetic survey (flown late 2014)

21 o Passive seismic peak H/V frequency highlighted the depth to fresh bedrock/base of cover o High amplitude response over main U target -> highlighted another potential target

22

23 o Passive seismic clearly mapped the paleovalley

24 o Two lines from the refraction survey. U-shape is subdued owing to diffraction on flanks

25 The passive seismic method is an innovative use of existing technology that will impact the future of exploration in regolith covered areas and direct detection of paleochannel and other basin-related mineral deposits Tromino seismometers are small, lightweight, fully self-contained, easy to use, two units can leap-frog during surveying to collect 40 stations per day per field operator, data processing to be carried out by specially trained geoscientists using Grilla software which comes with the Tromino The Tromino instrument and software package is the only industry ready passive seismic system available