Signals and Noises

A familiar graph in the geosciences is the Greenland ice-core isotope plot.

 

v16n1_icecore1_S2N

GISS O16 isotope plot

A first pass ‘look’ at the plot would interpret the data as noise, the red line region, and signal, the blue line region.  Remember the X-Axis is Depth, not age.

One explanation might be that the ice core labelled as “noise” might be very recent ice and that the abrupt change from red to blue marks an icy unconformity below which there is signal. The depth of the red-blue transition is approximately 1250 metres down, so when you work from down to up. and courageously assign an age to the core, then what age is the red-blue transition assigned?

Given the calving rate of the Greenland Ice Cap, (and has anyone actually estimated that?), it seems likely that the ice-conveyor operates to the depth of 1250 metres above which ice is more or less rapidly replaced, leaving behind fossil ice below the icyformity.

When did Greenland ice up? At the end of the Roman Period that in the revised chronology would have been ~950AD.

Update: Electrical resistivity sounding of the East Antarctic Ice Sheet, Sion Shabtaie, Charles R. Bentley (Source)

Electrical resistivity soundings using a Schlumberger array have been carried out at Dome C, East Antarctica (74°39′S, 124°10′E, elevation 3400 m), to sound the entire 3500-m depth of the ice sheet. Changes in density and temperature are largely separated in the ice at Dome C, so activation energies for both firn and ice could be determined: we find an activation energy of 0.25 eV in both solid ice and firn between −15°C and −58°C. A common value of the activation energy points to a single transport regime in which the charge carriers and conduction paths are the same in firn and ice. To evaluate the variation of resistivity with density, we have considered five dielectric mixture models that fit the available data on the high-frequency dielectric constant of firn. Only Looyenga’s equation fits the field data for dc resistivity. In the upper 900 m of the ice sheet, where impurity concentrations are known from core samples, we find no correlation between resistivities and the concentrations of salts or acids. Instead, we find it likely that resistivities are correlated with the crystal size, hence with the Holocene-Wisconsin boundary in the ice column. A pronounced increase in resistivity, to a value comparable with that in temperate glacier ice, occurs deep within the ice sheet. We attribute this to a large increase in the size and irregularity of the ice crystals, which destroys the continuity of the impurity shells surrounding the ice crystals that we believe supply the conduction paths (Shabtaie and Bentley, 1994a). High resistivity does not imply removal of the impurities from the system; moderate concentrations of impurities can be accommodated by locating them in disconnected domains. (my boldening LH)

So layering marked by acids or bases, isn’t producing the resistivity layering from the radar surveying.

About Louis Hissink

Retired diamond exploration geologist. I spent my professional life looking for mineral deposits, found some, and also located a number of kimberlites in NSW and Western Australia. Exploration geology is the closest one can get to practicing the scientific method, mineral exploration always being concerned with finding anomalous geophysical or geochemical data, framing a model and explanation for the anomaly and then testing it with drilling or excavation. All scientific theories are ultimately false since they invariably involved explaining something with incomplete extant knowledge. Since no one is omniscient or knows everything, so too scientific theories which are solely limited to existing knowledge. Because the future always yields new data, scientific theories must change to be compatible with the new data. Thus a true scientist is never in love with any particular theory, always knowing that when the facts change, so too must he/she change their minds.
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6 Responses to Signals and Noises

  1. johnm33 says:

    Here’s a zoomable cross section, http://i.imgur.com/J5oAtH0.jpg
    and this is a reworked version http://forum.arctic-sea-ice.net/index.php?action=dlattach;topic=154.0;attach=9206;image
    The base looks like frozen waves to me, and if deep frozen followed by hail, snow and rain to bind it it’s no wonder that given the ‘atoll’ like mountains/islands around the perimiter it’s filled with snow. I think that the base was formed by a collosal flooding of the north and that huge areas of eurasia were filled with frozen water in the same event, and had Greenland not been surrounded by mountains the ‘waves would have eroded down and formed a very similar permafrost layer to that in Eurasia/Canada.
    I’m still thinking a more remote time, possibly around 8000 years bp.

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    • Note the vertical exaggeration of 75 times. Remove that and display the data in real scale to see what it actually represents.

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      • Whoops. second image does exactly that. My error – (thinks to himself, must not jump gun, must not jump gun,…..).

        Whatever, given it is a radar image, the waves in the horizontal traces are not physical ones but, dare I say it, EM waves, if such an idea is physically real in the first place.

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  2. Always helps to update the knowledge base.

    Ground radar maps the dielectric values, here of ice, versus depth. Layering correlates with dielectric contrasts, or layers that are more conductive (lower resistivity) and layers that are less conductive (higher resistivity). It is basically electromagnetic profiling and a geophysical technique I used with equivocal success in the field. That system was DIGHEM used by Geoterrex, now part of the global Fugro Group. EM techniques are difficult to useless to deploy in regions where highly conductive surface layers result in an electrical “skin effect” producing abundant electrical noise and a barrier to depth penetration. However EM works well in the Canada since most of it is crystalline outcropping basement but not very well in the deep, conductive regolith of Australia. And, unlike gravity interpretation, there are no multiple solutions to the acquired data. So when it works, it’s tremendous, and when it doesn’t, too bad. That’s mineral exploration.

    Layering in ice is thus ice with low conductivity versus ice with high conductivity. High electrical conductivity means ionised ice, or ice with impurities of such volume that it behaves geophysically as a distinct layer of size or thickness capable of being a good reflector. A thin layer of conductive ice would not be a good reflector compared to a thicker layer of ionised ice, everything else being equal. Ice with impurities in it would be more conductive that pure ice.

    So what does the radar layering in the ice actually represent? Especially the upper-most ice marked by the “noise” label?

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  3. Ian MacCulloch says:

    Your comments are correct about EM sensu stricto. However, developments in the last decade in the dark art of EM seismic technologies have opened up a whole new world for explorationists. It offers high resolution from surface to 4,000 metres. Developed by Aquatronic out of NZ and SA it was primarily used for aquifer mapping. It then extended into oil field mapping and is now being used to map potential zones in the Bendigo Field and at other locations dotted around Central Victoria. I have used it myself on shallow gold bearing cemented gravels below 15 metres of basalt cap and it is quite a tool. Aquatronic have also developed an android app for shallow aquifer mapping using force generated by a sledge hammer on a striker plate. Should help resolve the Liverpool Plains imbroglio by providing abundant and accurate data at a very low cost. Every farmer should have one.
    https://play.google.com/store/apps/details?id=com.atsgs.ATS&hl=en

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    • I used a hammer seismograph for the geology appraisal for the Cordeaux Rail tunnel 1977/78, or I should say the field assistant used the sledge hammer when I worked for SMEC :-).

      Mind you looking for kimberlite at the surface or near surface in Australia reduces to looking for clay in clay, as it were. Clay in Ss is a bit easier.

      “every farmer…” – meh, another gadget …..

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