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Why is there a logarithmic grid when these wireline density and neutron porosity logs have linear scale labels, and when these logs are usually displayed on linear scales?

Part of wireline log printout

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    $\begingroup$ What is your question? $\endgroup$ – Pont Apr 12 '16 at 9:48
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    $\begingroup$ Reading between the lines (ha), I think the question is, "Why is there a logarithmic grid when the logs have linear scale labels, and when these logs are usually displayed on linear scales?". It's a fair question. I think it's a mistake, and it would make me question the quality of the data. $\endgroup$ – kwinkunks Apr 12 '16 at 13:45
  • $\begingroup$ Could you give some information on the origin of the data and what is displayed? $\endgroup$ – daniel.neumann Apr 13 '16 at 8:28
  • $\begingroup$ @kwinkunks Judging by the latest edit, your guess was extremely accurate! :) $\endgroup$ – Pont Apr 13 '16 at 8:42
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    $\begingroup$ Maybe it helps if you tell us what we actually see in this picture, so that we know how the quantities involved scale... $\endgroup$ – Jan Doggen Apr 13 '16 at 13:23
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It's a mistake.

You probably know this, but for anyone who's interested, these wireline logs are:

  • DRHO — density ('RHO') correction (delta, or 'D') in g/cc.
  • DPHI — porosity ('PHI') from density ('D'), as a fractional proportion.
  • NPHI — porosity from the neutron log ('N'), as a fractional proportion.

All of these logs are usually displayed on a linear scale. Indeed it wouldn't make sense to display them on a log scale, which is mainly for displaying resistivity logs, e.g. the induction log. Even if they were, it wouldn't make sense to show the usual 4 octaves (0.2 to 2000 for resistivity), given the range of the data and the scale of porosity values geologists are interested in.

It's not clear if the data have actually been displayed on a log scale, or if it's just that the grid has been erroneously added. I would treat the data with suspicion.

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