Samples, Sites, and Localities

Starting with v2.1, PaleoMag has recognized samples, sites, and localities as distinct entities. Previous versions referred to "sites" while meaning, in the present parlance of paleomagnetism, localities. A few leftover references might still grace this manual and the program.

Basically the hierarchy used is that an individual trimmed core (or other distinct piece of rock) is a sample. This is the building block of all else. Sites are collections of samples that represent a single measurement of the earth's ancient magnetic field. Samples from within a single volcanic flow are one example; another common (but more debatable) grouping is collecting samples from a single stratigraphic horizon (e.g., a single sedimentary bed). Averaging samples within a site should not average secular variation (aside from what averaging might occur within a sample). A locality is a more nebulous term, generally representing a single structural block or a single location where samples and sites are closely spaced. Within this program a locality is defined as the collection of samples within a single folder and listed in a single .SAM file.

PaleoMag provides two methods of defining sites: either explicitly within the .SAM file, or on the fly within the program through the "Define Sites..." (or "Redefine Sites...") command. This command, defined above, will group samples into sites using the stratigraphic level using a parameter called the "site width." This value should be less than the smallest stratigraphic distance between samples in adjacent sites but greater than the greatest distance between samples within a single site. If desired, this grouping can be saved to the .SAM file for future use (or editting). If a single site width does not exist, it is best to manually edit the .SAM file to define the sites. Each site can have a 2 character code (case sensitive) which follows the stratigraphic level; see the File Formats section below. Note that raw files from the CIT IBM-PC system lack the strat levels in the .SAM file; it is best to first run PaleoMag on such sites to update the .SAM format.

Once sites have been defined, plots that groups measurements from multiple samples (e.g., Least Squares Equal Area or Strat plots) can be displayed either by sample or Fisher mean, which is displayed. Through the "Include Sites..." option, the user can restrict the visable sites to those with an alpha-95 below some value; sites with only a single measurement (which obviously lack an alpha-95) can also be hidden. Note that the alpha-95 calculated is usually only a crude approximation for the small number of samples in most sites.

When displaying equal-area plots by statistics are done on the site means and not from the collection of individual samples. Note that each site is considered to have equal weight.

A dirty trick for getting an idea of the mean from a noisy locality without making any least-squares fits is to define sites, perhaps making each sample a unique site, and then select "Locality Equal Area Plot..." and choose several demag levels where the characteristic component is the only component present along with the desired samples. When displayed least-squares fits for each sample's demagnetization curve.

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