polespotter

polespotter(cmd0::String="", arg1=nothing; kwargs...)

Find stage poles from fracture zones and abyssal hills.

(Warning: Manual translate by Claude. Needs revision)

Description

polespotter reads line segments (abyssal hill fabric lineaments and/or fracture zone lineaments) and performs one of three types of scans. In spot mode it computes great circles for each individual segment along these lines. The bisector great circles to fracture zones and the great circle extensions of abyssal hills are expected to intersect at potential rotation poles. The assumption is that abyssal hill lines are meridians and fracture zones are parallels with respect to the rotation pole.

Line density may be computed and returned via a grid, the great circle lines may be returned via standard output, and the intersections of the great circles may be saved to file. In line mode it will determine which line segments are compatible with a given trial pole, while in pole mode it will compute chi-squared misfits for all the poles defined by the grid.

Optional Arguments

  • A or abyssalhills or hills : – hills=“filename”
    File with multiple segments of abyssal hill lineaments. These are assumed to reflect the great circle direction towards the rotation pole in effect when the seafloor was formed.

  • D or step or spacing : – spacing=value
    Sets the line increment for all great circles produced, in km [5]. Actual spacing will be adjusted to give an integer number of steps along the full circle.

  • E or sigma or uncertainty : – uncertainty=(a=val, f=val) | uncertainty=“a1”
    Provide different 1-sigma angular uncertainty (in degrees) in the orientation of abyssal hills or fracture zones. Use a (or abyssal) for abyssal hills [1] and f (or fz) for fracture zones [1]. These sigma values are then used to form weights = 1/sigma.

  • F or fracturezones or fz : – fz=“filename”
    File with multiple segments of fracture zone lineaments. These are assumed to reflect small circles about the rotation pole in effect when the seafloor was formed.

  • G or outgrid or save : – outgrid=“filename”
    Specify name for output grid. For spot mode we accumulate great circle line density for the grid. Each bin crossed by a great circle is incremented by 1, multiplied by cos(latitude), the length of the fracture zone or abyssal line segment, and any overall weight set via uncertainty. In pole mode we return the chi-squared misfit surface. Not used in line mode.

  • I or inc or increment or spacing : – inc=x_inc | inc=(x_inc, y_inc) | inc=“xinc[+e|n][/yinc[+e|n]]”
    Specify the grid increments or the block sizes. More at spacing

  • N or normalize or norm : – normalize=true
    Normalize the grid so max value equals 1 [no normalization].

  • R or region or limits : – limits=(xmin, xmax, ymin, ymax) | limits=(BB=(xmin, xmax, ymin, ymax),) | limits=(LLUR=(xmin, xmax, ymin, ymax),units=“unit”) | …more
    Specify the region of interest. More at limits. For perspective view view, optionally add zmin,zmax. This option may be used to indicate the range used for the 3-D axes. You may ask for a larger w/e/s/n region to have more room between the image and the axes.

  • S or mode or scan : – scan=:spot | scan=:pole | scan=:line | scan=(spot=true, …)
    Set the scan mode for this run. Choose from :line, :pole, or :spot:

    • Line mode (scan=(line=“plon/plat”, midpoint=true)): Accept a trial pole location and determine how compatible each data segment is with the predictions of small circles (for fracture zones) and meridians (for abyssal hills). By default reports summary statistics. Use midpoint=true to instead report misfit information for each mid-point along all multi-point line segments.

    • Pole mode (scan=:pole): Search for all poles on the given grid and determine the weighted chi-square misfit to all given line constraints. Requires outgrid, region, and inc.

    • Spot mode (scan=(spot=true, xfile=“crossings.txt”, lines=true)): Compute bisectors to fracture zones and meridians along abyssal hills and determine intersections of all great circles. Use lines=true to dump all great circles to stdout, and xfile to compute intersections and write locations to file.

  • V or verbose : – verbose=true | verbose=level
    Select verbosity level. More at verbose

  • bi or binary_in : – binary_in=??
    Select native binary format for primary table input. More at

Notes

  1. Each input line is expected to contain two or more points, and each consecutive pair of points defines a great circle line segment. For fracture zones, points should be digitized often enough so that the great circle between them can approximate the small circle.

  2. All line segments are given equal angular uncertainty [1, unless changed by uncertainty]. However, individual line segments can override this weight by adding a -Dsigma argument in the segment headers (in degrees).

Examples

To create a polespotting image from the abyssal hill and fracture zone fabric data in the files hills.txt and fractures.txt, on a 1x1 degree grid for the northern hemisphere, sampling the great circles every 10 km, and also dump the great circles to stdout:

using GMT
G = polespotter(hills="hills.txt", fz="fractures.txt", spacing=10, outgrid="poles.nc",
                region=(0,360,0,90), inc=1, scan=(spot=true, lines=true))

To search for the best pole location using all constraints on a 0.5 degree grid:

using GMT
G = polespotter(hills="hills.txt", fz="fractures.txt", outgrid="misfit.nc",
                region=(0,360,-90,90), inc=0.5, scan=:pole)

To test compatibility of line segments with a trial pole at (45, 30):

using GMT
D = polespotter(hills="hills.txt", fz="fractures.txt", scan=(line="45/30",))

See Also