Graphical Terrane Correction for Gravity Gradient Page: 8
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8 GRAPHICAL TERRANE CORRECTION FOR GRAVITY GRADIENT
merely to divide elevations and distances by any constant number.
That is, 1, - and a 5 2-3 may be used in place of sl, 82 s3 and
z, z2, z3 where s and z are respectively the distance and elevation of
any point and m is any number.
4. The charts of Figures 2 and 3 replace the Edtvds or the older
and more common Schweydar method for the calculation of the
gradient correction at distances greater than 5 meters from the
instrument, but if desired either of the older methods may be used
to a distance of 10 (or 20) meters and the present chart for greater
distances from the instrument. The 5 to 10 (and 10 to 20) meter
zones of the chart are neglected in that case and 20 per cent of the
effect of the first subzone correction is applied to the first subzone
of the 10 to 20 meter zone (or of the 20 to 40 meter zone).
5. This method is directly applicable without regard to the height
of the center of gravity of the instrument above the ground. The
Edtv6s and Schweydar methods use the quantity (Lh-z) in which h is
the height of the center of gravity of the instrument above the
level of the ground; that is, a constant, and z, the elevation of the
surface of the ground above the level of the base of the instrument,
is the variable. In the charts of Figures 2 and 3, the quantity
(h-z) is used directly; as a matter of practical field, work it is as
easy in leveling to refer elevations to the level of the center of
gravity of the instrument as it is to refer them to the level of the
base of the instrument. If the quantity (h-z) is measured directly,
the height of the instrument does not enter the calculations.
If desired, the charts of Figures 2 and 3 can easily be made over
into similar charts in which the elevations are referred to the level
of the base of the instrument and in which the z of the charts is re-
placed by (h-z); a new zero line of elevation and of gradient effect
is drawn at - h on the old chart, and. the elevations and piles of 0.1
E blocks are built up and. down from that line; if E'p and E", are
the respective values in E on the old and the new charts of any point
p and if E'_h is the value of E on the point - h in the same subzone
of the old chart:
E"= E'_;--E'p (9)
where E' % and E'p but not E"p are taken without regard to sign.
E"p is negative from z= - co to 0 of the new chart, positive from 0
to 2J", negative from 2h" to + co, and at a maximum at +th".
The rules of signs in (6) and (7) and in the form of Figure 4 should
6. The unit of vertical and horizontal distance in the charts can be
taken as any unit of linear measurement, not merely meters, without
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Barton, Donald C. Graphical Terrane Correction for Gravity Gradient, report, 1929; [Washington D.C.]. (digital.library.unt.edu/ark:/67531/metadc66454/m1/10/: accessed May 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.