Zero-Crossover Method
a
profile-based method of estimating the gravity source body parameters using the
truncated horizontal plate model approximation. Z.-C.M. is
applied to the gravity profile which is perpendicular to the upper edge of this
model. This gravity profile is upward continued over several constant intervals
and profiles of Second Horizontal
Derivative (2HD) are
calculated at each elevation. Differences in the lateral offset of the
zero-crossover points of 2HD of
successively upward continued gravity profiles are used to estimate dip,
vertical extent and location of the model boundary. Application of this method
requires that distance to the upper surface of the model (i.e., geological body)
is known. Tests indicate that the best results are obtained when both the length
of gravity profile and the finite-strike length of a model are at least three
times the maximum depth extent of the model. [156].
See also Upward Continuation.
Zero-Error Normalization
a data processing operator (LCT ZENิ algorithm) for adjusting closely spaced gravity and
magnetic datasets. Z.-E.N.
provides a 3-D line adjustment for each survey line to derive a minimum warp
surface that retains the high-frequency signals, supported by surrounding
neighbor control points, while suppressing apparent line-oriented noise events
(which have no surrounding support) and resolving the low-amplitude,
intermediate to long-wavelength level offsets between survey lines.
Zero-Length Spring
a basic
physical principle of operation of some gravimeters. The distance between points
of a quartz spring attachment is designed in such a manner that a projection of
the stress-strain curve passes through zero spring length for zero strain. In
other words, without a mass load the spring will have no extension. Gravimeters
with a spring of this type can be made very sensitive to strain (gravity) forces
with theoretically infinite period and linear deflection of a suspended weight
proportionally to the gravity field. Z.-L.S.
suspension supports a weight on the end of a horizontal beam in the LaCoste
& Romberg gravimeters. [36,
174].
See also Torsion Balance
and Weight-on-Spring.
Zero-Phase Meter Gravity
the gravity data after applying a soft
phase-only-inverse filter to the gravity field records in order to align all
data to a zero phase time lag. Z.-P.M.G.
is the basis for subsequent gravity reduction (correction) procedures. See Bouguer Gravity
and Free-Air Gravity.
Zone Chart
a template
used for applying Terrain Correction
or Isostatic Correction to the gravity data. Each zone within Z.C. is
represented by a circular ring divided by radial lines into compartments of
arbitrary azimuth. The smallest applicable zone (B) is bounded by the
inner radius of 6.56 ft and the outer radius of 54.6 ft, and divided into four
compartments. The largest zone (M) is bounded by the inner radius of
48,365 ft and the outer radius of 71,996 ft, and divided into 16 compartments.
In gravity exploration Z.C.
is commonly subdivided into the inner zone (with radii varying from 6.56 ft to
558 ft) and the outer zone (with radii varying from 558 ft to 21,826 ft), where
the surface topography is corrected by using the zone charts constructed on the
same principle as above, but with different scales. In practice, Z.C.
is laid over the topographic map of the exploration area with Z.C.
centered at the gravity Station being corrected. The differences in average elevation in
each zone and the station elevation are tabulated. The sign of this difference
is ignored as the correction is always positive, regardless of whether zones are
higher or lower than the station elevation. Z.C. is also
referred to as Hammer Chart
or Terrain Chart.
[223,
238
]. See also Inner Zone Terrain Correction and Outer Zone
Terrain Correction.
Zooming
an
interactive display option which magnifies the selected region of a display
(zoom-in) for better viewing or decreases it (zoom-out) back to the original
view. [223].