The cycles may appear on different logs in different regions
(so far we have had success with gamma, density, resistivity,
magnetic susceptibility, and sonic logs), and it is possible that
different logs from the same well will show energies at different
Milankovitch periods.
The climate system also varies on sub-orbital time scales, and
this climate variability is similarly reflected in the composition
and physical properties of the sediments. In regions where the
sedimentation rate is high enough, or conversely in logs with
sufficient vertical resolution, the millennial scale variability
can also be documented.
There are several prerequisites to successfully identifying any
climate cycle through spectral analysis. First, log display makes
a difference. A log plot that shows broad compaction trends may
obscure fine-scale Milankovitch cycles. Second, accurate sedimentation
rates are needed for confirmation that any detected periodicity
is at Milankovitch frequencies. Magnetic reversal stratigraphy gives more
accurate sedimentation rates than paleontology, because the latter
has errors at both datums that blow up when calculating a sedimentation
rate. Lacking precise sedimentation rates, one will need to detect
at least two Milankovitch periods (preferably in more than one
log) before any confidence can begin to be placed in them. Third,
a high sedimentation rate is needed for logging tools with 0.5 m
vertical resolution to detect high frequencies (e.g. 19Kyr, 23Kyr,
and 41Kyr). Fourth, beware of cycles caused by local sedimentary
phenomena (e.g. turbidites) rather than climate: the depth period
of the latter will change with the sedimentation rate, but not
the former. Keep in mind that the 41Kyr cycle is the only truly
constant period for all ages. Eccentricity strength varies somewhat
between 95Kyr and 123Kyr as a function of time, and precession strength
varies between 19Kyr and 23Kyr (although ideally one would find separate
eccentricity peaks and separate precession peaks). Also, the shortest
periods are the most likely to be smeared by small changes in
sedimentation rate within a log interval.
Spectral analysis is the most common means of characterizing
periodicity in logs and can be undertaken with either depth or
age as the independent variable. Ultimately, however, a conversion
from depth scale to age must be performed in order to understand
the driving forces behind the variability. There are a large number
of programs available on various platforms that easily allow spectral
analysis to be performed. Perhaps the easiest and most commonly
employed (and accepted) method is to use the Macintosh program
Analyseries to perform the analysis. In order to generate power
spectra in Analyseries you need to do the following:
