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| Titel |
Advanced analysis of thermal data observed in subsurface wells unmasks the ancient climate |
| VerfasserIn |
Lev Eppelbaum, Izzy Kutasov |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089073
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| Publikation (Nr.) |
 EGU/EGU2014-3261.pdf |
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| Zusammenfassung |
| Conventional methods of studying the ancient climate history are associated with statistical
processing of accomplished meteorological data. These investigations have focused attention
on meteorological records of air temperature, which can provide information on the only last
100–200 years. Number of the records is absolutely insufficient and their areal coverage is
limited, some oldest meteorological stations may have been affected by local warming
connected with urban and industrial growth. At the same time significant climate
changes are accompanied by the corresponding variations in the Earth’s surface (soil)
temperature. This effect is based on the known physical law that temperature waves at the
surface propagate downward into the subsurface with an amplitude attenuation and
time delay increasing with depth. Earth’s temperature profiles, measured by precise
temperature logging T(z) in boreholes to depth of about 80-300 meters, have a
“memory” on what has happened on the surface during approximately several last
centuries.
Knowledge of the past climate in archaeology is necessary not only for tracing some
ancient events and more deep understanding some historical facts, but also for estimation
of past harvests, analysis of some physical conditions of different constructions
built in the past, and in many other fields (Eppelbaum, 2010; Eppelbaum et al.,
2010).
The first attempts to recover the past ground surface temperature history (GSTH) from
measured T(z) profiles date back to the mid-1960s, however only after Lachenbruch et al.
(1988) pointed out that the magnitude and timing of the ground surface warming in Alaska is
consistent with models of the recent warming, the method became popular (Cermak et al.,
1996).
Let us assume that tx years ago from now the ground surface temperature started to
increase (warming) or reduce (cooling). Prior to this moment the subsurface temperature
is:
Ta(z,t = 0) = T0a + Î z,
(1)
where T0a is the mean ground surface temperature at the moment of time t = 0 years; z is
the vertical depth and Î is the geothermal gradient. It is also assumed that the host medium is
homogeneous with constant thermal properties. Now the current (t = tx) subsurface
temperature is (in case of warming):
Tc(z,t = tx) = T0c +f (z),
(2)
where T0c is the current (at the time (date) of temperature logging) mean ground surface
temperature; and f(z) is a function of depth that could be obtained from the field data. In
some cases the value of T0c can be obtained by extrapolation of the function Tc to z = 0.
However, in most cases, the value T0c can be estimated by trial and error method: Assuming
an interval of values for T0c, calculating for each T0c value of the temperature profiles Tcfor
various models of change in the ground surface temperature (GST) with time and, finally,
finding a best match between calculated and field measured Tc profiles. In our study we
found that a quadratic regression can be utilized to estimate the value of T0c = a0 (Kutasov
et al., 2000):
Tc(z,t = tx) = a0 + a1z +a2z2,
(3)
where a0, a1, and a2 are the coefficients.
We will consider four different models (Eppelbaum et al., 2006). Apparently each
of these models is more suitable (applicable) under concrete physical-geological
conditions.
In the first model we assumed that txC years ago the GSTvalue suddenly changed from
T0 to T0c. The current temperature anomaly (the reduced temperature) is
TR (z) = T0c + f(z) - T0 - Î z
(4)
and the solution is
( )
TRC = TR = ΔT0Φ *(x) --z– ,t = txC,
2 at
(5)
ΔT0 = T0c - T0,
(6) |
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