Geology Reference
In-Depth Information
is the
thermal conductivity
of the relevant wallrock, which
is usually determined by laboratory measurement.
Temperature gradients within about 20 m of the
Earth's surface are strongly affected by diurnal and sea-
sonal changes in solar heating and do not provide reliable
estimates of heat flux. Porous strata can also strongly in-
fluence temperature gradients by the ingress of connate
water and because their contained pore fluids act as a
thermal sink. Heat flux measurements are commonly
made to assess the potential of an area for geothermal
energy utilization.
Shale
1st
white specks
600
2nd
white specks
Sandstone
Impure
siltstone
Carbonate
Anhydrite
Shale
Carbonate
minor
chert
Shale
11.10 Magnetic logging
11.10.1 Magnetic log
The normal
magnetic log
has only limited application.The
magnetic field is either measured with a downhole flux-
gate or with a proton magnetometer (Section 7.6) or a
susceptibility meter is utilized. Anomalous readings
indicate the presence of magnetic minerals.
1200
Carbonate
minor
anhydrite
Salt
11.10.2 Nuclear magnetic resonance log
The
nuclear magnetic resonance
(or
free fluid index
)
log
is used
to estimate the hydrogen ion concentration in formation
fluids and, hence, to obtain a measure of porosity. The
method of measurement resembles that of the proton
magnetometer, but with the formation fluid taking the
place of the sensor. A pulsed magnetic field causes the
alignment of some of the hydrogen ions in a direction
different from the Earth's field. A receiver measures the
amplitude and decay rate of the precession of the protons
as they realign in the geomagnetic field direction when
the polarizing field is inactive. The amplitude measure-
ments provide an estimate of the amount of fluid in the
pore spaces and the rate of decay is diagnostic of the type
of fluid present.
Carbonate
1800
Precambrian
3 4.5 6
Interval velocities (km s
-1
)
Shale
Fig. 11.14
A continuous velocity log. (After Grant & West 1965.)
11.9 Temperature logging
Temperature gradients may be measured through a
borehole section using a sonde on which a number of
closely-spaced thermistor probes are mounted. The
vertical
heat flux H
is estimated by
11.11 Gravity logging
In situations where density is a function of depth only,
the strata being substantially horizontal, stepwise mea-
surement of the vertical gravity gradient with a
gravity log
can be used to estimate mean densities according to the
calculation given in Section 6.9.
A specialized borehole gravimeter of LaCoste and
Romberg type (Section 6.4) is used for gravity logging.
d
d
q
Hk
z
=
(11.13)
z
where d
q
/
d
z
is the vertical temperature gradient and
k
z
