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The heat probe is the latest of a series of probes developed
under the direction of Dr.
E. Davis and colleagues at the Pacific
Geoscience Centre of the Geological Survey of Canada (Davis
et al., 1977). It is a "Lister-type" probe that
measures temperature gradients and in-situ thermal conductivity
of sediments both in lakes and coastal areas (water depth
in 100's of meters) and in the deep ocean (water depths to
6,000 m). The probe is capable of making multiple measurements
over a single deployment of 12-24 hours and data acquisition,
timing, and heat pulse firing are computer controlled.
Temperatures and temperature gradients within the sediment
are measured by thermistors which are located at a known (30
cm) spacing within a small-diameter tube held in tension parallel
to a solid steel strength member. Measurement of the thermal
conductivity of the sediment is accomplished by (1) allowing
the probe to remain at rest in the sediment to allow dissipation
of the frictional heating generated by the penetration of
the probe, and then (2) heating the probe and surrounding
sediment by application of a fixed amount of energy to a heater
wire that parallels the thermistors within the sensor tube.
Analysis of the temperature decay following this period of
energy input (or "heat pulse") yields the conductivity
of the sediment.
The Heat Probe consists of (1) instrumentation, consisting
of 1-cm diameter sensor string tube, electronics data logger,
heat pulse system, batteries, and pingers all contained within
cylindrical pressure housings, (2) mechanical components,
including weight stand and 6cm solid steel bar which extends
continuously from wire termination at the top to the sensor
tube support fin at the bottom, and (3) software with modules
for communication, data analysis and graphic display. Further
technical specifications are given in Appendix A.
Instrumentation
Instruments are contained in two interconnected pressure
cases, one containing computer, measurement electronics, batteries
and data logger and the other with pinger and battery. The
probe contains sixteen (16) sensors (Fig. 1).
Figure 1. Annotated
example of screen print of 16-sensor output.
Sensors 1-11. Temperatures, in milliKelvins (mK),
measured within the probe at locations 30 cm apart, with sensor
1 the deepest, closest to the bottom of the sensor tube. These
sensor temperatures appear as a cluster of 11 lines with the
same value (water temperature during probe descent), until
they divide into separate values when the probe enters the
sediment. They respond very sharply to the heat pulse, and
respond to frictional heating on probe penetration and pullout.
Sensor 12. Water temperature, in mK, measured by an
external thermistor located at the top of the weight stand.
On the displays, it generally tracks the 11 sensors in the
sensor tube until the probe penetrates the sediment, then
Sensor 12 remains constant at the bottom water temperature.
Sensor 13. The internal temperature of the probe electronics,
in mK. This internal temperature is used to monitor internal
conditions of the pressure case.
Sensor 14. The reference constant resistor, appearing
at the very top of the display. It should remain constant,
illustrating the stability of the electronic system. It is
drawn on top of a straight line and is therefore only visible
when the system is unstable.
Sensor 15. The tilt sensor, indicating the tilt of
the probe from the vertical, in degrees, up to a maximum of
37.6°. A tilt greater than 37.6° is displayed as 37.6°.
There are three horizontal lines near the top of the display,
representing tilt values of 0°, 10°, and 20°.
The tilt display changes from a maximum value (when the probe
is horizontal on deck) to 0° when the probe is launched
vertically into the water.
Sensor 16. The water pressure (depth) sensor, as measured
by a quartz crystal sensor. At the surface, the water pressure
appears at the middle tilt of the Sensor 15 reference line
(10°). Note that for approximately three minutes before
penetrating the sediment, the probe is held approximately
50 m above the bottom.
Electronics (Data Logger, Heat Pulse
System, batteries, and pinger)
The controller is a 16-bit microprocessor and its operating
program and normal default start-up configuration (number
of channels, scan rate, etc.) is stored in ROM so that the
system can be turned on and used immediately. Data from the
sixteen sensors is passed through A/D converters and stored
in RAM where there is sufficient memory to store 24 hours
of continuous data. An on-board battery provides back up should
there be a failure of the main battery system. The sealed
lead-acid battery systems are recharged via an external connector
at the end of each 24-hour operation period (without opening
the pressure case). Old data are retained after data retrieval,
and are overwritten only after an explicit command requiring
verification. Data are retrieved via an external asynchronous
3-wire RS-232 connection at 9600 baud or via a simple delayed-ping
acoustic telemetry scheme utilizing pinger with separate pressure
case and power supply.
The heat pulse is software triggered only after the probe
has penetrated the sediments and has remained stationary for
a defined time interval (typically 7 minutes). This time interval
allows the temperatures measured after penetration to approach
equilibrium. Successful penetration and stationary period
is represented by a constant pressure, as indicated by the
pressure sensor. Pressure changes restart the time interval.
However, once one heat pulse has been completed, further heat
pulses are inhibited until the pressure sensor has indicated
a pressure change of nominally ±10 meters. Heat pulses
are also inhibited while the depth is less than 30 meters,
to prevent heat pulses from occurring on deck. Power to the
heater wire is applied for a programmable length of time (typically
20 seconds) beginning only after the probe has remained stationary
for the desired time interval. The total power-time product
delivered to the probe heater wire during a "heat pulse"
is known, repeatable, and stable, with a target value of 600
joules/meter of probe length. Twenty properly regulated heat
pulses per deployment are available.
Software: Communication and data
analysis
Data are downloaded from the heat probe to the computer via
the external asynchronous 3-wire RS-232 connection at 9,600
baud using the communication software PROCOMM, and data are
graphically displayed using PRO40 (Figure 1). This program
is also used pick penetration times and heat pulse parameters.
Once data are arranged according to penetration time and heat
pulse, data reduction is performed using a computer program
(HFRED). This method was developed and described by Villinger
and Davis (1987), following the method first suggested by
Lister (1979). One shortcoming of this method is that the
thermal conductivity is measured in the horizontal radial
direction from the probe, whereas the value in the vertical
direction is desired. As a result, anisotropy in the thermal
conductivity of shale (including unconsolidated shale) and
some other sediments can be considerable.
Mechanical Components
Figure 2 is a photograph of an assembled heat flow probe,
resting in its cradle under the A-frame used to deploy it.
The out-rigged thermistor string can been seen above and parallel
to the solid steel strength member that penetrates the sediment.
A diagram of the major components of the heat flow probe is
also shown.
Figure 2. Heat flow
probe
The length of probe is dictated by the "stiffness"
of the sediments under investigation. A probe length of 3.5
meters is generally used for deep ocean work. This 6-cm diameter
solid alloy steel bar extends continuously from the wire termination
at the top through the weight stand and down to the sensor-tube
support fin at the tip of the heat flow probe. The driving
weight is provided by a 500 kg weight stand, constructed of
galvanized steel and lead fill, which also houses and protects
the instrument pressure cases. Access to all external connectors
on the instrument cases is possible without removing the cylinders
from the weight stand (i.e., for battery charging and RS-232
communications). The thermistor sensors and heater wire are
contained within a high-strength 10-mm diameter stainless
steel tube supported in tension 10 cm away from the strength
member. The one ton instrument is usually suspended from a
9/16" torque-balanced oceanographic wire. A universal
swivel termination at the top of the heat flow probe prevents
fouling of the wire around the weight stand with resulting
damage to the sensor string. In addition to an adequate winch
and A-frame deployment device, a tensiometer is used to determine
definitively when the probe has penetrated the sediment (i.e.
when the probe weight has come off the wire).
Operation
The heat flow system is modular in design and quickly assembled.
The strength member is inserted through the weight stand and
secured with a large nut and swivel/feegee assembly. The modular
electronics packages are inserted in each end of the pressure
housing. The assembled electronics package in its pressure
housing is inserted in the weight stand and secured in place.
The bulkhead connectors providing communications, battery
recharge and input from water temperature and pressure sensors
are accessible from the top of the weight stand. The probe
may be charged, checked and test run on the ship before being
placed in the water with real time feed back via the communications
package.
At most stations the probe is lowered to within 50 m of the
bottom, at a rate of 60 m/minute and held there for 3 minutes,
then lowered into the bottom where it remains for 20 minutes,
with the heat pulse occurring at the 10 minute stationary
midpoint. In order to guarantee probe penetration at each
site, the tensiometer is carefully monitored. Both during
insertion when weight comes off the wire and during pullout,
to ensure that pullout tension is much greater than the weight
of the probe and cable.
Ambient temperatures below the seabed are derived by ten
minute tracking of the heat decay induced by the frictional
energy of the core barrel and mathematically projecting the
resulting decline curve to infinity. The heat pulse is then
applied to the cored section enabling the conductivity of
each 30 cm section to be calculated using an identical ten
minute sampling procedure. Heat flow (Q) measured in mW/m2
is determined by combining the site thermal conductivity (k)
measured in W/m-K with the geothermal gradient (G) measured
in mK/m determined from the thermistors according to the following
relationship.
Water depth is measured by pressure and the angle of tilt
of the core barrel is monitored to obtain true vertical depths.
Output
For each station, the data are immediately graphically displayed
to evaluate effectiveness within minutes after probe data
acquisition (Figure 1). Generally the resolution of the thermistors
in the sensor tube is greatly amplified, so that fine details
in the sediment temperatures can be viewed to ascertain that
the data is of high quality.
A Bullard plot, a graph of sediment temperature (T) reported
in mK versus the thermal resistance (R) reported in m2K/W,
is output for each station. Thermal resistance is the integral
of the inverse of thermal conductivity over the appropriate
depth interval. Each plot illustrates the relative temperature
as a function of the thermal resistance integrated from the
top sensor downward through the sediment. The reciprocal of
the slope of the resulting best-fit line is the heat flow
(thermal resistance is plotted as the vertical Y parameter
because it tracks depth; while heat flow is DT/DR), and under
the ideal conditions of a constant vertical heat flow in a
conductive regime, with isotropic thermal parameters, this
would be a straight line.
The variance between sets of electronics is minimal as Bullard
plots for a twice-sampled station indicates (Figure 3). The
line slope from the Bullard plot is used to determine the
most representative heat flow value for each station. At this
station, the electronics were changed when the reference resistor
measurements became unstable. The station was repeated with
new electronics, and close heat flow results of 17.5 and 15.5
mW/m2 were obtained.
For the heat flow probe, the tilt sensor signal is recorded
every 10 seconds, and during the time that the probe is in
the sediment, it is generally constant. The magnitude of tilt,
calibrated from 0 to 20° relative to vertical, is displayed
on the sensor output print screen (Figure 1). To obtain the
vertical heat flow, the tilt correction is the inverse of
the cosine of the tilt angle. For stations with measured tilt
less than 5° (which produces a correction factor of less
than 1.004, or an increase in the heat flow of up to 0.4%),
the correction is ignored. Tilt is almost always less than
5°.
Figure 3. Comparison
of heat flow measured by different electronics at a common
station.
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