INTRODUCTION
Soil exploration in the offshore arctic
environment imposes severe and unique demands upon the exploration
methods. The constant threat of weather shut down requires
rapid methods. The high cost of arctic programs additionally
requires methods that provide uniformly high quality data.
Finally, because of the complex thermal regime, sampling
invariably introduces change in thermal state prior to any
laboratory testing.
For these reasons, rapidly deployable,
multichannel in situ penetration testing systems have been
developed based upon the electric Cone Penetrometer Test
(CPT). The multichannel penetration testing systems include
piezometric, thermal and electrical conductivity sensors
in addition to cone end bearing and friction sleeve measurements.
Data collected with these systems have allowed for fuller
characterization of the complex dependence of mechanical
properties on thermal and physiochemical states of saline
saturated permafrost soils.
CPT SENSORS
The electric CPT instrument (subtraction
type) used as the base module of the multichannel system
has a 15 sq cm end area and 200 sq cm friction sleeve as
shown in Figure 1.
FIGURE 1 CONE PENETROMETER
This particular design has proven robust
enough for general applications, and with suitable choice
of load cell configuration, provides good resolution in
soft soils. The oversized end area, in addition to providing
increased load resolution, acts as particularly efficient
friction reducer, thus increasing penetration depth for
a given total pushing force. Biaxial inclinometers are included
inside this instrument for monitoring instrument drift from
vertical.
The two load transducers are composed
of 120 ohm foil strain gages connected as an active 240 ohm
four arm Wheatstone bridge. Two opposing legs are in axial
strain and the other two are Poisson’ gages. The bridge
is temperature compensated to .02%/C°, from -20°C
to +50°C. The sensitivity is a nominal 2.2 mv/v with a
tolerance of +/-10%. Excitation is 10v with remote load sensing.
Power density is 3.65 Watt/sq inch and the thermal time constant
of the bridge is 90 sec.
The biaxial inclinometers consist of
two strain gage accelerometers, each forming an active half
bridge, with two units in quadrature forming a full bridge.
Accelerometers are air damped.
The piezometric sensor added to the
basic instrument to allow measurement of generated fluid
pressures is shown in Figure 2. Typical data collected using
this sensor are also shown in Figure 2. This particular
design has the transducer ported through the mid-point of
the cone tip to a circular porous stone filter. This design
allows for measurement of pore pressures at their maximum
level. The effects of filter location on measurements must
be accounted for when comparing different piezocone data
bases (1).
FIGURE 2. CONE PENETROMETER TEST
WITH PIEZOMETRIC SENSOR
The piezometric transducer is a piezoresistive,
gage pressure type. It consists of an active four arm bridge
deposited on a silicon diaphragm. The transducer dead volume
is .0003 in-3. The compensated temperature range
is -16°C to +93°C. Sensitivity is 0.63+/- 0.18 mv/psi
at 10v excitation. The thermal time constant is 4 sec.
The addition of a thermal sensor to
the CPT allows for measurement of dynamic temperatures generated
during cone penetration. By ceasing penetration and allowing
thermal dissipations to occur (analogous to pore pressure
dissipations) an equilibrium thermal profile may be obtained.
Two different thermal sensors were used
in the arctic explorations. The first design has a PTAT
transducer mounted behind a copper alloy cone tip. The transducer
is thermally coupled (but stress isolated) through thermal
grease to the copper alloy tip. The sensor and data collected
are presented in Figure 3.
FIGURE 3. CONE PENETROMETER TEST
WITH THERMAL SENSOR
This PTAT transducer consists of a hybrid
bridge semiconductor chip, with current output proportional
to absolute temperature. It is scaled to provide 1 mv/°K
at 10v excitation.
This first tip sensor design allows
for rapid dissipation of generated heat to obtain ambient
temperatures, but precludes using the piezometric sensor
for simultaneous pore pressure measurement. Thus, a second
thermal sensor used in arctic explorations has a PTAT transducer,
bonded to the inside of the cone instrument. While the thermal
mass coupled to the transducer with this design is greater
than that with the first design, this configuration allows
for simultaneous pore pressure measurement. However, thermal
equilibration requires longer time periods.
The second PTAT transducer consists
of a semiconductor chip with current output proportional
to absolute temperature. The Input is 5 to 30 volts, with
output of 1 ? amp/°K. Output may be scaled as desired
with one resistor.
Electrical conductivity measurements
were added to the CPT to investigate their potential for
assessment of physiochemical properties of penetrated soils.
This sensor was mounted behind the cone instrument and allowed
the measurement of soil electrical conductivity as shown
in Figure 4. Data collected with this instrument are also
shown in Figure 4.
FIGURE 4. CONE PENETROMETER TEST
WITH ELECTRICAL CONDUCTIVITY SENSOR
The downhole portion of the conductivity
sensor consists of a four wire-two electrode configuration
excited at 2000 Hz. Alternating current is used to control
polarization. The voltage across the electrodes is measured
and compared with an inphase current through a reference
resistor. This comparison is performed digitally. Scale
can be changed by selecting different reference resistors.
The two brass electrodes are insulated from the instrument
housing using Teflon rings.
IN SITU PORE FLUID CONDUCTIVITY/THERMAL
PROBE
To rapidly obtain measurements of in
situ pore fluid salinity, without the thermal and mechanical
disturbance caused by drilling fluids, a push-in probe was
developed. This In Situ Pore Fluid Conductivity/Thermal
Probe is shown In Figure 5. The probe is pushed into the
soil while maintaining a positive pressure inside the probe.
At a selected sampling depth, the internal pressure is released
at the ground surface, and soil pore fluid flows into the
internal conductivity cell due to the positive fluid gradient
between the soil and the inside of the probe.
FIGURE 5. IN SITU PORE FLUID CONDUCTIVITY/THERMAL
PROBE
A two ring electrode conductivity cell
mounted inside the probe is used to measure pore fluid conductivity.
The electrode configuration is similar to that as described
previously. A PTAT transducer is bonded to one of the electrodes
in order to provide a measurement of pore fluid temperature.
Both temperature and conductivity are required to define
fluid salinity.
Another PTAT transducer was mounted
to a copper ring located above the filter on the probe exterior.
This ring is in good thermal contact with the surrounding
soil. Thermal equilibrium measurements were obtained during
the time period required to fill the internal conductivity
cell.
CONCLUSIONS
Multichannel data acquisition during
a CPT is a reliable and economical method of obtaining valuable
in situ data during the fast paced field explorations common
to remote arctic frontier areas. Proven systems have been
utilized with up to six channels of active data acquisition
– cone end bearing and friction resistance, piezometric
pressures, thermal response, electrical conductivity, and
biaxial inclination. Data obtained with these multichannel
In situ penetration testing systems have provided:
- continuous detailed information on
soil type, strength, and deformation characteristics,
based on CPT and piezometric data;
- data describing in situ thermal regimes
from CPY thermal transducers, without the long equilibration
times required using borehole approaches;
- continuous data on soil physiochemical
properties, including promising results for indication
of ice content, using the CPT electrical conductivity
sensor; and
- data on in situ pore fluid temperature
and salinity, without the invariable liquid-solid phase
changes and mixing encountered in borehole sampling.
It is important to note that the entire
penetration test system (including uphole electronics) was
calibrated under expected thermal conditions in the Earth
Technology cold room laboratory this process is of importance
in any arctic field program to preclude thermally induced
changes in sensor performance between laboratory calibration
and actual field testing.
Further research is being supported
by Earth Technology for development of other push-in probes,
Including a dielectric CPT for ice content assessment, a
CPT push-in pressuremeter, and BAT in situ fluid samplers
(2) for permeability measurements or long term pore fluid/dissolved
gas sampling.
ACKNOWLEDGEMENT
The authors wish to thank Sohio Petroleum
Company, and especially Dr. Jeff Weaver of the Sohio Technology
Center, for their support of many of these activities.
REFERENCES
- Douglas, B. J., and Strutynsky, A.
I., "Cone Penetrometer Test, Pore Pressure Measurements
and SPT Hammer Energy Calibration for Liquefaction Hazard
Assessment", Contract Report to the U.S. Geological
Survey, August, 1984.
- Torstensson, B., and Johansson, T.,
"A New System for Groundwater Monitoring," AHS/AISH,
International Symposium on Hydrochemical Balances of Freshwater
Systems, Uppsala, Sweden
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