P-wave velocity Calibration (5)
The P Wave velocity of the ultrasonic pulse through the sediments inside the
core liner is given by:
TX
TX
VP = X/TT
5
where:
X is the sediment thickness inside the core liner as measured.
TT is the pulse travel time in the sediment.
Introduction
There are now a large number of laboratories around the world that log sediment and rock cores using automated, or semi-automated core logging devices.
GEOTEK alone has supplied over 40 automated Multi Sensor Core Logging systems to laboratories in 13 countries, in a variety of configurations, since 1990
and there are probably numerous Ôhome grownÕ systems in use.
Core logging provides large data sets which are valuable for many different types of scientific studies. Parameters measured during automated core logging
are generally limited to those that can be measured in a non-destructive manner. To maximise the value of core logs, users should be aware of how to collect
the best data and the limitations of the individual measurement techniques.
The purpose of this poster is to address some of the issues of logging protocols and calibration which are sometimes overlooked. By using appropriate logging
protocols and calibration procedures the resulting data sets will provide users with the best information possible to complement other core analysis methods.
RX
RX
The measured total travel time, TOT, is given by the equation given in Figure 6. PTO includes the pulse travel time through the liner, the pulse travel
time through the transducer faces, the delay caused by picking a point on the wave form which is about 1 cycle after the onset, as well as any small
electronic delays in the system circuitry. PTO will vary depending on both the core liner being used and whether whole core or split core measurements
are being made.
Determining PTO
Cut a short length of liner (about 30 cm) of the type (and preferably the same manufacturerÕs batch) being logged. Fill the liner with distilled water and
place between the P-Wave transducers as if logging a normal core. When using whole cores the whole core liner should be capped, taped and inserted
between the transducers. When using split cores a split liner should be used with the upper transducer inserted just beneath the water surface. The
Sound Velocity in Distilled Water
parameters D, W and Vdw should be noted and PTO can be calculated as outlined in Figure 6.
at Atmospheric Pressure
1540
TX
It is important to recognise at the outset that the quality, and hence value, of core logs is to a large extent dependent on the physical quality of the cores being
logged. No matter how sophisticated the individual measurement techniques are or how skilled the operator is, poor quality core will result in poor quality
logs. Log quality is a function of:
1 - Quality of the recovered core.
Problems:
a) Poor recovery, and disturbed sediment cores.
b) Disturbance to cores as a result of bad handling and storage.
Butt error distance
Solutions:
a) Use the correct equipment for the prevalent conditions, accounting for weather, operator skill and the shipÕs capabilities.
b) Once a core is recovered treat it with care and ensure correct storage (i.e. the right way up and at the correct temperature).
1
3 - Core splitting (Figure 2).
Problems:
a) Inconsistent core thickness and cross-sectional area.
b) Poor sensor contact and variable thickness due to uneven core surfaces.
Solutions:
a) Split the liner consistently and correctly align adjacent sections.
b) Ensure that the core surface is as flat as possible.
TOT
6
TT = (D - W)/Vdw
TT
D = Total thickness
W = Liner thickness
Vdw = Sound velocity in distilled water at known temperature
PTO
1500
1480
1460
1440
1420
0
5
10
15
20
25
30
35
40
The basic equation for calculating bulk density from gamma ray attenuation measurements is:
ρ = 1/µd * ln(Io/I) where:
Air/Gas
good P-wave signal
small density errors
large density errors
no P-wave signal
ρ = sediment bulk density
µ = the Compton attenuation coefficient
d = the sediment thickness
Io = the gamma source intensity
I = the measured intensity through the sample
In practice many experimental factors need to be addressed in order to obtain
valid bulk density measurements for example; beam spreading, the attenuation
through the liner wall and the effect of water in the sediments which has a
significantly different attenuation coefficient to the sediment minerals.
small density errors
good P-wave signal
significant density
and P-wave errors
2
5 - Corer endcaps (Figure 3).
Problems:
a) Loss of acoustic contact for p-wave transducers with errors compounded by inaccurate core diameter measurements.
b) Endcaps necessitate the use of a larger magnetic susceptibility loop.
c) Endcaps increase the overall length of the core (1-2% depending on the care taken to ensure a good fit).
Solutions:
a, b & c) Ideally cores should be logged without endcaps, internal endcaps are a viable second choice (i.e. for soupy cores) and a third alternative is to
make cut outs in the endcaps as shown in Figure 3. One solution, often employed, is simply to discard the data around the end caps. This is hardly
satisfactory as the user could be discarding up to 10% of the data. Most users who adopt this technique normally find that the most interesting lithological
boundaries occur within the end-cap region of each section!
Logging Strategy
7
Consequently the simplest and most reliable method for the calibration and calculation of gamma density is to use an empirical approach which has been
shown to provide excellent results. The technique relies on calibrating the system using both the liner in which the core is contained and the fluid which
the sediment contains. For example, when using a whole core with water saturated sediments a calibration section should be made which consists of a
cylindrical piece of aluminium of varying thickness surrounded completely by water in a sealed liner (see Figure 8). For a dry split core the calibration
should be done with aluminium in a dry half liner.
Gamma counts should be taken through the calibration sample for long count times (e.g. 100s) at different Aluminium thickness' and plotted as as a graph
of ρ*d vs. ln I. The resulting graph may deviate from the theoretical straight line because of the factors cited above (see Figure 8). To accommodate any
variations from a straight line a second order polynomial equation can be fitted to the graph of the type:
y = Ax2 + Bx + C
10.2
y = -0.0046515x 2 - 0.48585x + 10.284
10.0
Whole Core
Split Core
9.8
9.6
9.4
y = 0.00083501x 2 - 0.088x +10.483
8
where:
x=ρ*d
y =lnI
9.2
4
6
8
10
12
14
16
Average Density * Thickness
If the test data is best represented by a
straight line then simply set A=0. The
coefficients A, B & C in the above equation
can then be entered directly into the gamma
density processing panel. This calibration
and calculation will then make all the
18 necessary empirical adjustments.
Magnetic Susceptibility Calibration (9)
The procedure and strategy for logging cores can depend on many factors including:
3
Loop Sensor
The magnetic susceptibility sensor is electronically set to measure a single standard sample of a stable iron oxide which has been tested and analysed by
the manufacturer (Bartington Instruments Ltd). Therefore, all magnetic susceptibility sensors supplied should record exactly the same value for any given
sample, and that value should be the same as a measurement made on a different measuring system. In that sense the magnetic susceptibility system is
calibrated absolutely. Since the calibration has been set electronically it should not alter. A calibration sample is provided which can be used to check the
long term consistency of the calibration. Any changes in the calibration are a fault in the system's electronics which only the manufacturer can rectify.
Note: Magnetic susceptibility measurements are temperature sensitive, so it is important to maintain a stable temperature environment during
measurement.
Modified Endcaps
Standard Endcaps
P-wave velocity and gamma density can be measured either horizontally or vertically on whole cores, and vertically on split cores. The best configuration is
dependent on the type and degree of core disturbance (see figure 2). Depending on the sensors available the following procedure should be used as a model:
1) Whole core Ð log P Wave, Gamma Density (horizontal orientation) magnetic susceptibility (loop sensor), electrical resistivity and natural gamma (short
count time).
2) Whole core - relog natural gamma (long count time) and any or any other parameters (if required).
3) Split core (without surface film) Ð log image data
4) Split core (with surface film) - log magnetic susceptibility point sensor and additional P-Wave and Gamma density if required.
Different sensors have incompatible time requirements (see Figure 4); natural gamma measurements, for example, take a great deal of time as the natural
radioactivity of marine sediments is usually very low - it may therefore be more sensible to log natural gamma separately to the other parameters. The time
taken for gamma attenuation and magnetic susceptibility can be reduced but at the price of data degradation. When considering the time to allow for logging
the spatial resolution of measurements must be taken into account and any knowledge of the sediment/rock sequences being logged should be used to
selectively log the relevant sections.
Typical
Spatial
Acquisit.
Comments
Resolution Resolution
Time
Constants
acceleration due to gravity (g) = 9.81 ms-1
density of aluminium = 2710 kgm-3
mass susceptibility (χ) of H2O = -0.719 x 10-6 cm3 g-1(cgs Units)
= -0.903 x 10-8 m3 kg-1(SI Units)
Gamma
Density
0.01 0.05 g/cc
2 mm to
10 mm
2 - 30 s
P-Wave
Velocity
2 m/s
20 mm
0.5 s
Magnetic
Susceptibility
20 - 30 mm
(loop)
0.1*10 cgs
5 mm (point)
-6
P-wave Velocity (SI Unit = ms-1)
1 ms-1 = 104 cmµs-1
1 kms-1 = 103 ms-1
Electrical
Resistivity
Density (SI Unit = kgm-3)
1000 kg/m-3 = 1 gcm-3 = 1 Mgm-3
Core
Imaging
8 bits per
channel
Natural
Gamma
Total or
Spectral
(K, U, Th)
4
0.05 Ωm
1 - 10 s
5 mm
(contact)
20 mm
(no contact)
1s
300 dpi
3 m/min
150 mm
Depends on
collimator
diameter and
gamma count rate
Depends upon
core size
Depends upon
loop/core size &
integration time
Point Sensor
The protocol for inter calibrating the new point sensor with the loop sensor is
not well established. A calibration piece is supplied which can be used to check
for any major problems. The user is advised to perform some comparative
measurements between the point sensor and the loop sensor. Values of
corrected volume specific magnetic susceptibility (Kloop) should be obtained
from a whole core using a loop and then a comparative set of uncorrected values
obtained from a split core using the point sensor (Kpoint). Fit this data to a linear
relationship:
9
Kpoint= A * (Kloop) + B
These values of A & B can be used directly in the processing panel of the GEOTEK MSCL software.
Volume specific magnetic susceptibility
The data obtained from the magnetic susceptibility system provides uncorrected, volume specific magnetic susceptibility, Kuncor.
Kuncor is dimensionless
(*10-5 SI units)
To obtain the corrected volume specific magnetic susceptibility (K) the data must be corrected for the relative effect of size of the core and the size of the
loop sensor being used:
K = Kuncor/Krel
(*10-5 SI units)
The relationship between core loop diameter (Dl), core diameter (d) and Krel has been determined experimentally (see Figure 10) and is used
automatically in the processing panel for magnetic susceptibility
Relative response to varying core and loop diameters
in the GEOTEK MSCL software.
1.4
1.2
Krel= 2.087 * (d/Dl) 2 - 0.218 * (d/Dl) - 0.0049
1.0
30 s to
1 hr!
Requires large
digital storage
space
Depends on
core size &
count time
d = Sediment diameter
0.8
Dl = Loop diameter
K rel
Useful Constants &
Conversions
Response characteristics
0.6
0.4
0.2
mass specific χ (SI Unit =
mass specific χ (cgs Unit = cm3 g-1)
χ (SI Units) = 4π x 10-2χ (cgs Units)
m3
PTO = TOT - TT
Temperature (¡C)
4 - Liner quality.
Problems:
small density
intermittent P-Wave signal
a) Core liner that is not perfectly round.
and P-wave errors
very small density errors
b) Variations in liner thickness.
Solutions:
a) Use liners as soon as possible after purchase and ensure that adjacent sections are marked so that they can be aligned during splitting and logging.
b) It is a fair assumption that liner thickness will not vary greatly although the user should be aware that these variations cause errors.
Magnetic Susceptibility
volume specific K (dimensionless):
Κ (SI Units) = 4π x 10-6Κ (cgs Units)
2
Velocity = -0.041747(T2) + 4.7703T = 1404.2
Gamma Density Calibration (7)
intermittent P-Wave signal
very small density errors
a) The information required
b) The equipment/sensor systems available
c) Core type/quality
d) Time available
e) Spatial resolution required
f) Previous knowledge of sequence
1520
PTO must be known to calibrate the p-wave system.
Ln (cps)
Air/Gas
2 - Core sectioning.
Water/
Core Liner
Problems:
no P-wave signal Fluid
a) Errors in measuring section lengths and butt error distance (see Figure 1).
large density errors
b) Compounded length problems.
Solutions:
a) Sections should be cut perpendicular to the core length.
large density errors
no P-wave signal
b) Adjacent sections should be oriented during sectioning and aligned during logging.
RX
Distilled Water
Velocity (m/s)
Core Quality and Handling
TOT = Total Travel Time
PTO = P-wave Travel Time Offset
TT = Travel Time in Distilled Water
kg-1):
10
Core imaging
Mass susceptibility = Volume sus.
Density
0.0
0
0.2
0.4
0.6
0.8
1
Mass specific magnetic susceptibility
The volume specific magnetic susceptibility takes no account of
the density of the sample being measured. Consequently it is
possible to have variations in K down the length of a core that
reflect changes in density only rather than changes in proportion
of magnetically susceptible minerals to other minerals. Mass
specific magnetic susceptibility, overcomes this problem by
taking into account the density of the sample being measured.
For further, more detailed, information on the measurement of
magnetic susceptibility the user is referred to the manuals
supplied by Bartington. The nomenclature used in the
processing panel of the GEOTEK MSCL software is different
from that used above:
d/Dl
CMS is the corrected magnetic susceptibility which can be either Kuncor, K or χ depending on the users requirements. MS is the measured magnetic
susceptibility, Kuncor. The user can enter numbers into the boxes; A, B, Den and LD. When using a loop and no correction to MS is required then set
A=1 and B=0. Den is used to obtain mass specific susceptibility and LD is used to correct for the size of loop being used.
P-wave velocity &
Core diameter
Magnetic Susceptibility Loop sensor/Point sensor
Electronics Rack
Core pusher
core section 1
GEOTEK
MULTI-SENSOR CORE LOGGER
6327
core section 2
GEOTEK P-WAVE LOGGER
2471
6358
8632
3472
© GEOTEK Ltd.
Natural Gamma
Electrical
Resistivity
Gamma density
Nene House, Drayton Fields
Daventry, Northants, NN11 5EA.
Tel: +44 (0)1327 311666 Fax: +44 (0)1327 311555
e-mail: info@geotek.co.uk
http://www.geotek.co.uk/
June 2000 MSCL lab wall poster