The
borehole geophysical logging plays a vital role by providing information on the character
of the rocks and fluid penetrated by boreholes. This information are useful for
litho-structural interpretation of the aquifer systems as well as for evaluation of
hydrogeologic characteristics and the water quality distributions in the aquifer system.
The multi parameter
analog or digital geophysical logging of water wells is a post drilling approach to
optimize the design and development of the wells. The prime objectives of the geophysical
logging are
1. to identify distinct
boundaries between different hydrogeological units
2. to decide the proper
setting of the screen depths against productive aquifers and casing against the
collapsible formations and also sealing against bad quality of information water. The
other objectives include defining and delineating the aquifer geometry and its framework
in space and time.
Geophysical logging
provides measurements of various physical properties of different formations and their
fluid contents penetrated by the borehole during the drilling. The measuring sensors,
commonly known as probes are lowered into the borehole to measure continuous changes in
physical properties vertically and a continuous record so obtained is know as a
geophysical log. The process is defined as geophysical logging. It can be run in all
boreholes including those cased with metal or plastic casing and filled with water brine,
mud, drilling foam or air. The greatest return of information is derived from open
(uncased) boreholes filled with formation water or mud. In plain cased boreholes
investigation of the geological formation is limited to the nuclear logs and with plastic
casing induction logs way be used. The logging equipment consists essentially of four
units i.e. down hole instrument probe; cable winch; power and processing modules and data
recording units.
The probes contain
appropriate sensors to enable specific properties to be measured, which provide output in
the form of electric signals. These signals are transmitted to the surface recording
system through the cable and winch and which may either be in analog of digital form. The
cable has double outer layer of high tensile steel or polyurethane, which serves the dual
purpose of supporting the probe and conveying electrical power as well as signals to and
from the probe. The winch serves to raise or lower the probe into the borehole and to
measure its prices depth position. This is achieved by passing the cable round a measuring
sheave of known diameter linked to an accurate depth measuring system. The surface
instrumentation typically consists of two sections, one provides power and the other
processes the electric signals from the probes for recording purposes. Data recorder units
enable encoding the signals from the probe or surface modules and drive the plotter to
produce field. Logs. The digital logging system has capabilities to convert analog signals
to digital series data, which can be stored on magnetic tape for further analysis.
Fig: Geophysical Log of
Exploratory Borehole at Thukral in Ujjain District M.P.
Formation Logging:
Formation logs respond to
the physical properties of the geological formations around the well and the fluids they
contained.
Electrical Log:
The electrical conduction
in the rock formation is primarily electrolytic. Since most of the grains of the
formations are insulators hence electrical conduction is mainly through the interstitial
water which usually contains some dissolved salts.
Resistivity Logs:
In this log the
electrical resistivity of the formation around the borehole is measured. In a borehole
drilled with mud, the mud cake is formed against the side of the borehole wall. The
resistance of this mud cake is different from that of the rock-formation. Behind the mud
cake, flushed-zone is formed where the mud filtrate replaces the natural formation water.
At some distance from the borehole wall the rock formation is uncontaminated and has the
true resistivity of the formation, which could be obtained by proper interpretation of
geophysical logs. The extent of the mud-flushed zone depends upon the drilling technique
rock type ground water conditions etc.
The simplest electrical
measurement is a single point resistance (SPR) log, the electrical resistance of the
ground being measured between one surface electrode and one down hole electrode.
The case of
multi-electrode probes enables resistance measurements to be made of known or assured
volumes of earth and hence the measurements are calibrated in terms or resistivity. Common
electrode arrangements are the 16" (Short) and 61 inch (long) normal and the lateral
arrays. The short normal is designed to measure the resistivity of the invaded zone of the
formation close to the borehole well the long normal is used to obtain the resistivity of
the undisturbed formation beyond the invaded zone. The radius of investigation is
approximately equal to the electrode spacing.
The other resistivity
logs have been devised to investigate deeper into the formation in order to obtain a more
accurate value of the formation resistivity. These are the focussed current tools such as
the guard and laterolog, which are specifically designed to measure true readings and
relatively high formation resistivity through conductive borehole fluids.
Applications:
Determination
of formation water quality and level.
Determination of bed
thickness and type.
Determination of
porosity.
Correlations between
boreholes
Detection of
casing/open hole boundary
Spontaneous
Potential Log:
The spontaneous
potential (SP) log is a record of the naturally occurring electro-chemical and
electro-kinetic potential of the formations in the boreholes. It could be obtained with a
single moving electrode (usually of lead) in the borehole and a reference electrode at
surface. It is a relative measurement of the DC voltage in the boreholes with no zero
being recorded. Reading against shale/clays formations are relatively constant and are
referred as ‘shale baseline’. Opposite permeable formations the SP curve
typically shows deflections to the left (negative SP) or to the right (Positive SP) of
shale-base line depending upon the relative salinity of the drilling mud and formation
water.
Applications
Radiation Logs:
(a) Natural Gamma-ray
Log :
It is a measure of the
natural radiation emitted as a result of the disintegration of the radioactive elements
like uranium thorium and potassium. These are concentrated in minerals such as feldspar,
mica and glauconite, which in turn are prevalent in clay and shale. The clay minerals
formed during the decomposition of igneous rocks have very high absorption and ion
exchange capacities. They are therefore, able to absorb the heavy radioelements released
during the decomposition of other minerals. Due to such process there always is an
unusually heavy concentration of radioelements in shale and clays as compared to sand.
These natural radioactivities of sands and clay/shale could be measured through natural
radioactive logging to distinguish and delineate these formations.
The probe normally
consists of a scintillation counter, commonly a sodium iodide crystal and photo-multiplier
plus associated electronics.
Applications:
(b) Neutron-neutron
(Porosity) Log:
The neutron is a direct
measurement of the hydrogen content of the formation. The neutron probe contains a source
of high-energy neutrons (commonly americium-beryllium) with thermal neutron detectors at
fixed distance away from the source.
Applications
(c) Gamma-Gamma
(Density) Log:
It measures the
attenuation of back-scattered gamma radiation as a function of electron density of the
rock surrounding the borehole. The probe contains a source of gamma radiation (such as
cobalt 60 or Cesium 137), which is placed at a set distance from the detector. Shielding
prevents radiation from the source directly reaching the detector; consequently most of
the radiation arriving at the detector is via the formation. The gamma radiation is
scattered by the atoms in the formation such that the number of gamma rays arriving at the
detector is inversely related to the density of the rock. Either bow springs or caliper
arms run the probe down the side of the borehole wall.
Applications
Measurement of
bulk density
Derivation of porosity
Identification of
lithology
Location of cavities
and cement outside the borehole lining.
(d) Sonic Log
The acoustic (or sonic)
log provides a measure of the time of travel of compress ional waves over a given interval
of formation adjacent to the borehole. The probe consists of one or two transmitters one
or more receivers spaced a fixed distance apart and their associated electronics. The
acoustic transmitters are pulsed at regular intervals and the time is measured for each
pulse to reach the receivers.
Applications:
Fluid Logging
These logs record
the properties and movements of water within the borehole.
Temperature Log :
It measures the
temperature of the borehole fluid surrounding the probe. The probe contains a thermal
detector, thermistor or solid state device plus electronics. As well as recording the
absolute fluid temperature, differential temperature between two depths using two sensors
or more commonly made by digital techniques. Differential logs are particularly useful as
small changes in the temperature gradient are sharply accentuated.
Applications:
Detection of
fluid movement within the borehole
Identifications of
zones of inflow/outflow (including casing leaks) within the borehole
Determination of
geothermal gradient.
Provide data for
correcting other logs such as conductivity and resistivity.
Conductivity Log :
It measures the
electrical conductivity of the borehole fluid. The probe contains a series of encased
electrodes of inert metal of carbon. An alternating current is passed between one pair
electrodes and the resultant voltage across another pair is measured. Conductivity may
also be measured by electromagnetic methods whereby coils are used in place of electrodes.
Applications:
Flow Log:
It measures the
fluid velocity in the borehole. There are several means of measuring vertical flow in
boreholes depending on the magnitude of the velocity anticipated. The most common is an
impeller type, which consists of a turbine whose revolutions against time are counted. Low
flow measurements may also be made using specialized techniques such as heat pulse and
tracer methods.
Applications :
Determination of
flow rates and direction within the borehole.
Identification of
permeable zone.
Location of casing
leaks.
Construction Logging :
There are several probes
are available to measure the physical characteristics of the borehole construction such as
diameter, depth to casing location of casing joints and integrity of the cement grout.
Caliper Log :
The caliper probe, which
measure the diameter of the borehole drilled, is a mechanical device consisting of one of
four spring-loaded arms, which are held against the borehole wall. Variation in the
movement of these arms gives the mean diameter of the borehole drilled.
Applications:
Location of
casing, type and breaks
Location of diameter
changes in the borhole.
Location of
fractures/fissures and other opening.
Identification of soft
and hard formations.
Provision of data for
correcting other geophysical logs.
Calculation of borehole
volume.
Casing Collar Locator:
This log measures
the changes in the magnetic flux due to the presence of metal. The device consists of
magnetic and coil arrangements. A change in the magnetic flux caused by moving magnetic
material is an induced voltage. The log is run at constant speed and a changes in the
record indicated casing collars etc.
Applications :
Location of
steel casing collars and breaks.
Location of other
magnetic hardware such as rising mains, pumps and debris.
Control log for other
geophysical logs which are affected by the presence of casing.
Cement Bond Log :
It is a sonic
where the amplitude of the received acoustic signals is measured. Steel casing suspended
freely (unbonded) in a fluid filled borehole transmits elastic energy at a velocity of
5200 m/s with little attenuation over the transmitter receiver spacing. When cement is
present and properly bonded around the casing, the elastic is dissipated before reaching
the receiver. The received amplitude is therefore a function of bonding.
Applications:
Closed Circuit Television
Log:
It is a visual record
of physical features in borehole.
Applications
This log not only measure
the depth of visual features but is useful for checking the condition of casing or screen
examining geological features in the borehole walls, examining collapses and obstructions,
for determining inflows and outflows and for examining installed pipe work.
GEOPHYSICAL
CHARACTERISTICS OF DIFFERENT HYDROGEOLOGICAL UNITS
Representative resistivity
values for different formations
in parts of Jabalpur district.
Probable
lithological
Resistivity ranges |
horizons
ohm-m |
| Dry sand and clay (Topsoil) |
10 - 160 |
| Shale |
10 - 50 |
| Saturated sandstone |
40 - 100 |
| Sandstone |
80 - 300 |
| Dolomite |
120 - 1200 |
| Slate |
100 - 200 |
Representative resistivity values for different hydrogeological units
in parts of Mandsaur district.
| Resistivity
range(ohm-m) |
Inferred lithology |
| 5 - 20 |
Intertrappean/ Red bole |
| 15 - 70 |
Saturated Weathered,
Fractured Vesicular basalt. |
| 60 - 100 |
Dry weathered or fractured
vesicular basalt. |
| More than 100 |
Massive basalt. |
Resistivity
values of granitic formations.
| Depth in mbgl |
Resistivity in ohm-m
|
Lithology |
| 1-3 |
10 - 50 |
Weathered granite |
| 3- 10 |
25 - 1000 |
Fractured/jointed granite |
| More than 10 |
More than 4000 |
Compact and Massive granite |
Seismic
velocities in different hydrogeological unit
| Formation |
Seismic velocity in
km/s |
| Black cotton soil : |
0.25 -
0.4 |
| Weathered/fractured basalt : |
1.15 |
| Massive basalt |
more than 4.0 |
| Vindhyan Sandstone |
3.5 - 4.1 |
| Weathered granite |
0.5 |
| Fractured and jointed
granite |
1.0 - 1.5 |
| Compact granite |
more than 4.0 |
Resistivity
values of different hydrogeological unit in parts of Raipur district.
| Lithology |
Resistivity value
(ohm-m) |
| Taranga shale |
75-350 |
| Chandi limestone |
1000-3000 |
| Gunderdehi shale |
75-900 |
| Charmuria limestone |
1000-4000 |
| Cavernous/fractured
limestone |
150-600 |
| Fractured shale |
25-100 |
|
GEOPHYSICAL STUDIES - Subsurface
Geophysical Logging
