Section 1.8
Geophysical Studies of Aquifers
Note: This section requires the student to engage in actual geophysical studies, either on
the shallow coastal aquifer of Gaza or the shallow perched aquifers of the West Bank.
The student is also introduced to techniques of down-well geophysical logging in the
deep aquifers. The background theory to the field study is derived from Clarke (1988)
1.8.1. Introduction
Information concerning the geology, water levels at various horizons and hydraulic
conductivity may be obtained from geophysical tests. Resistance tomography may be
used for imaging shallow aquifers. Geophysical logging is a method of measuring
features in a borehole or in the geological formation adjacent to the borehole.
1.8.2.Imaging the subsurface
It is possible to determine the depth and extent of saturated rock in a shallow aquifer by
resistance tomography. The sharp discontinuity between the unsaturated zone and the
saturated zone is distinguished by varying resistance to seismic vibrations generated at
the surface and recorded by a dot matrix computer display. This allows the profiling of
local shallow aquifers, particularly the extent and thickness of perched aquifers. This
method may be employed to depths up to 200 m. Only 50% of the distance between
dipoles can be imaged in this technique. Geophysical logging usually backs up data
obtained by resistance tomography.
1.8.3. Geophysical logging
A logging unit comprises a monitoring console, a set of tools and a winch with the
necessary conductor cable. Each tool is designed to measure one or more variables as it is
lowered down the borehole on the end of the cable. Measurements are sent as electronic
signals from the tool up the cable to the console as a continuous record of the parameter
being studied. Computers usually store the signalled information in data loggers.
Analogue tracers may convert the signals to a chart as illustrated in Figure 1.8.1.
The logs most commonly used in the water industry include
1. formation logs
2. structural logs
3. fluid logs
Their interpretation is usually qualitative in nature.
1.8.3.1. Formation logs
The formation logs include resistivity, self potential (SP), gamma ray, neutron and
gamma-gamma . The resistivity and SP logs are commonly run together as an electric
log, which can be valuable in identifying lithologies, but suffers from a lack of precision
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Figure 1.8.1. Geophysical well log.
[Note: It was not possible to get good resolution electronically – this figure and 1.8.3 will be sent as hard copy]
Source: Clark (1988)
and its limitation to fluid-filled, uncased holes. The deflections on an SP curve are
generated at the junction between a permeable bed, an impermeable bed and the drilling
fluid, so that the log can be used to detect boundaries between sands and clays. The
resistivity logs measure the resistivity of a particular geological formation which depends
to some extent on the groundwater salinity, but also characteristic of rock type:
High resistivity
↓
↓
↓
↓
Low resistivity
crystalline rocks
Limestones, etc.
Sandstones
Unconsolidated sands
Silts
Clay & shale
1.8.3.1.1. Gamma-ray log.
The gamma-ray log measures the natural gamma-ray emissions from formations and can
be invaluable in identifying various lithologies for comparison with a percentile log
(Figure 1.8.2). The natural gamma count generally reflects the shale content of
sedimentary rocks because most gamma rays come from radioactive potassium isotopes
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in clay minerals. The great advantage of this log is that it can be used in dry or cased
boreholes.
Figure 1.8.2. Correlation using a gamma-ray log.
Source: Clark (1988)
1.8.3.1.2.The neutron and gamma-gamma logs.
These use small radioactive sources to emit neutrons and gamma rays respectively. The
use of such sources makes the logistics of logging much greater because their storage,
transport and use on-site are subject to rigorous safety rules. These radioactive logs,
therefore, are much less frequently used than others.
The neutron tool emits high-energy neutrons, which lose their energy on collision with
atomic nuclei in the rock formation, and eventually become captured. The lost energy is
emitted as gamma rays. The greatest energy loss is when the neutrons hit hydrogen nuclei
(protons), and therefore the energy loss and emissions of gamma ray is dependent on the
amount of hydrogen (as in water) in the rock formation. The % water in a saturated
formation is the formation porosity, and so the gamma-ray emissions, measured by a
detector on the tool, can be calibrated in terms of the porosity. Other neutron tools have
detectors for slow or fast neutrons instead of gamma rays, but are based on the same
effect of neutron capture by hydrogen nuclei.
The gamma-gamma tool is used to measure the density of the rock formation. Mediumenergy gamma rays are emitted from a source in the tool and lose energy through
collisions with electrons in the formation. A detector on the tool measures the residual
gamma rays from the emitter and these can be related to the formation density. In a
mono-mineralic rock formation, this formation density can be used to obtain the
formation porosity (Ø) by using the known mineral specific gravity
Ø= ρ1 – ρ2/ρ1 – 1
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where ρ1 = specific gravity of mineral
ρ2 = density of rock formation
1.8.3.2. Structural logs
The structural logs can be useful in formation evaluation, but their main purpose is to
check the physical status of the borehole. The most common structural logs are
 caliper
 casing collar locator
 closed-circuit television (CCTV)
1.8.3.2.1. Caliper
The basic calliper tool is lowered to the bottom of the well, where three or four springloaded arms are extended by servo-motor. The tool is then pulled to the surface, and the
diameter of the hole is measured as twice the average radius measured by the arms. Many
modern calliper tools record the radii of the different arms to give a more complete
geometry of the hole. The calliper tool identifies zones of caving, zones of thick mud
cake, fractured casing, and fissures in the hard rock.
1.8.3.2.2. Casing collar
The casing collar locator detects small electric fields generated at junctions at different
metals, and can be used to detect casing joints or faults in casing.
1.8.3.2.3. CCTV
The CCTV gives a visual display of the well walls, but suffers from the fact that the
picture quality can be destroyed by cloudy water. The visual inspection is good for
detecting cracked casing, fissures in rocks or debris in the borehole. CCTV must be used
as an accompaniment to the geophysical logs. It has very limited use on its own.
1.8.3.3. Fluid logs
The fluid logs measure properties of liquids in the borehole and, in shallow water wells.
These logs are usually run after the well has been cleaned out, so that they can measure
the properties of natural groundwater. Fluid logs include data on temperature,
conductivity, differential temperature (ΔT) and conductivity (ΔC). Fluid logs also include
flow measurements.
The temperature of a static column of water in a borehole will rise with depth according
to the geothermal gradient. Groundwater flow across or up a borehole will disturb this
temperature gradient. ΔT measures the change in temperature over a fixed depth interval,
so that, with a uniform gradient ΔT is constant However, a temperature change, caused
by an inflow of water to the well creates a peak on the ΔT log (Figure 1.8.1).
The conductivity log measures the conductivity of the water in the well and gives some
indication of its quality. A change in conductivity ( a peak on the ΔC log) will mark an
inflow or outflow of water to the well. In a well where heads vary through the thickness
of the aquifer, there will be flow up or down the water column. An impeller flow meter
can measure high flows, but at low-flow velocities ΔC of a few cm/sec, a heat pulse flow
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meter may be needed. The interpretation of this full suite of flow logs can provide
invaluable data on the hydraulic regime in the borehole and surrounding formations.
1.8.3.4. The basic suite of log data
Geophysical logs can be used to provide information to help in pumping test analyses or
regional hydrogeological studies. They are also useful in checking completion details of
wells in drilling contracts. A suite of logs also represent a base set of data against which
future well performance or status can be measured. For all these reasons it is
recommended that every borehole drilled should have a basic set of logs run to
completion. This set should include:
1. Gamma-ray, calliper and flow meter
2. Temperature + ΔT
3. Conductivity + ΔC
1.8.3.5. Limitations of geophysical logs
The limitations of the use of various geophysical logging tools in boreholes with or
without casing, or containing water columns or not, are summarized in Table 1.8.1.
Table 1.8.1 Application of borehole geophysics
Log.Type
Geophysical
log
Formation
Resistivity
self-potential
gamma
neutron
gamma-gamma
Calliper
casing collar
locator CCTV
Flow-meter
Temperature/
Temperature
Conductivity/
Conductivity
Structural
Fluid log
Cased/
screened
borehole
No
No
Yes
Yes
Yes
No use
Yes
Yes
Uncased
borehole
Dry
borehole
Yes
Yes
Yes
Yes
Yes
Yes
No
Yes
Yes
Mud/waterfilled
borehole
Yes
Yes
Yes
Yes
Yes
Yes
Yes
in clean water
Not in mud
Yes but of
limited use
Yes
Yes
Yes
Yes
No
No
No
No
Yes
Yes
Yes
Yes
No
Yes
No
Geophysical logging is a specialized operation and should be done by a geophysical
contractor or trained operator. It is important that students observe on-site geophysical
surveying and receive instruction by the professionals. It is possible to obtain actural
geophysical well logs from Palestinian wells. Interpretation of well logs or other
geophysical data requires considerable practice and experience. The comprehensive
analyses of a suite of logs need a trained and experienced geophysicist. It is better to have
modern digitally recorded logs. Some computer programs are designed to interpret as
well as make graphic representations of the data recorded.
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Figure 1.8.3. Caliper log for Arrub observation well
Source
Figure 1.8.4. Geophysical logs for Hebron 1
Source: Mekorot
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