HYDROGEOPHYSICS

 


UPSL has the technological experteese and necessary equipment to contact state of the art Hydrogeophysical investigations either for the discovery of new aquifers  or for the evaluation of their pollution potential. It has successfully directed and completed the GEOMONITOR  program under the Life EC finance. In addition to the reach seismic gear that UPSL uses for hydrogeophysical investigations, the following equipment are also heavily used in various relevant projects.

Borehole Geophysics

UPSL  has a suite  of  borehole logging equipment on the market today. Using state-of-the-art tools we can provide borehole logging services for a variety of applications. 

 

APPLICATIONS 

  • Map geologic strata

  • Stratigraphic correlation

  • Identify aquifers

  • Thickness of beds

  • Identify clay-containing layers in overburden

  • Fracture characterization

  • Assess water quality, flow, elevation

  • Assess well completion

  • Inspection of well casing

  • Identify contaminants

 

EQUIPMENT 
UPSL uses equipment manufactured by OYO Geospace and Robertson Research.  The GEOLOGGER 3030 system is a highly portable winch and uphole module equipped with 660 feet of armored cable.  A suite of lightweight, small diameter downhole probes is used with the GEOLOGGER 3030.

 

Borehole Geophysical Logging

LOG

PARAMETERS MEASURED

APPLICATIONS

CALIPER

Borehole or casing diameter.

Fracture identification, lithologic changes, and well construction.

NATURAL GAMMA

Natural gamma radioactivity.

Lithology and estimation of clay content in overburden.

FLUID TEMPERATURE

Temperature of borehole fluid.

Indicates geothermal gradient, and water flow in borehole or between borehole and fractures.

FLUID RESISTIVITY

Resistivity of borehole fluid.

Indicates water flow within borehole, or between borehole and fractures; and water quality.

SINGLE POINT RESISTANCE

Resistance of materials between probe and ground surface electrode.

Lithology, fracture identification, and location of well screens.

NORMAL RESISTIVITY

Apparent resistivity of material.

Lithology, and water quality.

SPONTANEOUS POTENTIAL (SELF POTENTIAL or SP)

Electrical potentials between probe and surface electrodes.

Lithology, water quality, and in some cases, fractures in resistive crystalline rock.

EM CONDUCTIVITY (INDUCTION)

Electrical conductivity in medium surrounding borehole.

Location of contaminant plumes, conductive clay units, or bedrock fractures. Monitor water quality changes over time.

FLOWMETER: IMPELLER or HEAT-PULSE

Continuous or point measurements of water flow in borehole.

Identification of permeable zones and apparent vertical hydraulic conductivity and flow direction.

BOREHOLE VIDEO

Provides visual record of lithology, fractures, well construction.

Lithologic logging; identification of fractures; examination of casing or well construction.



DATA PROCESSING AND PRESENTATION 
Data collected by the downhole probes are digitally stored during acquisition in a laptop PC.  Low-resolution field printouts are produced while the data is being acquired, allowing the operator to review the data for completeness.  Later, appropriate scales are chosen and filters may be applied, and high resolution printouts are made.  Presentation quality logs from several probes are merged on the final printouts.

GEOELECTRIC EXPLORATION 

UPSL has the state of  the art equipment for 2D and 3D geoelectric exploration and tomography aquifer and pollution plume surveys: 

SuperSting R1 IP
single channel Memory Earth Resistivity and IP Meter

Sting R8

The SuperSting R1 IP is a state-of-the-art single-channel portable memory earth resistivity meter with memory storage of readings and user defined measure cycles. It provides the highest accuracy and lowest noise levels in the industry.
This new instrument is based on technology developed for the famous SuperSting R8/IP multi-channel instrument. It pushes the performance levels of single channel systems forward by a large step.
With the high power transmitter good data can be recorded in difficult locations where time-consuming stacking was the only alternative before.
SuperSting R1/IP uses the patented Swift Dual Mode Automatic Multi-electrode cable. For users of the existing Sting/Swift system wanting to upgrade the instruments their cable investment can be reused with this new instrument since the old cables can be used also with SuperSting R1/IP.
The controller for the cable is now completely built into the SuperSting R1/IP main instrument so there are no extra boxes to carry and connect in the field.

Key Benefits

  • High power transmitter.

  • Field adapted rugged construction. Built to last in real conditions.

  • Easy to use menu driven system.

  • The best accuracy and noise performance in the industry!

  • Large capacity internal memory for storage of measurement results.

  • User programmed measure cycles can be loaded into memory from a PC and later executed in the field.

  • Directly controls the Swift Dual Mode Automatic Multi-electrode system (patent pending)!

  • Induced Polarization mode records 6 individual IP chargeability windows.

PRELIMINARY TECHNICAL SPECIFICATION:

Measurement modes

Apparent resistivity, resistance, self potential (SP), induced polarization (IP), battery voltage

Measurement range

+/- 10V

Measuring resolution

Max 30 nV, depends on voltage level

Screen resolution

4 digits in engineering notation.

Output current

1mA - 1.25 A continuous

Output voltage

800 Vp-p, actual electrode voltage depends on transmitted current and ground resistivity.

Output power

100W

Input gain ranging

Automatic, always uses full dynamic range of receiver.

Input impedance

>20 Mohms

SP compensation

Automatic cancellation of SP voltages during resistivity measurement. Constant and linearly varying SP cancels completely.

Type of IP measurement

Time domain chargeabilitiy (M), six time slots measured and stored in memory

IP current transmission

ON+, OFF, ON-, OFF

IP cycle times

1 s, 2 s, 4 s and 8 s

Measure cycles

Running average of measurement displayed after each cycle. Automatic cycle stops when reading errors fall below user set limit or user set max cycles are done.

Resistivity cycle times

Basic measure time is 1.2, 3.6, 7.2 or 14.4 s as selected by user via keyboard. Autoranging and commutation adds about 1.4 s

Signal processing

Continuous averaging after each complete cycle. Noise errors calculated and displayed as percentage of reading. Reading displayed as resistance (dV/I) and apparent resistivity (ohmm or ohmft). Resistivity is calculated using user entered electrode array co-ordinates.

Noise suppression

Better than 100 dB at f>20 Hz
Better than 120 dB at power line frequencies (16 2/3, 20, 50 & 60 Hz)

Total accuracy

Better than 1% of reading in most cases (lab measurements). Field measurement accuracy depends on ground noise and resistivity. Instrument will calculate and display running estimate of measuring accuracy.

System calibration

Calibration is done digitally by the microprocessor based on correction values stored in memory.

Supported configurations

Resistance, Schlumberger, Wenner, dipole-dipole, pole-dipole and pole-pole.

Operating system

Stored in re-programmable flash memory. New versions can be downloaded from our web site and stored in the flash memory.

Data storage

Full resolution reading average and error are stored along with user entered coordinates and time of day for each measurement. Storage is effected automatically in a job oriented file system.

Memory capacity

Resistivity mode 70.000 readings, Resistivity/IP mode 25.000 readings

Data transmission

RS-232C channel available to dump data from instrument to a Windows type computer on user command.

Automatic multi-electrodes

The SuperSting is designed to run dipole-dipole, pole-dipole, pole-pole, Wenner and Schlumberger surveys including roll-along surveys completely automatic with the Swift Dual Mode Automatic Multi-electrode system (patent pending).
The SuperSting can run any other array by using user programmed command files.
These files are ASCII files and can be created using a regular text editor.
The command files are downloaded to the SuperSting RAM memory and can at any time be recalled and run. Therefore there is no need for a fragile computer in the field.

User controls

20 key tactile, weather proof keyboard with numeric entry keys and function keys.
On/Off switch
Measure button, integrated within main keyboard.
LCD night light switch (push to illuminate).

Display

Graphics LCD display (16 lines x 30 characters) with night light.

Power supply, field

12 V DC external power, connector on front panel.

Power supply, office

Mains operated DC power supply.

Operating time

Depends on survey conditions and size of battery used.

Weight

10.9 kg (24 lb), instrument only.

Dimensions

Width 184 mm (7.25"), length 406 mm (16") and height 273 mm (10.75").

 

 

 

SALINE WATER INTRUSION MONITORING AND CONTROL

UPSL has developed a methodology for the automatic monitoring and control of saline water intrusion in coastal aquifers.

 

 

The flowchart below depicts the work-program applied by UPSL in collaboration with LandTech Enterprises during a deep aquifer exploration project in Libya:

 

 

 

Planning the survey

The optimal ground water surveying method is no doubt drilling. This method ensures that all necessary information is being brought up from the geological formations. However, in order to obtain a desired degree of information from the subsurface of a project area, drilling alone is normally not a feasible alternative.

There are a number of efficient and inexpensive geophysical surveying methods available to the project hydrogeologist. It is worth noting at this point that these are, without exception, indirect methods. This implies that neither method measure directly what we are actually looking for. With geophysical surveys, the target features are therefore invariably associated features. This implies that unless we understand the water context of these features, our geophysical surveys will be less than meaningful.

Some examples may illustrate this:

  • A water-filled fracture may have a deposit ("skin") consisting of a conductive mineral, deposited by moving water. This last feature is picked up by VLF instrumentation, not the water itself.

  • A dyke can be picked up from its host rock by seismic refraction; water can often be found in the transition between these two bodies; the presence of water is only indicated by association and deduction.

In addition, a number of complications and limitations apply. For example, during interpretation of a resistivity survey, a thick resistive layer may have the same signature as a thin low resistive layer; the principle of equivalence. Highly conductive layers may limit the depths of investigation short of the target features. A thorough knowledge of a method's limitations and assets is vital.

As a rule, considerable effects of synergy can be achieved if more than one method is applied. For example, your resistivity survey indicates a layer with high conductivity. This could mean either a saturated clay lens, or porous material (gravel) but with saline water; both occur in the area. A refraction seismic survey indicates the same layer as having a speed typically lower than that of clay. Conclusion: The layer consists of gravel, with saline water, i.e. an aquifer which is not suitable for water supply purposes.

Identification of important target features is a prerequisite for planning and implementation of geophysical groundwater surveys. Considerable efforts and research should be invested in this part of the project. A surprising number of groundwater exploration projects tend to skip this important aspect.

Choosing the appropriate method

The next logical step is then to find the most appropriate method that fits the project's Term of Reference, budget, as well as local conditions, the identified target features, appropriate technology levels, logistics, etc.

A systematic approach is encouraged in the selection of adequate methods. There are many considerations; some few examples of pitfalls are illustrated below:

  • A VLF survey is highly productive but would provide little useful information if the target aquifer is a porous gravel aquifer.

  • A magnetic survey could be next to useless over homogenous and unfractured sandstones.

  • A particular method may prove inappropriate with regards to technology transfer within the project context.

  • Data acquisition and processing could be too expensive for the project budget

  • Use of explosives for seismic surveys could prove to be impractical.

A simple Decision Support System (DSS) can be established on a spreadsheet for this purpose.

Note that all blue fields are protected ("Tools-Protect/Unprotect", no password is necessary.) Yellow fields are data entry fields. Red characters are weights; the relative weight (importance) for each table can be entered as integer values 0-1.0; the sum of these must be equal to 1.0. The bottom matrix indicates the results of this hypothetical case study: Rankings are presented as percentages. Note that a particular alternative is flagged NO GO if the product is zero, i.e. an "unacceptable" condition.

Desicion Support Calculator Desicion Support Calculator

The table above illustrates a simplified way of , an array of methods are presented. The list could have included also other alternatives but the above list was considered realistic for most ground water projects . 

Exploration methods: Brief outlines 

Let us recapitulate what parameters might be interesting to us, and what properties can be measured with the above methods.

Very Low Frequency (VLF) is an electromagnetic based method operating in the 8 kHz band; requiring either military transmitters already in operation and positioned around the globe, or dedicated transmitters set up in the survey area, operated for the duration of the surveys. In the first case, the method is easy to use, it has a high level of productivity, and rented equipment is easy to obtain. Drawbacks include low signal to noise ratio, and impractical transmitter orientations.

Where dedicated transmitters have to be set up in order to provide adequate signal strengths and orientations, the overall costs increase and the productivity goes down. Recent developments in data acquisition and processing techniques can rapidly provide impressive graphics; however the application of the method in groundwater prospecting is generally limited to water bearing fractures where fracture geometry, water chemistry and the electric characteristics of the host rocks are favourable. It is important to note that the method is not suitable for porous aquifers such as sands, gravels etc.

VLF (Very Low Frequency)

VLF is an abbreviation for Very Low Frequency, and includes electromagnetic waves within the 3-30 kHz frequency bands. The principle of VLF is simply the study of interaction of these radio waves with geological elements in the ground. This interaction induces secondary fields which can be measuredat and above the surface of the earth. This, in turn enables measuring VLF waves and their interactions with earth materials to be used for applications such as exploration of subsurface geological structures.

VLF waves have several unusual characteristics. Firstly, they penetrate relatively deep into the ground (and the sea). For this reason, a series of military VLF transmitters around the World have been set up for submarine communication purposes. The signals from these stations can also be used for civilian purposes, including the geophysical prospecting for water and minerals. Secondly, the range of these signals is global.

A characteristic near-vertical VLF wave front from one of these military transmitters will be affected by geological features (anomalies) such as a conductive ore body, or a water filled fracture. The degree of disturbance (actually expressed as angles) over this anomaly can be measured, processed, and interpreted. But, as always with geophysics, one must know what to look for; i.e. be able to see the observed features and signals in a geological context.

The VLF method is very fast, inexpensive, and particularly well suited for hard rock prospecting. Porous unconsolidated media such as sand is not suitable for this method unless you are looking for very large metal sheets buried in the ground. Conductive media such as wet clays will effectively mask anything that may lie beneath.

Considerable advances in data processing has been achieved during the last years; the method has considerable potential in hard rock terrain and when used in conjunction with GPS and proper processing software.