Why Measure Soil Resistivity?

Why measure soil resistivity?

Soil resistivity measurements have a threefold purpose. First, such data is used to make sub-surface geophysical surveys as an aid in identifying ore locations, depth to bedrock and other geological phenomena. Second, resistivity has a direct impact on the degree of corrosion in underground pipelines. A decrease in resistivity relates to an increase in corrosion activity and therefore dictates the protective treatment to be used. Third, soil resistivity directly affects the design of a grounding system, and it is to that task that this discussion is directed. When designing an extensive grounding system, it is advisable to locate the area of lowest soil resistivity in order to achieve the most economical grounding installation.

Effects of soil resistivity on ground electrode resistance

Soil resistivity is the key factor that determinates what the resistance of a grounding electrode will be, and to what depth it must be driven to obtain low ground resistance. The resistivity of the soil varies widely throughout the world and changes seasonally. Soil resistivity is determined largely by its content of electrolytes, which consist of moisture, minerals and dissolved salts. A dry soil has high resistivity if it contains no soluble salts.

Resistivity (approx), Ohm-centimeters

Soil Minimum Average Maximum
Ashes, cinders, brine, waste 590 2370 7000
Clay, shale, gumbo, loam 340 4060 16,300
Same, with varying proportions of sand and gravel 1020 15,800 135,000
Gravel, sand, stones with little clay or loam 59,000 94,000 458,000

Factors affecting soil resistivity

Two samples of soil, when thoroughly dried, may in fact become very good insulators having a resistivity in excess of 109 ohm-centimeters. The resistivity of the soil sample is seen to change quite rapidly until approximately 20% or greater moisture content is reached.

Moisture content % by weight Resistivity top soil Resistivity sandy loam
0 >10 >10
2.5 250,000 150,000
5 165,000 43,000
10 53,000 18,500
15 19,000 10,500
20 12,000 6300
30 6400 4200

The resistivity of the soil is also influenced by temperature. Next figure shows the variation of the resistivity of sandy loam, containing 15.2% moisture, with temperature changes from 20° to -15°C. In this temperature range the resistivity is seen to vary from 7200 to 330,000 ohm-centimeters.

Temperature C Temperature F Resistivity (ohm-centimeters)
20 68 7,200
10 50 9,900
0 32 (water) 13,800
0 32 (ice) 30,000
-5 23 79,000
-15 14 330,000

Because soil resistivity directly relates to moisture content and temperature, it is reasonable to assume that the resistance of any grounding system will vary throughout the different seasons of the year. Since both temperature and moisture content become more stable at greater distances below the surface of the Earth, it follows that a grounding system, to be most effective at all times, should be constructed with the ground rod driven down a considerable distance below the surface of the Earth. Best results are obtained if the ground rod reaches the water table.

In some locations, the resistivity of the Earth is so high that low-resistance grounding can be obtained only at considerable expense and with an elaborate grounding system. In such situations, it may be economical to use a ground rod system of limited size and to reduce the ground resistivity by periodically increasing the soluble chemicals content of the soil.

Chemically treated soil is also subject to considerable variation of resistivity with temperature changes. If salt treatment is employed, it is necessary to use ground rods, which will resist chemical corrosion.

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At Heritage Geophysics  you will find the latest in resistivity meters, electromagnetics, magnetometers, induced polarization, seismographs, and even magnetic resonance sounding systems.

To read more about geo resistivity, resistivity meters, or resistivity equipment, visit our website: http://www.heritagegeophysics.com.

CORIM Continuous Profiling Resistivity System

terraplus.ca

Features

  • Continuous Profile Resistivity Measurement
  • Real Time Data Display
  • Investigation Depth of 3 Meters
  • Rapid Data Collection

General

The CORIM System consists of a trolley that supports the system controller and power supply, along with distance measuring device and notebook PC, attached to up to seven capacitive electrodes, and a final transmitting electrode, each with their electronics. The system may be pulled by hand, or by a vehicle, at speeds up to a few kilometers per hour.

CORIM Continuous Profiling Resistivity System. Get yours at heritagegeophysics.com.

CORIM Continuous Profiling Resistivity System. Get yours at heritagegeophysics.com.

With it’s continuous data collection capability the CORIM system may find applications for measurement of earth resistivity in cases where other geophysical techniques may not be appropriate, such as areas of high clay content, or excessive cultural noise. The CORIM system can be expected to make measurements with a 3-4 meter depth limit, with a resolution of about 0.3 meter.

The electrode carpets consist of one meter spaced capacitive plate electrodes with a receiver unit mounted on each. Each receiver then connects to the system controller in the trolley. One of the electrode carpets functions as the transmitter and contains an independent transmitter with its own battery pack. A spare transmitter battery pack may be kept on the trolley and recharged from the main system, so that it is ready when needed.

Mounted on one of the trolley wheels is an optical pulse encoder to measure distance and trigger readings every 0.2 to 10 meters. The acquisition software displays vehicle speed and warning messages when the speed is too fast. Control of the system is from the PC via a serial port connection. System status, including battery voltages, are displayed on the PC.

During system setup the Compensation routine may be run. This measurement calculates the local SP effect and zeros this for data analysis. The Compensation measurement may be made several times on a profile in order to correct for changes in surface conditions.

As data acquisition begins the apparent resistivity values calculated for each electrode carpet, or channel, are continuously displayed, either digitally, or as a series of profiles. A marker function allows the operator to mark fiducial points for later reference.

The CORIM View software is used to filter and display the final data. Data may be displayed and printed as either a series of line profiles or as a pseudosection. Data may be displayed as either raw data, or as Compenstation(SP) corrected data.

Standard components

6 Receiver dipole carpets, 1 transmitter dipole carpet, 6 receiver electronics, 1 transmitter electronics, transmitter battery, receiver controller, main rechargeable battery, battery chargers for main, and transmitter batteries, distance encoder, trolley, spare parts kit, software and manual.

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Resistivity imaging systems allowing automated electrode sampling for 3D surveys, marine surveys, and borehole ERT. Systems from 24 to more than 4,000 electrodes. Order your today at Heritage Geophysics.

Are you interested in resistivity equipment, resistivity imaging systems, or resistivity instruments? To continue reading about these subjects, visit our website: http://www.heritagegeophysics.com.

Geophysics: The Tools of the Trade (Magnetic)

by Dr. Peter Hood | northernminer.com

Exploration activity in Canada last year fell markedly from the 1989 level, which was about 70% or so down from the record 1987 and 1988 levels.

Taxation changes have greatly affected the ability of the junior mining companies to raise money, resulting in a generally depressed level of grassroots activity.

Companies offering ground geophysical surveys have been hit particularly hard. But, rather than sitting idle, these companies have been busy developing newer techniques as you’ll discover in the following pages…

The opportunity for prospectors and exploration personnel to become familiar with the use of ground geophysical equipment under actual field conditions is rather infrequent. In order to provide such a training opportunity, the Haileybury School of Mines in Haileybury, Ont., organized a 4-day field school in Kirkland Lake in June, 1990. Introductory lectures were presented each morning, and the rest of the day was spent using the various geophysical instruments on a test range about seven kilometers south of Kirkland Lake. More than a dozen instrument and contracting companies supported the field school by demonstrating their equipment and presenting discussions that included the compilation of the day’s survey results.

In this opportunity we will reproduce the presentation about magnetic equipment.

Magnetic

Bartington Instruments of Oxford, England, makes a magnetic susceptibility system with a measurement range of 1-9999 x 106 cgs units. The MS-2 gives digital readings in about one second on a large LCD. The MS-2 has a range of coil sensors that produce an alternating one-oersted magnetic field at the sample. The change in frequency due to the change in inductance of the coil is measured. The MS2B laboratory sensor can accept one-inch cubes and cores and can operate at two frequencies. The MS2D field sensor is used for in situ volume susceptibility measurements in the top 2-metre subsurface.

Gem Systems has introduced an option to the 0.1 nT GSM-19 Overhauser magnetometer-gradiometer series that allows the operator to record magnetic and/or gradient field data every 0.5 seconds as the operator walks along the survey line. The GSM-19 has undergone various software improvements, such as increased cycling time of five or 10 readings per second; decimal station spacing; and 60 Hz or 50 Hz filters. Additionally, the memory capaciy has been standardized to 128 kilobytes.

This latter improvement became necessary to accommodate the volume of data that can be accumulated by the GSM-19 in the “walking” option. A second “hip-chain” option will automatically assign grid co-ordinates to readings measured and stored by the walking option. This provides tighter ground control to the magnetic data collected using the walking mag option. The GSM-19 can also use other magnetometers, such as the Omni Series and GSM-10/18, as diurnal base stations.

Geoscan Research of Bradford, U.K., is manufacturing three microprocessor-controlled fluxgate gradiometers having 0.5-metre separation of the sensors. The FM9 basic version has a 5 nT resolution and a range of +/- 20,000 nT. The FM18 has a 0.5 nT data storage resolution and a 4,000-reading memory. The FM36 has a 0.05 nT data storage resolution and a 16,000 reading memory. Both the FM18 and FM36 keep track of the survey position. The ST1 sample trigger permits rapid sampling of up to eight readings per metre for detailed archeological work.

Geotech has produced two new versions of its proton precession magnetometer. The Model M100S is a 1-nT ground magnetometer that can be converted to a base station. The data for the M100S can be recorded by connecting it to Geotech’s miniature hand-held data acquisition computer.

Geotech has also developed a 0.1-nT proton magnetometer, the M1000 that incorporates auto-tuning and can be used with a wide range of sensors. Versions are available for airborne, portable marine and base station use. The ground memory magnetometer version will interface to portable GPS and VLFEM receivers.

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See a lot more about magnetometer, magnetometers, or ground magnetometer, by visiting our website: http://www.heritagegeophysics.com.

SYSCAL Pro Resistivity & IP Equipment for Sounding, Imaging and Monitoring

geo.au.dk

The Syscal Pro offers an entirely new level of performance to the working geophysicist needing to extend capabilities. Consideration of the system at its most basic level reveals a unit with unmatched power. The Pro offers a maximum 1,000V transmitting pulse (2,000V peak to peak), at up to 2.5A, for a total of 250W of transmitting power. Contractors can use this power to reach greater depths, and to speed surveys, since fewer stacks will be required.

The standard Pro system offers GPS input and a continuous data acquisition mode, so it is ready to go for marine surveys.

For the research geophysicist a Syscal Pro Switch system may be used to monitor lab experiments with the alarm function. In this mode an optional adapter box is used so that bare wire connections may be easily made. Standard are 20 programmable IP windows, and input voltage of 15V on the first channel, and up to an additional 15V total on channels 2-10. The standard system also offers a low current option that extends the A/D low range for accurate low current measurements.

To buy your SYSCAL Pro resistivity meter, please go to heritagegeophysics.com.

To buy your SYSCAL Pro resistivity meter, please go to heritagegeophysics.com.

SYSCAL Pro Switch main features:

  • The SYSCAL Pro Switch is the more versatile or the family of SYSCAL Pro electrical resistivity meters. It combines a transmitter, a receiver and a switching unit in one single casing. It is supplied by a 12V battery.
  • The measurements are carried out automatically (output voltage, stacking number, quality factor) after selection of limit values by the operator, and are stored in the internal memory.
  • The output specifications are 800V (1 600V peak-to-peak) in switch mode, 1 000V (2 000V peak-to-peak) in manual mode, 2.5A, and 250W with the internal converter and a 12V battery.
  • The SYSCAL Pro Switch uses multi-core cables for controlling a set of electrodes connected in a line or in several lines. The standard number of electrodes: 24, 48, 72, 96, 120, can be increased through Switch Pro units for 2D or 3D ground images.
  • The ten channels of the system permit to carry out up to 10 readings at the same time for a high efficiency.
  • The Induced Polarisation chargeability (IP) is also measured through 20 windows for a detailed analysis of the decaying curves displayed on the graphic LCD screen.
  • The SYSCAL Pro Switch unit can be operated with cables in boreholes, or cables pulled on the ground by a vehicle or on the surface of the water by a boat for continuous acquisition surveys.
  • The SYSCAL can be used for time lapse readings (monitoring).

SYSCAL Pro specifications:

TRANSMITTER

  • Max voltage: 800V in switch mode
  • Max voltage: 1 000V in manual mode
  • Max current: 2.5A, typ. accuracy 0.2%
  • Max power : 250W with internal DC/DC converter and 12V external battery; 1200W with external AC/DC and Motor Gene.
  • Option 25mA max for readings on samples
  • Pulse duration: 0.2s, 0.5s, 1s, 2s, 4s, 8s
  • Internal 12V, 7Ah battery, plug for ext. batt.

RECEIVER

  • Automatic ranging, 10 input channels
  • Input impedance: 100 Mohm
  • Max voltage channel 1: 15V
  • Max voltage sum of channel 2 to 10: 15V
  • Protection up to 1 000V
  • Typ accuracy: 0.2%, resolution: 1 microV
  • 50 to 60Hz power line frequency rejection
  • Stacking process, SP linear drift correction
  • Reading of current, voltage, standard dev., 20 IP windows (preset or selectable),
  • Internal 12V, 7Ah battery

GENERAL

  • Memory: 40 000 readings USB & SD card link
  • GPS input for coordinates
  • Fiber glass casing, weather proof
  • Temperature range:
  • 20 to +70°C
  • SYSCAL Pro Switch 48: 31x23x36cm, weight: 13kg, cable w/ 24 take-out: 23kg

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Go to: http://www.heritagegeophysics.com, to learn more about resistivity meters, resistivity equipment, or magnetometers.

Soil Resistivity Measurement

fluke.com

Why Determine the Soil Resistivity?

Soil resistivity is most necessary when determining the design of the grounding system for new installations (green field applications) to meet your ground resistance requirements. Ideally, you would find a location with the lowest possible resistance. But as we discussed before, poor soil conditions can be overcome with more elaborate grounding systems.

The soil composition, moisture content, and temperature all impact the soil resistivity. Soil is rarely homogenous and the resistivity of the soil will vary geographically and at different soil depths.

Moisture content changes seasonally, varies according to the nature of the sub layers of earth, and the depth of the permanent water table. Since soil and water are generally more stable at deeper strata, it is recommended that the ground rods be placed as deep as possible into the earth, at the water table if possible. Also, ground rods should be installed where there is a stable temperature, i.e. below the frost line.

For a grounding system to be effective, it should be designed to withstand the worst possible conditions.

How do I Calculate Soil Resistivity?

The measuring procedure described below uses the universally accepted Wenner method developed by Dr. Frank Wenner of the US Bureau of Standards in 1915. (F. Wenner, A Method of Measuring Earth Resistivity; Bull, National Bureau of Standards, Bull 12(4) 258, p. 478-496; 1915/16.)

The formula is: Divide ohm—centimeters by 100 to convert to ohm—meters. Just watch your units.

Example: You have decided to install three meter long ground rods as part of your grounding system. To measure the soil resistivity at a depth of three meters, we discussed a spacing between the test electrodes of three meters.
To measure the soil resistivity start the Fluke 1625 and read the resistance value in ohms. In this case assume the resistance reading is 100 ohms. So, in this case we know:
A = 3 meters, and
R = 100 ohms
Then the soil resistivity would equal:
r= 2 x p x A x R
r = 2 x 3.1416 x 3 meters x 100 ohms
r= 1885 Ωm

How do I Measure Soil Resistance?

To test soil resistivity, connect the ground tester as shown below.

Setup for soil resistivity testing using the Fluke 1623 or 1625.

Setup for soil resistivity testing using the Fluke 1623 or 1625.

As you can see, four earth ground stakes are positioned in the soil in a straight line, equidistant from one another. The distance between earth ground stakes should be at least three times greater than the stake depth. So if the depth of each ground stake is one foot (.30 meters), make sure the distance between stakes is greater than three feet (.91 meters). The Fluke 1625 generates a known current through the two outer ground stakes and the drop in voltage potential is measured between the two inner ground stakes. Using Ohm’s Law (V=IR), the Fluke tester automatically calculates the soil resistance.

Because measurement results are often distorted and invalidated by underground pieces of metal, underground aquifers, etc. additional measurements where the stake’s axis are turned 90 degrees is always recommended. By changing the depth and distance several times, a profile is produced that can determine a suitable ground resistance system.

Soil resistivity measurements are often corrupted by the existence of ground currents and their harmonics. To prevent this from occurring, the Fluke 1625 uses an Automatic Frequency Control (AFC) System. This automatically selects the testing frequency with the least amount of noise enabling you to get a clear reading.

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Heritage Geophysics offers a broad line of magnetic susceptibility meters for use on rock outcrops, and also for field samples.

Click on the this link: http://www.heritagegeophysics.com, to continue reading about susceptibility meters, resistivity instruments, or ground magnetometer.

Resistivity Methods: 2D & 3D Electrical Imaging

epa.gov

Extract from “Resistivity methods”

Following on the 1D applications of resistivity imaging theory, comes the 2D and subsequently 3D applications. The 2D profiles take the above sounding techniques and integrate them into a 2D plane transecting the desired target area. The most common configuration of the 2D survey employs dipole-dipole electrode configurations. An example of the data aquisition geometry for a 2D profile is presented in figure 1.

Figure 1. Two dimensional measurement configuration for a dipole-dipole resistivity profile. Pseudosection plotting location indicated inred.

Figure 1. Two dimensional measurement configuration for a dipole-dipole resistivity profile. Pseudosection plotting location indicated inred.

Figure 1 shows a transmitting current dipole (I) followed by a series of potential dipoles (V) which measure the resulting voltage gradient at each station along the line. Subsequent measurements are completed by sequentially moving the current dipole down the line. However, alternative resistivity measurements can be made using towed surface or marine arrays, which would maintain the above configuration, and build up the 2D image by moving the entire measurement array for each series of measurements. In both cases the resulting image plots the apparent resistivity with depth, which is then contoured (commonly krigged) using a commercially available program. The color contoured image displays the distribution of apparent resistivity values and associated gradients within the area of interest. In order to convert the apparent resistivity data to true resistivity, the data are inverted. Figure 2 displays an example of a measured apparent resistivity pseudosection at the top, followed by a calculated apparent resistivity pseudosection, and resulting in the inverted true resistivity 2D section. The numbers presented at the bottom of the inverted section display goodness of fit criteria used to assess the accuracy of the calculated resistivity model. Finally note that the surface elevations have been included in the final model, accounting for variations in measurement geometry due to changing topography.

Figure 2. Examples of measured apparent resistivity, calculated apparent resistivity, and inverted resistivity sections.

Figure 2. Examples of measured apparent resistivity, calculated apparent resistivity, and inverted resistivity sections.

Figure 3 shows an example of an inverted 2D cross borehole ERT data set.

Figure 3. An example of an inverted cross borehole ERT data set .

Figure 3. An example of an inverted cross borehole ERT data set .

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On Heritage Geophysics you will find 2D and 3D resistivity instruments. Visit us today!

Click on the this link: http://www.heritagegeophysics.com, to continue reading about resistivity instruments, resistivity meters, or resistivity equipment.

Earth Resistivity Meter

by John Stanley | geotech1.com

Abstract

An earth resistivity meter can be used to identify the composition of various earth strata and the depth at which each strata occurs and by detecting changes in earth composition, to point to the existence of buried objects.

An earth resistivity meter may be used to locate archaeological objects to assist in finding conditions favourable for alluvial gold or gemstones, or even for such prosaic duties as determining where to locate a septic tank!

These instruments are not expensive compared with most electronic instrumentation. Nevertheless at $1000 or so they are way above the budget of most amateur archaeologists or rock-hounds.

But for such people all is not lost — it is possible to construct a simple dc operated resistivity meter for a mere fraction of the price of commercial units.

For this to be possible we have to accept a few operating limitations — primarily of operating depth — for whereas a commercial unit may be used to depths of 100-200 meters, our unit is limited to 15 meters or so. But unless you are hoping to locate oil bearing deposits in your backyard the limitation on operating depth should not be a problem.

The basic instrument is extremely simple — four equally spaced electrodes are placed in line in the earth. An accurately known current is caused to flow from one outer electrode to the other and a measurement is taken of the voltage between the two inner electrodes.

Having measured both voltage and current, a simple formula is used to establish depth and composition of the strata.

Professional earth resistivity meters use alternating current across the earth electrodes in order to eliminate the effects of the small galvanic voltages caused by the earth.

This effect cannot be totally eliminated with dc instruments but it can be minimized by switching the battery across the electrodes in alternate polarities — a center position of the switch (SW2) meanwhile short-circuits the two center electrodes between readings to discharge the galvanic potential.

Another common application of the resistivity meter is in searching for buried objects such as large water mains, buried stream beds or underground sewerage tunnels. The method used is simply to decide approximately at what depth the object is likely to be found, and divide the distance by 0.6 to give a suitable electrode separation. Maintaining this same separation, the array of all four electrodes should be progressively moved in a line over the ground being explored. Readings of resistivity should be made at each point and the value plotted against distance moved. The distance between each reading point should be no greater than half the dimension of the object to be located; in fact the closer the readings are taken, the greater will be the resolution.

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Visit our website: http://www.heritagegeophysics.com, for further information about resistivity meters, resistivity instruments, or other resistivity equipment.

StrataVisor NZXP Exploration Seismograph

geomatrix.co.uk

The StrataVisor NZXP is a high-performance exploration seismic system in a compact, weatherproof chassis. The NZXP can operate as a field PC, as a stand-alone seismic recorder with 3 to 64 internal channels. The NZXP expands easily to larger channel systems by connecting other NZ seismographs or lightweight Geode modules. This flexibility lets you collect data for all applications in all environments – you can even rent extra channels when needed.

To order your StrataVisor NZ today, visit our website: heritagegeophysics.com.

To order your StrataVisor NZ today, visit our website: heritagegeophysics.com.

Examine your data at all phases of acquisition to ensure data quality. Customizable windows show real-time noise monitor, amplitude spectra and seismic traces so you see problems instantly. A log file keeps track of all parameter changes and customizable alarms alert you to critical issues. You can even do preliminary processing in the field with industry-leading reflection, refraction and tomography software included with every system.

The StrataVisor NZXP console includes a brilliant daylightvisible color screen, waterproof keypad and built-in printer. Low-power circuitry and a standby mode extend battery life and reduce weight. The StrataVisor NZ is backed by a 2 year parts and labor warranty. All this from a company with factory trained service centers world wide and over 35 years of superior support to geoscience professionals.

Features:

  • Get the best data: professional, ruggedized 24 bit seismic recorder suitable for all seismic surveys: reflection, refraction, downhole, VSP, marine or monitoring.
  • Flexible configurations: houses from 3 to 64 channels. Expands seamlessly up to 1000 channels by connecting Geode in-field distributed modules or connecting other NZs.
  • Work in all conditions: Military-grade CPU, shock-mounted chassis, reliable in harsh environments, operates in extreme temperatures, humidity and dust. Exceeds MIL 810E vibration spec.
  • Widest bandwidth: 20 kHz bandwidth (0.02 µs to 16 ms sampling) for ultra-high resolution engineering surveys or recording low frequencies for earthquake monitoring.
  • Field friendly: brilliant full-sun-visible color screen and built-in plotter – available also as rugged field computer without seismic channels.
  • Put your client at ease: built-in geophone and line testing, full waveform noise monitor. Optional automated internal in-field instrument testing and enhanced geophone and line diagnostics.
  • Use any source: Sub-sample trigger ensures accurate stacking; hardware correlator eliminates delays doing Vibroseis or pseudo-random Mini-Sosie sources.
  • Software available for ALL applications:
    • Reflection (includes processing software)
    • Refraction (includes processing and analysis)
    • Downhole, crosshole, VSP
    • Event triggering for earthquakes, microseismic, blast monitoring and surveillance
    • Marine surveys
    • Continuous recording, GPS synchronization
    • Vibroseis, pseudo-random sources
    • Passive and active surface-wave surveys

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See a lot more about seismographs, seismometers, or resistivity equipment, by visit our website: http://www.heritagegeophysics.com.

How to Select Magnetic Field Instruments

globalspec.com

What are Magnetic Field Instruments?

Magnetic field instruments are devices used to measure the magnetic field or flux around permanent magnets, coils, and electrical devices. They include meters, gauges, sensors, recorders, and other instrumentation.

Selection

Selecting a specific magnetic field instrument depends upon the type of device needed, the technology it implements, its form, its outputs and interfaces, and various performance specifications.

Device Type

Magnetic field instruments include magnetometers and Gaussmeters (Tesla meters). Magnetometers and Gaussmeters are sometimes used interchangeably for describing devices used to measure magnetic field strength. However, the two can be differentiated based on the type of field strength they measure. Gaussmeters are considered devices for high field strengths while magnetometers are used for low field strengths. Specifically, Gaussmeters are said to take magnetic field measurements above 1 mT (milliTesla), while devices measuring fields below this value are considered magnetometers.

Instrument technology

Magnetic field instruments include several types of sensing technologies. Depending on the range of sensitivities the devices can be designed for, they can be considered Gaussmeters, magnetometers, or both.

Gaussmeters

    • Hall Effect devices convert the energy stored in a magnetic field to an electrical signal by developing a voltage between the two edges of a current-carrying conductor whose faces are perpendicular to a magnetic field.
      • Magnetodiodes are two-terminal Hall effect devices similar to a conventional bipolar diode. The voltage-current characteristic of a magnetodiode is sensitive to a magnetic field.
      • Magnetotransistors consist of a bipolar transistor implemented on a semiconductor surface. They are three-pronged devices consisting of an emitter region, an elongated base region, and a collector region. The presence of a magnetic field in the base region creates a Hall effect voltage which produces a pulse on the transmission line.

Magnetometers

Magnetometers are magnetic field instruments for high-sensitivity applications detecting low-strength fields. They can be classified as vector or scalar devices based on their ability to sense field direction in addition to field strength.

    • Scalar: These magnetometers measure magnitude only.
      • Proton precession devices use liquids such as kerosene and methanol that have high densities of hydrogen atoms.
      • Optically-pumped instruments polarize a gaseous alkali with a specific wavelength of light. An RF signal is modulated to determine its optimum depolarization frequency – this depolarization frequency varies with the ambient magnetic field.
      • Overhauser or nuclear precession devices combine an electron-rich liquid with hydrogen and then subject the mixture to a radio frequency (RF) signal.
    • Vector: There magnetometers measure both magnitude and direction.
      • SQUIDs or superconducting quantum interference devices consists of two superconductors separated by thin insulating layers to form two parallel Josephson junctions. They are most commonly used to measure the magnetic fields produced by brain or heart activity.
      • Atomic SERF magnetometers achieve very high magnetic field sensitivity by monitoring a high density vapor of alkali metal atoms precessing in a near-zero magnetic field. They are among the most sensitive magnetic field sensors available.
      • Flux gate or coil instruments measure differences in the magnetic field at the ends of a vertical rod and plot this information on a grid.
      • Magnetoinductive devices consist of a coil that surrounds a ferromagnetic core whose permeability changes within the earth’s magnetic field.

All inclusive technologies

  • Magnetoresistive instruments measure electrical resistance as a function of the applied or ambient magnetic field. They can be built as magnetometers for more sensitive applications, or as Gaussmeters for stronger magnetic fields.

Form

Magnetic field instruments can either be in handheld or mounted form. For field applications and those requiring portability, handheld form may be necessary. Mounted forms are usually bigger devices incorporated into a larger transportable unit or vehicle, or are used in fixed lab or building environments.

Outputs and interfaces

It is important for a magnetic field instrument to have outputs and interfaces that are usable for the operator and compatible with other incorporated systems. Magnetic field instruments differ in terms of electrical outputs. Analog current levels such as 4 – 20 mA are suitable for sending signals over long distances. Analog voltages are simple, usually linear functions. Modulated analog output signals are encoded, but still analog in nature. Examples include sine wave, pulse wave, amplitude modulation (AM), and frequency modulation (FM) signals. Several digital outputs are available. RS232, RS422, and RS485 are common serial, digital protocols. Popular parallel protocols include the general-purpose interface bus (GPIB), a standard which is also known as IEEE 488. Other digital outputs for magnetic field instruments include transistor-transistor logic (TTL) signals. Outputs that change the state of a switch or alarm are also available.

Performance specifications

Magnetic field instruments can be selected based on a number of different specifications related to device performance.

  • Flux density measurement is the range through which the sensor or instrument is designed to measure, often corresponding to the linear output region of the sensing technology.
  • Sensing accuracy is the required measuring accuracy of the device.
  • Resolution is the smallest increment of measurement possible with the device. An instrument with higher resolution can make smaller measurements.
  • Bandwidth is the frequency range over which the device meets its accuracy specifications. Accuracy degrades with lower frequencies unless the device is capable of dc response. Accuracy also degrades near and above resonance frequencies, where its output response rolls off.
  • Operating temperature is the temperature range over which the device must operate.

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Are you interested in getting a magnetometer, resistivity meter, or other resistivity equipment? To continue reading about these subjects, visit our website: http://www.heritagegeophysics.com.

Introduction to Borehole Geophysics

ny.water.usgs.gov

Borehole geophysics is the science of recording and analyzing measurements of physical properties made in wells or test holes. Probes that measure different properties are lowered into the borehole to collect continuous or point data that is graphically displayed as a geophysical log. Multiple logs typically are collected to take advantage of their synergistic nature–much more can be learned by the analysis of a suite of logs as a group than by the analysis of the same logs individually. Borehole geophysics is used in ground-water and environmental investigations to obtain information on well construction, rock lithology and fractures, permeability and porosity, and water quality. The geophysical logging system consists of probes, cable and drawworks, power and processing modules, and data recording units. State-of-the-art logging systems are controlled by a computer and can collect multiple logs with one pass of the probe.

Why log?

Borehole-geophysical logging can provide a wealth of information that is critical in gaining a better understanding of subsurface conditions needed for ground-water and environmental studies. Geophysical logs provide unbiased continuous and in-situ data and generally sample a larger volume than drilling samples.

  • Delineation of hydrogeologic units

The different hydrogeologic units found in the subsurface display a wide range of capabilities to store and transmit ground water and contaminants. Borehole-geophysical logging provides a highly efficient means to determine the character and thickness of the different geologic materials penetrated by wells and test holes. This information is essential for proper placement of casing and screens in water-supply wells and for characterizing and remediating ground-water contamination.

  • Definition of ground-water quality

The quality of ground water is highly variable and ground-water contamination may be caused by man-made or natural sources. Integration of borehole-geophysics logging with water-quality sampling provides a more complete picture, whether the objective is to develop a water-supply well or remediate a contaminated aquifer.

  • Determination of well construction and conditions

Wells are the access points to the ground-water system, and knowledge of their construction and condition are important whether they are being used for ground-water supply, monitoring, or remediation. The location and condition of casing and screen can be rapidly evaluated with geophysical logging.

What are the common geophysical logs and how are they used?

Common geophysical logs include caliper, gamma, single-point resistance, spontaneous potential, normal resistivity, electromagnetic induction, fluid resistivity, temperature, flowmeter, television, and acoustic televiewer.

  • Caliper logs record borehole diameter. Changes in borehole diameter are related to well construction, such as casing or drilling-bit size, and to fracturing or caving along the borehole wall. Because borehole diameter commonly affects log response, the caliper log is useful in the analysis of other geophysical logs, including interpretation of flowmeter logs.
  • Gamma logs record the amount of natural gamma radiation emitted by the rocks surrounding the borehole. The most significant naturally occurring sources of gamma radiation are potassium-40 and daughter products of the uranium- and thorium-decay series. Clay- and shale-bearing rocks commonly emit relatively high gamma radiation because they include weathering products of potassium feldspar and mica and tend to concentrate uranium and thorium by ion absorption and exchange.
  • Single-point resistance logs record the electrical resistance from points within the borehole to an electrical ground at land surface. In general, resistance increases with increasing grain size and decreases with increasing borehole diameter, fracture density, and dissolved-solids concentration of the water. Single-point resistance logs are useful in the determination of lithology, water quality, and location of fracture zones.
  • Spontaneous-potential logs record potentials or voltages developed between the borehole fluid and the surrounding rock and fluids. Spontaneous-potential logs can be used in the determination of lithology and water quality. Collection of spontaneous-potential logs is limited to water- or mud-filled open holes.
  • Normal-resistivity logs record the electrical resistivity of the borehole environment and surrounding rocks and water as measured by variably spaced potential electrodes on the logging probe. Typical spacing for potential electrodes are 16 inches for short-normal resistivity and 64 inches for long-normal resistivity. Normal-resistivity logs are affected by bed thickness, borehole diameter, and borehole fluid and can only be collected in water- or mud-filled open holes.
  • Electromagnetic-induction logs record the electrical conductivity or resistivity of the rocks and water surrounding the borehole. Electrical conductivity and resistivity are affected by the porosity, permeability, and clay content of the rocks and by the dissolved-solids concentration of the water within the rocks. The electromagnetic-induction probe is designed to maximize vertical resolution and depth of investigation and to minimize the effects of the borehole fluid.
  • Fluid-resistivity logs record the electric resistivity of water in the borehole. Changes in fluid resistivity reflect differences in dissolved-solids concentration of water. Fluid-resistivity logs are useful for delineating water-bearing zones and identifying vertical flow in the borehole.
  • Temperature logs record the water temperature in the borehole. Temperature logs are useful for delineating water-bearing zones and identifying vertical flow in the borehole between zones of differing hydraulic head penetrated by wells. Borehole flow between zones is indicated by temperature gradients that are less than the regional geothermal gradient, which is about 1 degree Fahrenheit per 100 feet of depth.
  • Flowmeter logs record the direction and rate of vertical flow in the borehole. Borehole-flow rates can be calculated from downhole-velocity measurements and borehole diameter recorded by the caliper log. Flowmeter logs can be collected under non-pumping and(or) pumping conditions. Impeller flowmeters are the most widely used but they generally cannot resolve velocities of less than 5 ft/min. Heat-pulse and electromagnetic flowmeters can resolve velocities of less than 0.1 ft/min.
  • Television logs record a color optical image of the borehole. In addition to being recorded on video-cassette-recorder tape, the optical image can be viewed in real time on a television monitor. Well construction, lithology and fractures, water level, cascading water from above the water level, and changes in borehole water quality (chemical precipitates, suspended particles, and gas) can be viewed directly with the camera.
  • Acoustic-televiewer logs record a magnetically oriented, photographic image of the acoustic reflectivity of the borehole wall. Televiewer logs indicate the location and strike and dip of fractures and lithologic contacts. Collection of televiewer logs is limited to water- or mud-filled open holes.

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