Golden Engineering X-ray generators are based on Pulsed X-ray technology. Pulsed X-rays generate a high intensity X-ray burst (pulse) in a very short period of time (10 to 50 nanoseconds depending on the model). The output dose of each pulse is 3-6 mR measured 12 inches from the front of the X-ray generator. The operator varies the overall dose of each exposure by changing the pulse setting. The pulse rate varies from 10 pulses per second to 25 pulses per second depending on the model. The generators can fire up to 200 pulses before a four-minute rest period.
Pulsed X-ray technology has inherent advantages for field operation compared to conventional constant potential X-ray machines or radioactive sources.
Performance and Packaging: Pulsed technology generates high output voltage using minimal input voltage. The result is extremely small single package generators with significant penetrating capability.
Safety and Simplicity: Minimal side and rear X-ray leakage means the operator safety stand-off distance is 20 feet (6 m) behind the unit. The generators contain no radioactive material. Radiation is only emitted from the generator while it is pulsing. The single package design eliminates the need for connecting cables between tube head, control module, and power supply. The Golden Engineering cold cathode tube requires no warm up and one input variable (pulse setting) simplifies the operating process.
Compatibility: Golden Engineering pulsed generators are compatible with most digital imaging systems. Conventional radiographic film requires more X-ray dose than digital imaging systems and may not be a viable option for light duty, pulsed X-ray generators. Contact us for more information.
The moisture and humidity testing system for floor covering related industries has been developed to help users avoid moisture related problems in flooring and to identify the cause of problems if they do occur.
The system involves testing the subfloor using both the Tramex CME non-destructive test and in situ Relative Humidity Hygro-i probes. The ambient conditions of the building are also measured and the moisture condition of many floor coverings can be checked.
ASTM F2659: Non-destructive testing
The CME instant test for concrete is completely non-destructive and specified by many floor-covering manufacturers around the world, in part because of its simplicity and the repeatability of the results.
The ease and speed of the CMEX allows for many tests, conforming to ASTM F2659, to be carried out over a large area in a short amount of time. Simply turn on the meter and push it onto the surface being tested and read the results on the clear display. The results are calibrated to show percentage moisture content by weight.
ASTM F2659 calls for 3 to 5 readings in the same location. If there are any variations in those readings, record the highest result.
This is repeated in at least 8 locations for the first 1000sq.ft. and 5 for every 1000sq.ft. thereafter, allowing the user to build up a moisture map of the entire slab and identify when and where to test further if testing, for example, to ASTM F2170.
ASTM F2170: Hygro-i in situ testing
The reusable Hygro-i relative humidity probe has been designed specifically for measuring the potential moisture condition of a slab or screed as per ASTM F2170.
To perform this in-situ type relative humidity test, simply drill a hole in the concrete 40% of the thickness of the slab.
Push the sleeve into the hole, insert the Hygro-i probe into the sleeve and cover with the cap. Leave for the standard test period. When taking readings, plug the interface into the Hygro-i probe and instantly read the relative humidity, temperature and dew point simultaneously on the clear display. When testing is complete the Hygro-i probes can be removed with the extraction tool and used repeatedly. The user must be mindful of all the proper safety precautions as mentioned in the ASTM standard.
Ambient Conditions: Hygro-i testing
In conjunction with the Hygro-i relative humidity probe, the CMEX also functions as a digital hygrometer. The meter displays the relative humidity, temperature, dew point and mixing ratio, allowing the user to assess the environmental ambient conditions within the building, making sure they are suitable for installing a floor covering. Using an Infrared Surface Thermometer in combination with the relative humidity results it becomes easy to identify condensation problems.
Moisture Content of Wood: Pin-type wood probe
The CMEX can also be used with the hand held pin probe, for measuring moisture in wood. This is especially useful when installing wood floor coverings.
By comparing the moisture content of wood with the ambient relative humidity conditions we can determine when the wood is in balance with the ambient conditions within the building.
Another way of identifying when wood is in balance with the building, is to discreetly take moisture content readings from wood somewhere already in service within the building. By taking this in-service moisture content reading it becomes more predictable if the wood floor covering is likely to move, to shrink or expand, or if it is in balance.
Excessive moisture in concrete floor slabs and screeds or a disequilibrium between moisture content in the building and the flooring materials can lead to major problems in many types of floor coverings.
From bomb detection through Special Forces to counter surveillance and customs, the security sector is facing ever growing challenges. Throughout the years, radiography has been a common imaging method for inspecting suspicious articles and explosive devices. For a long while, X-ray film was the most common (and practically the only) recording medium. The Digital Age brought about radical changes, and use of Digital Radiography (DR) expanded, while rapidly replacing conventional radiography methods.
Digital Radiography uses X-ray digital detectors instead of traditional film or Phosphor Plates (also known as Computed Radiography or CR). DR yields immediate and superior quality X-ray images at minimum time on target, with minimal radiation levels.
Digital Radiography vs. Film
Much like in a camera, using traditional film in radiography is time consuming and environmentally harmful. Film needs to be chemically developed, and is very limited in terms of image analysis and sharing with others.
Instead of film, DR uses a digital image capture device. Utilizing a wide dynamic range and high resolution, an immediate high quality image is generated. The retrieved image is displayed on a tablet and can be processed, enhanced, shared and digitally stored and accessed, all within a matter of seconds.
These attributes are particularly beneficial for the security industry as they:
Digital Radiography vs. Computed Radiography (CR)
CR makes use of phosphor crystals plates as a recording medium. The X-ray is absorbed and the exposed plate is then scanned with laser. The emitted light captured is converted into a digitized digital image.
Image readout must commence promptly as the amount of energy stored rapidly declines - the recorded image can substantially degrade during processing. Readout process for a single image takes about a minute and requires a dedicated bulky scanner.
With its unique penetration and detection capabilities, DR maximizes speed, safety, quality and overall performance, while making CR pale in comparison:
Emlid RTK GNSS receivers were recently used to carry out training and conduct UAV-based research in the Democratic Republic of the Congo (DRC). Drone mapping is no novelty for the DRC, but up to now most of the drone flying jobs are done by foreign contractors. Therefore the research groups from the Catholic University of Bukavu (UCB), Volcanic observatory of Goma (OVG) and the University of Lubumbashi (UNILU) saw the essential need for the region to have skillful people able to use drones for topographic mapping and land surveying.
With the assistance of the geography department of the UCLouvain (Belgium), the team went for support to ARES. Together, they launched a project aiming at creating a local center of excellence in UAV-based research in the DRC. The goals were to provide advanced training and to locally install a well-equipped UAV-laboratory.
The Congolese researchers used Emlid products to collect centimeter-accurate 3D positional data to study soil erosion processes in Bukavu and to quantify termite mound volumes in Lubumbashi.
Using a Reach M+ RTK module and a Reach RS+ allowed to carry out efficient training and collect precise and reliable data for the research. A pair of Reach RS+ was deployed to survey ground control points, and a Reach M+ was integrated on a drone for PPK mapping.
Interested? Contact us to give you more information about the GPS solutions we are offering.
The new FeedBack DataLogger from Tramex can help identify humidity issues, saving you from possible floor failures with anhydrite screeds. When it comes to talking about drying screeds and concrete floors in general, there seems to be an elephant in every room: ambient humidity.
Although it’s mentioned in every datasheet, handbook and national standard, ambient humidity seems to be overlooked or misunderstood by many architects, builders and flooring installers. As a rule-of-thumb, concrete slabs are expected to dry at a rate of 1mm per day (or an inch per month) and anhydrite screeds the same up to 40mm, or two days per mm when poured deeper (ie a 60mm screed will take: 40mm @ 1 day = 40 days + 20mm @ 2 days = 40 days which = 80 days in total).
‘Ideal conditions’, as stated by screed manufacturers in their guidance literature, are usually agreed on as being in the region of 20deg C and 40-60% RH. These are the optimum conditions to allow the moisture within the slab or screed to evaporate from the surface. The rate of evaporation will depend on ambient conditions. Warm, dry, flowing air will allow for faster evaporation. These conditions, while ideal, are obviously not the normal state of a building site in the UK for most of the year, except the three short months of summer, at which time doors and windows should be thrown wide open to create a good flow of dry air, lifting moisture from the material and carrying it away.
However, for the rest of the year, when temperatures are closer to 5deg C and humidity upwards of 70-80% RH, leaving doors and windows open will have the opposite effect, instead introducing more humidity into the environment and slowing the drying further. Then, with the addition of wet trades applying plaster to the walls, humidity in the air is raised even higher. At most times of the year, heaters and dehumidifiers are needed to artificially create those ‘ideal conditions’.
Building sites which aren’t artificially conditioned will maintain a high humidity level and, when temperatures drop (overnight for example), can easily reach dew point, resulting in condensation settling on the surface of the floor, thus wetting and re-wetting the screed. An obvious solution in this instance, and one which seems to be the go-to quick fix in the UK today, is the use of a damp-proof membrane (DPM). This will slow the rate of drying of the floor to a level which isn’t harmful to the floorcovering.
DPMs can be ideal for this scenario (a sort of get out of jail card) and also in the situation where an older floor which was installed many years before, is still showing high levels of moisture. Moisture in a slab or screed should continue to dry slowly over many years even with floorcoverings installed and so an older floor shouldn’t be expected to read as high as a new floor which is emitting its construction moisture.
If a reading which would be regarded as normal for a new floor, is found in an older floor, it could be an indication that there’s a breach in the damp-proof course (DPC) or even that one was never installed. Again, this can be an ideal situation for a DPM which will ensure moisture, intruding from below the slab, isn’t going to cause a failure. (Be sure, in this case, to select a DPM which is suitable for residual construction moisture as well as groundwater vapor).
Once a DPM is installed, however, it becomes even more important to monitor the ambient conditions on site leading up to the floor cover installation. This is because when the condensation point is reached, in normal circumstances as described above, most of the condensation is absorbed into the surface of the screed, whereas with a DPM in place, this condensation will sit on the surface with nowhere to go. This means even a small trace of moisture can cause problems for the adhesive. This consequence of the use of a DPM is often overlooked.
The Tramex Feedback datalogger, for example, is a suitable tool for monitoring these ambient conditions over the course of the drying stage of the floor and right up to and during the installation. Readings of ambient temperature, humidity and dew-point are recorded by the device and read from a smartphone or tablet using the Tramex Feedback app. Anhydrite screeds are sensitive to high ambient humidity conditions and readily absorb moisture from the air, slowing or blocking the drying completely. Removal of the laitance from the surface of the screed after the initial curing will allow the surface to release its moisture, whereas not sanding/abrading the surface will normally result in the laitance hardening and making it significantly more difficult to remove at a later stage.
While some anhydrite screed installers will return to site after the initial curing period and remove the laitance as part of their service and will hand a copy of the instructions over to the contractor, ensuring everyone is aware what type of screed it is and how to treat it, these highly professional screeders are the exception and unfortunately not the rule. The more common scenario sees the contractor arriving to site with no idea that this is an anhydrite screed and therefore how to treat it.
Knowing the screed is anhydrite will have important ramifications on several aspects, including choosing which type of DPM to use. DPMs designed for concrete and sand/cement screeds are usually not suitable for use with anhydrite screeds. Manufacturers are now producing DPM products for use specifically with anhydrite; however, most cannot be used when underfloor heating (UFH) systems are installed. Moisture testing of anhydrite screeds is another issue which causes confusion for contractors as these screeds do behave differently to concrete and sand/cement screeds.
The three main tests in use in the UK are the British Standard humidity box, non-destructive electronic moisture meters and the German (DIN) standard carbide method (bomb test or speedy test). The British Standard humidity box measures the ERH or equilibrium relative humidity of the screed. This is performed by affixing a specially designed sealed box to the surface of the screed with butyl tape (which, unlike silicone, doesn’t affect the RH readings) in a location of the floor which reads highest with a preliminary electronic concrete moisture meter test.
Ensure the box is out of the way of direct sunlight or drafts from open doorways. Equilibrium is achieved when the trapped air inside the box is no longer receiving humidity from or giving it to the screed. At this point a measurement should be taken. 75% RH or below is a commonly acceptable result. The length of time required for the airspace inside the box to achieve equilibrium with the slab or screed depends on its thickness and whether a screed is bonded to the sub-floor or not. In the case of anhydrite screeds, which are usually poured to a depth of between 40-60mm and are normally placed over insulation, British Standards recommend a first reading is taken after four hours, and equilibrium may be assumed when two consecutive readings taken at four-hour intervals show no change.
In practice this test method is often reported to be performed unsatisfactorily. For example, many testers will leave the box in place for too short a time and it’s rare to hear of anyone checking readings twice at four-hour intervals. The reason for taking subsequent readings four hours apart is to ensure the recorded reading is taken during a period of equilibrium. A small change in ambient temperature will have a dramatic effect on the readings, destabilizing the fine balance of equilibrium inside the box and sending the RH reading up or down depending on the temperature change.
This fluctuation in temperature can result in an unpredictable spike in RH (eg from between 72-82% as in figure 1) as the equilibrium is upset. Stability will only resume about three to four hours later. For this reason, users of this test will often take a reading in the morning which reads high and possibly another in the afternoon which reads low and wonder which is correct, causing further uncertainty. But this uncertainty can be overcome.
Verification of the humidity box test results is easier when used together with a datalogger such as the Tramex Feedback. The external probe is placed into the box and monitors the temperature and humidity readings for the entire duration of the test, thus showing clearly when equilibrium was achieved and what the correct reading was at the appropriate time.
Electronic moisture meter testing is non-destructive and provides instant readings. A purpose-built concrete moisture meter such as the Tramex CME4 and CMEX2 provides more helpful readings (of moisture by mass in concrete and cementitious screeds) than a general purpose, comparative moisture meter.
This method of testing allows the user to map a whole area, very quickly assessing the moisture condition and locating the highest reading points for further testing with more elaborate, time-consuming methods (such as the humidity box test already described) when such methods are required. When testing anhydrite screeds with an electronic moisture meter it’s essential the laitance has been removed from the surface of the screed to gain a meaningful reading. The laitance acts as a barrier or skin, trapping moisture at the surface of the screed, therefore producing a false positive reading on the instrument which is designed to take a correct measurement based on the drying curve of the slab/screed in normal drying conditions.
For the same reason it’s important ambient humidity conditions are within the normal range of between 40-60% to avoid condensation which can also lead to false positive readings. The CM test (known as the bomb test or speedy test in the UK) is the German national standard test and is required as a final certification of moisture conditions of slabs/screeds in many European states. The CM test involves removing a sample of the slab/screed with a hammer and chisel and crushing it using a mortar and pestle, then weighing the required amount and placing into an airtight chamber together with calcium carbide which, when in contact with moisture, produces acetylene gas.
The higher the concentration of moisture the more gas is produced which is read as pressure from the devices gauge. This test is ideal for certain proprietary and fast drying screeds which act by chemically binding most construction moisture and therefore cannot be tested with relative humidity or electrical impedance devices which will give high results. In theory the CM test is the most suitable for anhydrite screeds owing to the chemical nature of the test, showing only ‘free’ moisture which can cause floor failure. In practice, however, the test is easy to get wrong and requires a good deal of knowledge and skill to get exactly right.
The project involves a DT-X SUB echosounder currently being deployed in a yearlong project to assess biomass (fish and plankton) in Papua New Guinea. The system is configured with 38 and 200 kHz transducers and operating in a fully autonomous mode. The DT-X-SUB is powered by (3) Ocean Sonics battery packs and will operate on a 15% duty cycle to extend battery life and allow for 3-month deployments. The hardware is mounted in a custom mooring frame deployed in 400 m of water. The DT-X SUB system will measure demersal and pelagic fish biomass and the diel migration of plankton over the course of this 12-month study.
BioSonics DT-X SUB is a programmable, autonomous scientific echosounder often used where tether cables to the surface are impractical. The DT-X SUB is ideal for deployments on moorings, ROV/AUVs, towed vehicles, and/or seafloor observatories. Echosounder duty cycle can be programmed to extend battery life for long-term studies. A single DT-X SUB echosounder can operate up to nine split beam transducers of various frequencies.
The DT-X SUB allows for long-term data collection in previously inaccessible, deep water environments, and studies of temporal distribution and marine organism behavioral patterns. Learn more about the DT-X SUB. Ocean Sonics Battery Packs are easy to handle and service. Each pack uses 72 standard D-cell units, with options for Alkaline or Lithium. The packs are available in two depth ratings; 200m and 3500m rated with titanium endcap and bottom. Lightweight glass fiber composite case eliminates cathodic issues found with metal cans. The endcap connector guard doubles as a lever making the release of the endcap a low-effort and safe procedure and the levers act as carrying handles for easy transport. Learn more about Ocean Sonics Battery Packs.
If you would like to learn more about BioSonics integrated solutions for submersible, autonomous echosounder deployments, drop us a line. We'd love to hear from you!
The extraction of underground oil or gas usually also generates large amounts of water, called 'produced water'. If not properly managed, it can cause local soil and water pollution.
In this case study, a GEM-2 handheld conductivity meter was used to quickly identify problem areas. In this case, the conductivity variations were large enough that the operator could explore the areas of greatest interest (as constrained by the local terrain) rather than following a fixed grid. This type of opportunistic approach can be very efficient. The whole survey area covered several square miles, but the immediately mapped area shown here required less than 2 man-days to survey.
This case study confirms that maps of apparent electrical conductivity (EC) are very useful in locating potential soil contamination. In the case of brine contamination, the EC values correlate well with laboratory analyses of both soil conductivity and concentration of chlorides. More generally, any type of contaminant whose EC contrasts with the environment can be delineated.
GSSI, the world’s leading manufacturer of ground penetrating radar (GPR) equipment, announces the release of a major software update for the StructureScan™ Mini XT – the newest generation of GSSI’s popular all-in-one concrete inspection GPR system. The update expands StructureScan™ Mini XT capabilities with an increased depth range, improved Focus Mode, and a new Auto Drill feature.
The update increases StructureScan™ Mini XT’s depth range by 20% to up to 24 inches for greater visibility in survey situations involving thick structural concrete and slab on grade. Additionally, algorithm improvements enhance the StructureScan™ Mini XT’s gain at greater depths.
The improved Focus Mode uses input from the StructureScan™ Mini XT’s 2.7 GHz high-resolution antenna to resolve closely spaced and bundled targets within concrete, offering precise visualization where traditional GPR hyperbolas would condense data into a singular dot. Users can sweep between raw GPR data and the easy-to-read focused view. The new Auto Drill feature searches for potential obstructions to a planned core location. The innovative software tool uses a specialized algorithm to identify possible obstacles to drilling operations by analyzing a user-selected position and size (1/2’ to 6”) on a 3D grid.
What makes GPR such a good tool to investigate archaeological sites?
Before geophysical methods arrived on the scene, archeologists had enjoyed centuries of success using excavation and shovel test grids to narrow down the most likely areas in which to dig. This time-tested site surveying method involves laying out a grid and excavating a unit, typically 50 centimeters by 50 centimeters. Archaeologists sift through the material to determine whether artifacts are present, and if so, from which layers. They then move on to the next unit, which may be 10 to 20 meters away.
However, this method has one major problem – if 20 percent of shovel tests contain artifacts using 10-meter spacing, there is a great likelihood that researchers can jump right over a discrete feature. It takes time and effort to excavate shovel tests on the scale necessary to accurately narrow down artifact locations. This process is labor and time intensive – and it results in a high potential to miss something.
Take the example of early sites from the 1600s, which may not contain a lot of cultural material – just a few bits of pottery, pipe stems, or nails. It is very likely that you may not recover cultural material from a shovel test even if you are right in the middle of the site.
In recent decades it has become clear that GPR and other geophysical technologies could really help with surveying sensitive archeological sites remotely and non-destructively. Targeting what to excavate saves time, money and protects fragile artifacts. GPR can also aid investigations comparing the site’s natural soils with archeological components.
GPR and other geophysical technologies are not generally used as “first phase” methods; rather, they are used when other information is needed to help refine the site, usually after an initial shovel test finds artifacts that point toward something interesting. They may also be used after plowing an agricultural field turns up artifacts, or if researchers have a detailed historical map that suggests a house or farm used to be at a site.
Such methods can pinpoint the best places to excavate and indicate which areas should be avoided. This is especially useful for large multi-acre areas, where GPR can be used to build a high-resolution map of what the site might have looked at when it was occupied. Surveying a few acres in high resolution could help locate all the roads in a farm complex, as well as all the paths, activity areas (blacksmith shop, yards), and even individual buildings.
On a smaller scale – say a researcher finds a house and a well – the GPR can be used to produce a more localized survey across discrete features to get a better idea of their size and depth, and to determine if the walls are intact and if the cellar hole is filled with rubble or clean material.
What are the geophysical methods archaeologists use in the field?
Archaeologists use several geophysical methods, including GPR, electrical resistivity imaging (ERI), magnetometry, and electromagnetic induction (EM or EMI).
GPR works by sending a tiny pulse of energy into a material via an antenna. An integrated computer records the strength and time required for the return of any reflected signals. Subsurface variations create reflections that are picked up by the system and stored on digital media. GPR is considered the most accurate, highest resolution geophysical technology. It works best in dry sandy soils with little salt content; the technique is not useful on the coasts where there is a high salt content, for example salt marsh. Dense clay-based soils are difficult to penetrate with GPR, it cannot see through metal and is also incapable of identifying bone.
ERI is used for mapping the depth of soils and rock. It involves placing stakes in the ground and measuring electrical resistance. Technicians must set up a row of about 24-48 sensors (metal stakes) along the ground typically in a straight line; information is only collected along that one line. This tool works well in clay soil, but takes longer and costs more to get the required data coverage than GPR. One can collect 80 or more profiles of similar length with GPR in the same time it takes to collect 2-4 profiles using ERI.
Magnetometers are passive sensors that measure the strength and sometimes the direction of a magnetic field. By detecting irregularities in the earth’s magnetic field, a magnetometer can indicate the location of items made of ferrous material. Archaeologists use them to measure human activity that increases magnetism. For example, old fire pits have higher magnetic readings, as do bricks, storage pits, and even old trenches. Magnetometers do a good job of finding ferrous objects, but do not provided accurate depth information like GPR.
Electromagnetic induction (EM or EMI) devices measure the change in mutual impedance between a pair of coils on or above the earth’s surface. Most EM instruments are comprised of two or more sets of coils, electrically connected and separated by a fixed distance. EM devices can simultaneously examine soil conditions and locate objects found beneath the surface of the earth spatially, but do not provide good depth information.
It is important to emphasize that these methods are often complementary, because each is better at measuring different things. For example, magnetometers are often paired with ER surveys. But here’s the vital point: Only GPR can provide true depth information that can be calibrated. Unlike other available geophysical methods, a GPR survey can indicate where an anomaly or archaeological feature is in high resolution spatially, enabling archaeologists to say how deep it is below the surface. That’s a huge advantage.
How to use GPR for surveying?
Before even starting to scan, it is absolutely critical to obtain as much information as possible about the site. GPR surveyors should seek out any historical maps and make sure they have access to the results of walkover surveys showing concentrations of archaeological features and artifact density. GPR surveyors should also have an idea of what researchers expect the GPR to show them so they can get a sense of what they should be looking for.
As part of this information gathering, researchers should pay close attention to what the landscape looks like. Is it at the side of a mountain where it may be difficult to access? Is it clear of vegetation or densely vegetated? GPR equipment needs to be pushed in a straight line and the antenna sits on the ground, so if a site is overly vegetated it must be cleared before conducting a GPR survey. Essentially, anything one would not want to go over with a lawn mower would also be difficult for GPR equipment.
Other factors surveyors should know is the time period being investigated, results from the initial archaeological investigation, and the density of archaeological features to be mapped. A pre-contact Native American site may contain mainly debris from making stone tools or food remains, so there may not be much to image. A historical 17th century farm complex might contain at least one building or a cellar hole, or perhaps a large underground feature that can help orient the site, like a barn, well, privy or farm lane.
What type of planning can GPR surveyors do to guarantee an efficient survey?
GPR surveys are a great tool to fill in gaps between shovel tests to ensure a complete picture of a site. Surveying budgets are always tight, so the key is to collect as much data as possible within the time and budget allocated.
Context is everything, so the right surveying parameters will always be based on the type of site and the findings of the initial archaeological investigation. Collection parameters will vary by the type of site and the density of features. Ideally, one should carefully consider line spacing parameters and direction of lines based on the specific site features.
GPR surveys should always be collected on grids. Surveyors should place a larger grid over a feature so they can determine what is happening near the feature or is associated with it. A recommended practice is to bracket the area with space buffers to collect more information. This can be difficult, since projects are often restricted spatially by property boundaries. Development projects affected by the National Preservation Act’s Cultural Resource Management (CRM) requirements are generally restricted to the area of potential effect; surveyors do not usually have permission to survey beyond the project boundaries. For academic projects, one should keep surveying to collect as much data as possible in the allotted time. A GPR survey provides a digital archive of the recording process; even if the site is damaged or destroyed, the digital archive will remain.
The GPR surveyor conducts the survey and tells researchers where there are anomalies. In an ideal world, the GPR surveyor would later get feedback about the anomalies, with information on what was eventually found. This would enable surveyors to go back in and re-examine the data, providing a better sense of what particular data findings mean.
House sites and cemeteries are common geophysical survey locations. What are special considerations about using GPR at these site types?
Early American house sites are very feature rich, with numerous underground targets. Researchers are typically looking for former extensions of a house that have since been demolished, as well as kitchen wings, foundation walls, and even gardens and pathways. As noted earlier, it is important to conduct background research before surveying, including deed research and use of the Historic American Buildings Survey (HABS) and Historic American Engineering Record (HAER) collections.
Geophysical surveys can help with investigation of cemeteries – both formal ones with standing stones and informal cemeteries, with unmarked graves or single burials. GPR can be used to image internment, but, Hollywood portrayals to the contrary, it is incapable of identifying bones. Use of GPR is also hampered by the fact that there may be variable states of preservation across a particular landscape, depending on soil types and topographical features. One part of a cemetery from 1750 could be remarkably preserved, whereas a grave from a different part of the cemetery might be completely decayed.
Another factor is that older coffins were wood or brick, which are difficult to image with GPR. The technique targets hyperbolic reflectors (an upside down U); in the absence of those reflectors GPR surveyors rely on vertical disturbances in the soil profile that come from digging, which shows up fairly well in GPR data.
The accompanying graphic shows two-dimensional GPR data taken from a cemetery site. The data represents six burials, approximately 10 nanoseconds in depth.
Whats the added value in processing software?
Regular GPR profiles are interesting, useful, and powerful for people who can read them – but they are also limited. It is difficult to see the shape of a feature by just seeing a cross section. GPR data does not show up on the screen labeled – surveyors must interpret what the features are. This is where post-processing software can play a role.
Similar to software used to process camera images, post-processing allows researchers to downplay some features and highlight others. Noise is inherent in digital GPR data, and the post-processing software enables users to reduce or remove noise to accentuate what they are looking for. The accompanying figure shows a three-dimensional GPR data image that identifies nine anomalies that could represent burials. The data was processed with RADAN post-processing software.
One excellent software-based technique is called time-slicing, in which all the individual lines of data collected are stitched together using the assigned coordinates into a three-dimensional cube of the survey area. Horizontal slices (also called time slices) can isolate specific depths to show the soil layers and review lateral relationships and actual feature shapes. Time slices help researchers really see the shape of a feature, like a circular well or building foundation, or a long linear pipeline. The slices add an immense amount of interpretative data and are often the best way to illustrate findings to the general public.
To use the tool to its best advantage, GPR surveyors should collect the right information before beginning a GPR survey, carefully plan data collection parameters to get the highest resolution data at the highest percent coverage for each specific site, and use software based post-processing tools, especially time-slicing.
Magnetotellurics (MT) refers to a technique in which electrical resistivity is determined by making measurements of electric and magnetic fields related to naturally occurring currents (“tellurics”, caused mostly by lightning strikes) flowing in the ground. Typical MT frequencies are from 0.0005 Hz to 1,000 Hz. The ratio of the amplitudes of the electric and magnetic fields is used to calculate the electrical resistivity of the ground at a depth determined by the ground resistivity and the frequency of the measured signal. Higher ground resistivity and lower frequencies allow greater depth of investigation. For traditional low-frequency MT, typical depth of investigation is up to 20 km or greater, but generally targets within the first 100 meters cannot be resolved.
Audio magnetotellurics (AMT) is similar to standard MT in that it uses naturally-occurring currents, but the frequency band is limited to the audio range, generally from 0.1 Hz to 8,000 Hz. Depth of investigation for AMT is typically from 30m to 2 km. Geometrics’ AMT instrument (Geode EM3D AMT) is designed to investigate this depth range, operating in the frequency band of 0.1 to 10,000 Hz.
Controlled-source audio magnetotellurics (CSAMT), in its most common variation, does not use naturally-occurring currents, but instead only uses a man-made transmitter generating currents in the frequency range of from 1 Hz to 10 kHz. Geometrics CSAMT’ instrument (Geode EM3D CSAMT) uses a controlled-source transmitter operating in the frequency band of 0.1 Hz to 10,000 Hz. Depth of investigation ranges from about 20 m to 2 km.
Geometrics’ Hybrid-Source AMT (HSAMT) instrument (Stratagem EH4) uses the natural field signals from 0.1 Hz to 100,000 Hz, but also uses a controlled-source transmitter to supplement the natural-field low frequencies for a depth of investigation of 5m to 2 km. The Geometrics hybrid source transmitter provides 15 separate frequencies ranging from 800 Hz to 70,000 Hz.
Common applications for AMT (Geode EM3D AMT) and HSAMT (Stratagem EH4):
Minerals and ground water exploration to 1,500m depth.
Deep engineering site characterization.
Considerations and Limitations for AMT and HSAMT:
Data quality for AMT and the low-frequency bands of HSAMT depend on the availability of natural field sources. Natural AMT signal availability depends on the season, time of day, and weather.
Contamination by 50 Hz or 60 Hz power sources such as power lines, industrial machinery, or urban settings negatively affect data quality.
Advantages of AMT and HSAMT over similar techniques:
AMT acquisition is faster than traditional MT. Acquisition for low-frequency MT data requires up to 12 hours on a single station. Collection of high-frequency AMT data at 10 Hz and above can be done in less than 15 minutes.
HSAMT transmitter setup is much faster and easier. A traditional grounded dipole CSAMT transmitter can take several hours to set up. A dual-loop induction transmitter as a high-frequency source can be set up in less than 10 minutes.
HSAMT can resolve shallow targets. HSAMT up to 100 kHz can image targets as shallow as 5 meters. Traditional low-frequency MT cannot resolve targets in the upper 100m. AMT to 10 kHz can resolve targets as shallow as 20 meters in conductive earth.
AMT and HSAMT sensor setup is easier. Traditional low-frequency MT surveys require the magnetic sensors to be buried at least 20cm in the ground, which can take considerable time and may be impossible in frozen or otherwise hard ground.
High-frequency AMT or HSAMT magnetic sensors can often be used unburied. MT electric sensors use non-polarizing porous pot electrodes which must be buried in moist ground. The AMT electrodes can be metal stakes that are simply hammered into the ground.
Deliverables for AMT and HSAMT:
MT processing is used for MT, AMT, and HSAMT measurements. The processing generates impedance, phase, coherency, and other parameters of the earth’s response. 1-D and 2-D transformation and inversion software are used to generate 1-D soundings and 2-D depth sections of depth and true resistivity. 3-D inversion software is under development in several academic settings . An example of a 2-D section showing a conductive brine zone (red) is shown below.
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