Applications of Ground Penetrating Radar (GPR)

August 18, 2014 Edited by  
Filed under Geology, Physical Geography

Typical Antenna Diagram

Typical Antenna Diagram

In recent years, ground penetrating radar (GPR) has gained general acceptance in a variety of fields, including geology, engineering, and archaeology as an excellent method for capturing high-resolution imagery of geologic structures and subsurface objects. High-frequency electromagnetic pulses (EM) are emitted from an antenna to probe the Earth. These waves change velocity and are reflected when encountering materials with different dielectric properties like soil, bedrock, water, anthropogenic objects, and other anomalies. The changes in wave velocity are examined to calculate travel times and produce a viable profile image, known as a radargram. Multiple transects or survey lines can then be stitched together to produce a 3-dimensional view for various analyses.

Radargram

Radargrams of subsurface Reflectors

The appeal of GPR is that it provides a non-invasive geophysical method for subsurface exploration that is preferable to more costly excavation or drilling techniques. Current GPR systems are relatively user-friendly and are composed of an antenna to send and receive radio frequency pulses, a laptop or monitor to capture and view data, and a power source.

Student calibrating and initializing GPR software and 100MHz antenna for survey

Student calibrating and initializing GPR software and 100MHz antenna for survey

Depth penetration and resolution varies widely depending on the antenna used. Here in the U.S., common systems range from 100MHz (low-resolution; high penetration) to 1.3GHz (high-resolution; low penetration). The 100Mhz antenna has a maximum depth of ~60ft at a resolution of 1.6ft, while the 1.3GHz can only penetrate to a foot or two, but has a resolution of .035ft. This trade-off between depth and resolution requires investigations to tailor equipment to study parameters and intended purposes.

GPR antennas are either shielded or unshielded. The former are primarily used for medium to high resolution surveys, and are highly suited for urban and residential investigations, which have background noise and interference. Unshielded antennas encounter issues when cultural noise is a factor, so are limited in applications.

Cemetery with mix of graves from 1800-present

Cemetery with mix of graves from 1800-present scanned in survey

Because EM wave velocities change drastically when encountering void spaces and disturbed soils, GPR is particularly suited for grave detection and inventory surveys of old cemeteries and archaeological digs. Many of these sites have absent or crumbling headstones and older graves with unmarked locations. Buried objects and point features produce a reflection hyperbola (looks like an inverted U) on the radargram, while bedrock contacts and soil density changes show up as planar reflectors. Void spaces manifest as a “ringing” reflection. The user can interpret these features to produce a reasonable understanding of near-surface phenomena.

These geophysical indicators of buried objects in the radargram offer enormous potential in locating lost and historical graves, as well as the traditional ability to locate pipes and other man-made objects, without destructive digging, excavation, or drilling. As GPR technology evolves it is becoming more cost-effective and practical for a variety of research, industrial, and educational applications.

Grave detection survey with 100MHz Mala GPR

Grave detection survey with 100MHz Mala GPR

 

 

 

 

 



Mapping the Mysterious Carolina Bays

July 17, 2014 Edited by  
Filed under Geology, Physical Geography

3m DEM showing Carolina Bays, Hoke Co., NC

3m DEM showing Carolina Bays, Hoke Co., NC

Carolina Bays are terrestrial geomorphic features which are largely elliptical with long-axes trending in a general northwest-southeast direction, found throughout the Atlantic Coastal Plains from Florida to New Jersey. Physical characteristics of bays have been an integral part of the development of the cultural landscape, via subsequent location of roads, buildings, and agriculture. Bays also increase flood hazard potential with their high concentrations of clayey materials. Past estimates have put the number of bays at about 500,000, but new high-resolution datasets coupled with modern techniques may exponentially increase this inventory. Their origin in the Coastal Plain has been largely cryptic since they were first identified in 1848, and bays were generally overlooked within the geologic and academic community until aerial photography became widespread in the 1930’s. Attempts at age dating have varied, but most methods show ranges from the late Pleistocene, or 100,000 BP.

Geomorphically, bays are generally depressed only a few feet, with raised sand rims on the southeast side. They are elliptical in shape and vary vastly in size, while some are ponded or contain wetlands. The concentrated density of these features in the Carolina’s led to the popularization of the first part of the name, while the latter “Bay” refers to the dominant tree species. Sizes range from several thousand square meters to several square kilometers, covering 50-60 percent of land area in some regions. Overlapping and truncation is common, indicating an ongoing, rather than catastrophic or singular process.

Rose Plot of Bay Orientations, Hoke Co., NC

Rose Plot of Bay Orientations, Hoke Co., NC

Since the first recorded recognition of Carolina Bays in 1848, theories of their origin have been rampant and exhaustive, with much speculation and conjecture. The wide assortment of fields conducting research led to multiple theories ranging from meteorite and antimatter explosions, to more terrestrial origins, such as fish nests and upwelling of spring water. Origin theories also attempt to account for uniform bay orientation, as well as their variable sizes, distribution, ages, soils, vegetation, and geology. In fact, the origin of these mysterious surficial features is still hotly debated 165 years later. At present, multiple genesis hypotheses still attempt to explain the processes that created these odd, sinkhole-like features. The most credible is Kaczorowski’s (1976) dissertation-proposed wind and wave action theory, which implicates unidirectional winds towards the Pleistocene glaciers in the north, as the most likely agent accounting for their distinctive elliptical shapes. While there is no current identical comparative geomorphic process, Carolina Bays are similar to modern Alaskan features known as oriented lakes.

Landscape Morphometrics

Landscape Morphometrics

With multitudes of new spatial data, imagery, and techniques available, the future of Carolina Bay research is sure to change drastically. My own work in this area has created a database for morphometric analysis by digitizing Carolina Bays at the USGS quadrangle scale. Morphometrics, or the study of shape, then adds quantitative descriptions that can allow more robust comparisons and statistical analyses. Reducing object shapes to quantitative datasets allows for more objective study without user bias and interpretation. By analyzing the planimetric morphometry, underlying geomorphic processes that lead to bay formation may be better understood. Then principal components analysis is conducted to highlight trends and correlations within the feature class. Furthermore, analysis using nearest neighbor and spatial dispersion statistics, produces useful correlations within the data.  Length and width measurements from the mapped bays are used to determine a simple measure of form, the ratio of long to short axes. From this a product of symmetry (length ratio * width ratio) can be calculated to determine internal symmetry of the bays. Field methods are then employed to gather on-the-ground morphometric measurements and collect sand rim samples for OSL (Optically Stimulated Luminescence) aging of sediments. Another emerging tool in bay research is Ground Penetrating Radar, which offers huge potential for mapping and analyzing these features. Radio waves are reflected by different density substrates to produce a 2-dimensional image. Traverses of sampled bays in the study area elicit useful data and information for interpretation that may one day lead to a clear picture of how these curious features formed in the landscape.

 

Radargram of Karst Topography, Lycoming Co., PA

Radargram of Karst Topography, Lycoming Co., PA

 



Sinkholes: Illinois vs. Florida

Greenbrier County Sinkhole

Sinkholes in Greenbrier County, West Virginia

Recent sinkhole events in both Illinois and Florida made national news and highlighted a little-known geohazard, raising many questions and concerns of property damage and safety. Sinkholes are a common surface expression found mostly in regions of karst topography. Karst is a Slavic word for a large, flat field, which is typical of the landforms in Slovenia that contributed the name. The presence of sinkholes tells the geologist that a particular type of geology, hydrology, and environmental impacts can be expected. Most sinkholes are formed by the dissolution of calcite-bearing rocks. As precipitation (H20) makes its way through the hydrologic cycle, it picks up carbon in the atmosphere, soils, and rocks in dissolved form (CO2). This creates a mild corrosive known as Carbonic Acid (H2CO3), which can dissolve the mineral calcite found in limestone (CaCO3) and dolomite {CaMg(CO3)2}. Other sinkholes are formed by the dissolution of evaporites or anhydrites of copper (CuSO4), calcium (CaSO4), and gypsum {CaSO4 (2H2O)}. Regardless of their formation, the hazard exists when this process leaves a cavity beneath a thin soil or rock covering. The cavity continues to grow until a critical mass is reached where the roof can no longer hold the weight and it collapses. Likewise, this can occur when weight is added by someone or something (cars, infrastructure, golfers, etc.).

Maxwelton Cave

Author in Maxwelton Cave, West Virginia where many sinkholes deliver water and materials to the subsurface.

There are several types of sinkholes but most occur as either solution sinks, where rock is slowly dissolved but there is no connection to the subsurface, or collapse sinks, which overly cave systems and transport material to the subsurface creating an excavation with a throat. The former are prevalent in karst but are relatively harmless, while the latter are more rare but far more costly and dangerous, since they can extend several hundred feet vertically and spread laterally for hundreds of feet. The sinkhole that caused the death of Jeff Bush in Hillsborough County was of the collapse variety, slowly forming over hundreds or thousands of years, culminating in a brief collapse event. This sink was 20-30 feet wide and 30 feet deep. Unfortunately for residents, this is a common part of the landscape there, as much of Florida has karst topography. The limestones in Florida are porous and the water table is high, creating much dissolution that forms thousands of sinkholes and caves. Many of these will have a thin rock or soil mantle, which enhances the hazard, as we are often unaware of their presence until collapse initiates.

Suffosional Sinkhole

Suffosional Sinkhole in West Virginia

The Illinois event represents another type of sinkhole, known as suffosion or soil-piping. This occurs when water transports soil and overburden to the subsurface creating a cavity. While these occur naturally, they are aggravated by human influences in the watershed that change hydrology and drainage, such as pavement, rooftops, and other impervious surfaces. These runoff modifications can cause excessive soil and substrate to be transported to the subsurface, creating a sinkhole. Likewise, this process occurs when there are leaks or breaks in water pipes. Fortunately, Mark Mihal suffered only a dislocated shoulder when a suffosion sink opened up under his feet on the golf course. The most likely culprit is a leaking irrigation pipe commonly used to water the green.

So what can we do to prepare and mitigate damages and loss of life from sinkholes without expensive and technical seismic and geophysical equipment? Primarily you should be aware of where you live and the range of local geologic hazards. Those living in earthquake country have management and emergency preparedness plans. Living in karst similarly requires knowledge of human impacts and geohazards found there. Hazard mapping of these features in karst can offer awareness and contribute to local management and best practice plans to help mitigate property damage and loss of life. Potential hazard zones can be established to restrict or regulate development in high-risk areas. Only active awareness and participation within an integrated management plan in karst topography will help avoid future loss of life and property damage.



‘The testimony of rocks’ in science v. creationism

February 11, 2013 Edited by  
Filed under Geology

The ongoing battle between creationists and scientists is still raging. Polls conducted over the last 30 years have indicated that more than 40% of Americans believe that God created life fewer than 100 centuries ago (Gallup, 2012). A majority of this population also believes that scientists have been actively perpetuating an anti-faith conspiracy for centuries.

In the Geological Society of America‘s November issue of GSA Today, David Montgomery’s account of this debate condemns creationists for abandoning “faith in reason” and discarding a centuries-old theologic understanding that “rocks don’t lie.”

Click to read more of Montgomery’s account The evolution of creationism from GSA Today. Additional commentary is available here: Geology and creationism.



More jobs, fewer funds for the Geosciences

February 5, 2013 Edited by  
Filed under Geology

The geosciences are hiring. Thanks to booming mineral and petroleum industries and increasing awareness of climate change, geoscience jobs are multiplying faster than the number of qualified applicants in the United States, Europe, and Asia.

Despite this increased demand, universities across the globe are downsizing their geosciences programs. Last year, Open University, which boasts about 4,500 Earth Science students per year, cut all residential geoscience courses. The university’s reasoning? Read Steven Drury’s article for earth-pages  to find out.

The production of geoscientists: a cautionary tale from the Open University



NEW Monsoon in India Animation

 

Click here to view Monsoon in India  Animation



NEW Industrial Production Past & Present Animation

Click here to view Industrial Production Past & Present Animation



NEW Formation of Tornadoes and Mesocyclones Animation

January 2, 2013 Edited by  
Filed under Physical Geography

Click here to view Formation of Tornadoes and Mesocyclones Animation.



NEW Coastal Processes Animation

January 2, 2013 Edited by  
Filed under Geology, Physical Geography

Click here to view new Coastal Process Animation



Living Lectures

July 27, 2012 Edited by  
Filed under World Regional Geography

By Wiley author Barbara Murck

“We don’t need to go to lectures, we’re livin’ it.”
– Cali, one of our fantastic students in the University of Toronto’s Ecuador 2012 Summer Abroad course.

I’m passing along this quote, just in case you needed additional reinforcement on the value of field experiences for students. Cali said this spontaneously while we were hiking up the volcano Bartolome in the Galápagos Islands, and it quickly became – for me – the quote of the course.

Approaching our hike at Chimborazo, the tallest volcano in Ecuador, the highest point on the equator, and the farthest point from the centre of the Earth.

Certainly very few students (and very few profs) get the opportunity to visit the Galápagos, the Andes, or the Amazon, and we went to all three of them for this course. The trip was amazing – life-changing – for all of us, in many ways. Swimming with sharks, sea lions, and penguins, seeing a pygmy marmoset from less than 2 meters away, and hiking up the highest mountain on the Equator (Volcan Chimborazo) were some of my personal highlights.

The trip was more than just fun and eye-opening. It was also physically and mentally intense, and it had a very serious academic core. The ground we covered, both literally and academically, was impressive. We went from freezing in parkas and hats at 5300 m altitude in the Andes, to hot and sweaty in rubber boots in the Amazon, to snorkeling in the Pacific Ocean in San Cristóbal. Academically the breadth was just as significant. Between my own lectures and those of my Canadian and Ecuadorean co-instructors we covered everything from fisheries, tourism, and oil development to island biogeography, rainforest ecology, and invasive species to hotspot volcanism, subduction zones, and El Niño – and more.

Examining buttress roots with our guide in the Ecuadorian Amazon at Tiputini Biodiversity Station.

For me there were some “small” moments that really drove home the importance and impact of teaching in the field. In the Galápagos I gave a lecture on atmospheric and oceanic circulation systems that most of my students had heard already in their first Environmental Science course, and probably in other courses as well. But teaching about the trade winds in a classroom in Toronto is one thing; asking students to monitor the wind every morning and check the clouds brings them to a whole different level of understanding. When they realized that, yes, in the Galápagos the wind actually does blow steadily from the SSE every day, all day long, the significance of the trade winds and how the whole atmosphere circulation system fits together really solidified their understanding. Walking upright in a lava tunnel on Isabela Island and measuring thick sequences of pyroclastics in the Andes demonstrate the power of geologic processes in a very immediate and physical way. Seeing penguins that are endemic to the tropics, and understanding the role of the cold Humboldt and Cromwell currents in moderating temperatures on land makes a connection between life and the physical environment in a way that is virtually impossible in the classroom.

Bartolome Hike

Climbing the volcano at Bartolomé Island in the Galápagos. This is where Cali’s quote came from!

Although it was truly great to have the opportunity to go on this trip to Ecuador with our 32 wonderful students, I do actually believe quite strongly in the value of field experiences closer to home. We have instituted a program of field trips for our introductory Geography students. There are 300 students in the course, and the instructor takes 100 to 150 of them at a time (with help from Teachings Assistants) on a series of field trips in the Toronto area. They visit housing projects, look at shorelines, take soil samples, monitor the weather, and look at urban development. For some of them it seems like it must be the first time they have set foot outside of their own homes, the world is so new to them. This is not the case, of course, but it definitely is new for them to be looking at their world through academic eyes.

We can offer this type of experience to our students. We can’t get all of them to the Andes, the Amazon, or the Galápagos, but we can make an effort to ensure that every student gets outside – preferably a little bit out of their comfort zone. Ultimately, if we want our lectures to be meaningful we need to get students away from the lecture hall and into an environment where it all comes together and makes sense.