Radio-Echo Sounding on Temperate Valley Glaciers

• How does Radio Echo Sounding Work?
• Frequencies and Wavelengths
• Instrumentation
• Radio Wave Propagation in Ice
• Field Work
• Data Processing
• Results
• References

A radio-echo sounding system consists of two main components: 1) the transmitter, and 2) the receiver. The transmitter sends out a brief burst of radio waves of a specific frequency. The receiver detects the radio waves from the transmitter and any waves that have bounced, or reflected off nearby surfaces. The receiver records the amount of time between the arrival of the transmitted wave and any reflected waves as well as the strength of the waves (measured as an AC voltage).

The radio waves travel at different speeds through different materials. For example, radio waves travel very close to 300,000,000 meters/second (3 x 108 m/s) through air, a little less than double the speed in ice at 1.69 x 108 m/s.

See the next three tabs for more indepth explaination.

Frequencies & Wavelengths of Waves

Electro-Magnetic (EM) energy is made up of both particles and waves. A single wavelength is 2¼ or 360° of the wave's angular distance. When a wave travels through a material, the wavelength is the distance travelled through the material by 2¼ of a wave.

The number of times a wave oscillates over a certain amount of time is know as the frequency of the wave. The units of frequencies are Hertz (Hz) which is the number of complete wavelengths that pass a point in a single second. Therefore, 1 Hz = 1 cycle/second or 1/s.

The wavelength of a signal passing through a material depends on the frequency (f ) of the wave and the signal velocity (u ) through the material (a property of the material itself). As shown above, the units of frequency are 1/s, and the units of velocity are m/s. Since wavelength(l ) is measured in m, the equation to obtain wavelength is:

l = f * u
or wavelength = frequency * velocity

A higher amplitude wave of a given frequency carries more energy than a low amplitude wave. A signal can be detected only if its amplitude is greater than that of any background noise. For example, if you are listening to a radio in New York City, you can pick up a station from Seattle only if its signal is stronger than the EM noise caused by the sun, electric motors, local radio stations, etc.

The Transmitter

There are numerous radio-echo sounding devices used by various researchers thorughout the world. The components described here are those used by researchers at the University of Wyoming, which is based on that designed by Barry Narod and Garry Clarke at the University of British Coloumbia (Narod & Clarke, J. of Glaciology, 1995). It has been designed for use on temperate glaciers.

The transmitter emits a 10 ns (nanosecond) long pulse at a frequency of 100 MHz. The details of the pulse-generation circuitry can be found in Narod & Clarke, 1995. The frequency of the pulse is modulated for use on temperate glaciers by attaching two 10 m antennas. The resulting 5 MHz frequency is ideal for temperate glacier radio-echo sounding. The transmitter is powered by a 12 V battery.

The transmitter and battery are housed in a small tackle box which is attached to a pair of old skis. The antennas extend out the front and back of the tackle box. The forward antenna is carried by the person pulling the transmitter sled's tow rope, while the rear antenna drags behind. There is no focusing of the transmitted signal, so it propagates in all directions into the ice and air. In order to reduce "ringing" of the signal along the antenna, resistors are embedded every meter along the antenna. The total resistance of each 10 m antenna is 11 ohms.

The receiver begins with an antenna identical to that of the transmitter. As each pulse is sent out of the transmitter, some of the transmitted energy travels through the air and some through the ice. The velocity of radio waves in air is almost twice that in ice, so the receiver first detects the "Direct Wave" transmitted through the air between the transmitter and receiver. This triggers the oscilloscope to begin recording the signal. For the next 10 µs, the oscilloscope records the voltage of the signals that have reflected off nearby surfaces. The scope averages 64 of the transmitter pulses and reflected waves to generate a single trace. By averaging the scope reduces niose due to signal scatter and instrument noise in order to obtain a better trace to be recorded on the laptop computer. The entire receiver is placed in a small sled which is pulled by a tow rope. A third researcher monitors the signals on the oscilloscope and records the information onto the laptop. Both the scope and the laptop are powered by a 12 V battery which can be charged by a solar panel for extended surveys.

## Radio Wave Propagation in Temperate Ice

#### Background Information

As most people know, both water and ice are transparent to the visible light portion of the Electro-Magnetic (EM) spectrum. At the much lower frequencies (and longer wavelengths) of radio waves, liquid water is opaque while ice is still relatively transparent. This is why radio-echo sounding is used in the sub-freezing regions of the Arctic and Antarctic glaciers and ice sheets. There is little water present within these cold ice masses to scatter or block the radio signals. The lack of water has allowed researchers to use frequencies ranging from a few MHz for subglacial mapping, up to 200-500 MHz for crevasse detection near the ice surface. Frequencies in the GHz range are used for studies of snow structure and stratigraphy.

By definition, temperate ice exists at the pressure-melting point. This means that both ice and water phases coexist. The presence of liquid water presents a problem when trying to use radio waves in temperate glaciers because the water scatters the radio signals making it difficult to receive coherent reflections that can later be interpreted.

In the late 1960s through the mid-1970s, a number of researchers experimented with various frequencies and transmitter designs. Their findings concluded that frequencies between ~2 and ~10 MHz are best for temperate glaciers. 5 MHz pulse-transmitters are the most common used today.

The basic reason that a 5 MHz signal works in most temperate ice is that the resulting 34 m wavelength is far larger than the size of the majority of the englacial water bodies that scatter the signal. Unfortunately, the long wavelength of the signal seriously limits the resolution of the radio-echo sounding survey.

#### EM Wave Propagation Through a Dielectric Material

Radio waves travel through ice due to its dielectric properties. The dielectric constant of a given material is a complex number describing the comparison of the electrical permittivity of a material and that of a vacuum. As a complex number, the dielectric constant contains both real and imaginary portions. The imaginary part of the number represents the polarization of atoms in the material as the EM energy passes through it (Feynman, 1964). The EM wave propagation velocity is determined by its entire complex dielectric constant.

The propagation velocity of a radio wave in ice is determined by the dielectric properties of ice. Liquid water and various types of bedrock have unique dielectric constants. Since the dieliectric properties of a material are related to conductivity, concentrations of dissolved ions in liquid water will affect the dielectric constant (more free ions increase the conductivity of water). The dielectric constants of some materials are listed below:

 Material Dielectric Constant Reference Air ~1.0 Serway (1990) Ice (at 0ºC) 3.2 ± 0.03 Paren (Unpublished) Water ~80 Hasted (1961) Quartz ~4.3 Gregg (1980)

## Reflections of Waves

#### The Basic Concept

When a wave encounters an interface between materials of different properties, the wave may be refracted, reflected, or both. Snell's Law describes the reaction of light to a boundary between materials of different dielectric contrasts (or refractive index), based on the angle at which a ray perpendicular to the wave front hits the interface. The angle of the incoming ray (Angle of Incidence: ai) is equal to the angle of reflection (ar). The Angle of Refraction (aR) is determined by the ratio of the sines of the Angle of Incidence to the Angle of Refraction and the ratio of the dielectric constants for the upper and lower layers (e1 and e2).

There is a point where the Angle of Incidence is large enough (close to horizontal) that there is no refraction. This is called the Angle of Critical Refraction where all the incoming waves are either reflected or refracted along the interface. Ay angles larger than the Angle of Critical Refracion result in only reflection.

## Radio-Echo Sounding in the Field

#### Introduction

The appropriate field methods for gathering Radio-Echo Sounding (RES) data depend upon the objective of the survey. If a researcher simply wants a rough estimate of the glacier thickness, only a couple readings might suffice. If a high-resolution map of the glacier bed is desired, a dense grid of measurement points is necessary. Below is a description of the field techniques used to develop a high-resolution map of the glacier bed. It is important to remember that even after the field work is over there are many hours of data processing to be done. The techniques described here were developed to minimize the processing time and to maximize the resolution of the resulting map.

#### Mapping the RES Grid

When processing and interpreting the RES data after the field season, the researcher needs to know the topography of the glacier surface to correct for changes in the recorded wave travel times. The glacier surface topography is mapped using the Global Positioning System (GPS) or by traditional optical surveying. While GPS is faster, it does not have the vertical or horizontal resolution of optical surveying. The horizontal positions are necessary to locate the map with respect to other maps of the area, while the vertical coordinates are critical for the data processing and need to be accurate to within 0.5 m.

In order to reduce the possibility of spatial aliasing and to maximize the resolution of the RES survey, the traces should be recorded less than one-quarter wavelength apart. For example, a 5 MHz RES system produces a 34 m wavelength. Therefore the grid of RES traces should be less than 8.5 m apart.

A rectangular grid with the traces aligned at 90° to one another greatly simplifies the data processing. Unfortunately, field conditions do not always oblige such an orderly system and the grid is modified by the presence of crevasses, melt-water ponds, steep slopes, avalanche debris, etc. In such cases, detailed notes help to recreate the grid during the data processing.

#### Recording the Profiles

The transmitter and receiver occupy separate sleds. These may be pulled in-line or side-by-side depending on the design specifications of the instruments. The Univ. of Wyoming system is pulled side-by-side so that the transmitter and receiver are pulled parallel to one another. A single researcher pulls the transmitter on its homemade sled while another pulls the receiver sled. A third researcher walks beside the receiver sled to monitor the incoming signals on the oscilloscope and then record them to the laptop computer.

Some systems can continuously record traces to a computer and do minor amounts of pre-processing such as trace stacking (or averaging) and digital filtering to remove noise. The Univ. of Wyoming system is much simpler requiring the researchers to stop at each position in the RES grid and manually tell the computer to retrieve data from the oscilloscope. Although more time consuming, this method allows the researchers to monitor the condition of the incoming data and results in a smaller data set. Each trace recorded onto the computer is an average of at least 64 received pulses from the transmitter so that the signal-to-noise ratio is improved.

#### RES Field Work on the Worthington Glacier, Alaska

The Worthington Glacier is a small temperate valley glacier in the Chugach Mts. of South-Central Alaska. Radio-echo sounding surveys have been recorded there in support of ice-dynamics research by the Univ. of Wyoming and the Institute of Arctic & Alpine Research at the Univ. of Colorado.

#### Introduction

Processing the Radio Echo-Sounding (RES) data transforms the data from incoherent numbers to a data set that can be interpreted. Our processing methods are drawn from refection seismology techniques. These are outlined in Welch, 1996; Welch et al., 1998; and Yilmaz, 1987. We use a number of IDL (from Research Systems, Inc.) scripts to organize our data and usually create screen plots of each profile through each step of the processing to help identify problems or mistakes. We also use Seismic Unix (SU), a collection of freeware seismic processing scripts from the Colorado School of Mines. SU handles the filtering, gain controls, RMS, and migration of the data. IDL is used for file manipulation and plotting and provides a general programming background for the processing.

The processing steps below are listed in the order that they are applied. The steps should be followed in this order. Note that quality of the processing results are strongly dependent on the quality of the field data.

#### Data Cleaning and Sorting

The first step of data processing is to organize and clean the field data so that all the profiles are oriented in the same direction (South to North, for example), any duplicated traces are deleted, profiles that were recorded in multiple files are joined together, and surface coordinates are assigned to each trace based on survey data. These steps are some of the most tedious, but are critical for later migration and interpretation.

#### Static and Elevation Corrections

The data is plotted as though the transmitter and receiver were a single point and the glacier surface is a horizontal plane. Since neither is the case, the data must be adjucted to reflect actual conditions. The transmitter-receiver separation results in a trigger-delay equivalent to the travel-time of the signal across the distance separating the two. This travel-time is added to the tops of all the traces as a Static Correction.

The data is adjusted with respect to the highest trace elevation in the profile array. Trace elevations are taken from the survey data and the elevation difference between any trace and the highest trace is converted into a travel-time through ice by multiplying the elevation distance by the radio-wave velocity in ice (1.69 x 108 m/s). The travel-time is added to the top of the trace, adjusting the recorded data downward.

#### Filtering and Gain Controls

We use a bandpass filter in SU to elimitate low and high frequency noise that result from the radar instrumentation, nearby generators, etc. Generally we accept only frequencies within a window of 4-7 MHz as our center transmitter frequency is 5 MHz. Depending on the data, we will adjust the gain on the data, but generally avoid any gain as it also increases noise amplitude. We try to properly adjust gain controls in the field so that later adjustment is unnecessary.

#### Cross-Glacier Migration (2-D)

We 2-D migrate the data in the cross-glacier (or across the dominent topography of the dataset) in order to remove geometric errors introduced by the plotting method. Yilmaz (1987) provides a good explanation for the need for migration as well as descriptions of various migration algorithms.

#### Why is migration necessary?

The radar transmitter emits an omni-directional signal that we can assume is roughly spherical in shape. As the wave propagates outward from the transmitter, the size of the spherical wavefront gets bigger so when it finally reflects off a surface, that surface may be far from directly beneath the transmitter. Since by convention, we plot the data as though all reflections come from directly below the transmitter, we have to adjust the data to show the reflectors in their true positions.

We generally use a TK migration routine that is best for single-velocity media where steep slopes are expected. As you can see from the plot below, the shape of the bed reflector has changed from the unmigrated plots shown in the previous section.

#### Down-Glacier Migration (2-D)

In order to account for the 3-dimensional topography of the glacier bed, we now migrate the profiles again, this time in the down-glacier direction. We use the same migration routine and the cross-glacier migrated profiles as the input. Although not as accurate as a true 3-dimensional migration, this two-pass method accounts for much of the regional topography by migrating in two orthogonal directions. Radar Profile After Down-Glacier Migration

#### Interpretating and Plotting the Bed Surface

Once the profiles have been migrated in both the cross-glacier and down-glacier directions, we use IDL to plot the profiles as an animation sequence. The animation shows slices of the processed dataset in both the down-glacier and cross-glacier direction. By animating the profiles, it is easier to identify coherent reflection surfaces within the dataset. Another IDL script allows the user to digitize, grid, and plot reflection surfaces.

The resolution of an interpreted surface is a function of the instrumentation, field techniques and processing methods. Through modeling of synthetic radar profiles, we have shown that under ideal circumstances, we can expect to resolve features with a horizontal radius greater than or equal to half the transmitter's wavelength in ice. So for a 5 MHz system, we can expect to resolve features that are larger than about 34 m across. Since the horizontal resolution is far coarser than the vertical resolution of 1/4 wavelength, we use the horizontal resolution as a smoothing window size for the interpreted reflector surfaces. We use a distance-weighted window to smooth the surfaces.

The ice and bedrock surfaces of a portion of the Worthington Glacier obtained in the 1996 radio echo sounding survey. The 1994 boreholes are also plotted. (Plot by Joel Harper, U. of Wyo.) Click on the image for a larger version.

The ice surface and bedrock surface beneath the Worthington Glacier, Alaska. Resolution of both surfaces is 20 x 20 m. Yellow lines indicate the positiond of boreholes used to measure ice deformation.

### Pictures of the Worthington Glacier Area

Three arrays of Radio-Echo Sounding profiles have been recorded on the Worthington Glacier. The 1994 survey was recorded using different field methods than the field methods used in 1996 & 1998. The same eqpuipmet was used in all three surveys as well as the same data processing techniques.

#### 1994 Survey

The first profiles were recorded in 1994 and oriented parallel to the ice flow direction. The locations of these profiles were not measured accurately, and the profiles were recorded a few at a time over a period of about a month. The resulting glacier bed map was not very accurate, with a resolution of about 40 x 40 meters.

#### 1996 Survey

The 1996 radar profiles were recorded in the cross-glacier direction. The location of every fourth trace of each profile was measured with optical surveying equipment using a local coordinate system seen in the map below. The profiles were spaced 20 m apart and a trace recorded every 5 m along each profile. The resulting glacier bed map had a resolution of 20 x 20 meters.

#### 1998 Survey

In 1998 we used the radio-echo sounding equipment to look for englacial conduits that transport surface meltwater through the glacier to its bed. This study required the maximum resolution that we could obtain from the eqpuipment, so the profiles and traces were spaced every 5 m. Every fourth trace on each profile was surveyed to locate it to within 0.25 m and the entire RES survey was recorded in two days. The survey was repeated a month later to look for changes in the geometry of any englacial conduits found. The first RES survey was processed to produce a map of the glacier bed surface with a resolution of 17.5 x 17.5 m. The maximum resolution obtainable by an RES survey is half of the signal wavelength. Our 5 MHz system, therefore, can obtain 17 x 17 m resolution under the best of circumstances.

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