Science behind sensing hydrocarbon seepage using X-band radar

Thomas C. Bailey
Premier E&P Co.
Houston

Merrill I. Skolnik
Naval Research Laboratory
Washington, D.C.

Methods have historically concentrated on measurements taken in the earth, primarily in the soil, but beginning as early as the 1970s atmospheric measurements were introduced. In 1972 Robert Owen and J.M. Busby were issued a patent² for mapping hydrocarbons (implied to be methane) in the atmosphere using microwave radar in the X-band (8,000-12,000 MHz). In 1978 Luke Gournay was issued a similar patent³ (propane was indicated as the gas).

The first published assessment took place in 1978.⁴ The U.S. Department of Energy (DOE) Bartlesville, Okla., Energy Technology Center conducted a series of tests of seepage methods (soil gas, radiometrics, Landsat). They included radar at one site in Greenwood County, Kan., (Fig. 1 [24846 bytes]).

In 1982 Sun Exploration & Production⁵ undertook an assessment of several seepage tools (Landsat, helium, radiometrics, induced polarization) including radar. Sun surveyed parts of Cheyenne, Lincoln, Kit Carson, and Kiowa counties, Colo., along the Las Animas arch (Fig. 2 [33526 bytes]).

In 1986 the Naval Research Laboratory (NRL) Radar Division became interested in the nature of radar detection of gas seeps.⁶ NRL people spent time in the field with Robert Owen and concluded that radar was:

1. Detecting an above-surface effect different from the echoes obtained from the surrounding natural clutter (such as trees, bushes, and high ground);

2. The radar echo was likely to be due to seeping gases;

3. The radar was operated in a modified conventional manner rather than the unconventional manner described in the original patents that postulated a frequency translation on reflection; and

4. The full potential of radar for detection of gas seepage had not yet been realized.

In the first of two articles the authors will show that the original theory proposed for radar sensing of seepage is incorrect. There is, though, a plausible scientific explanation.

The second article will examine, based on a large radar survey, how well the method appears to work.

Radar equation

Radar operates by transmitting an electromagnetic signal (usually a repetitive series of short pulses) which, when incident on a reflecting object, returns some of the incident energy back to the radar where the presence of the target is recognized and its location in range and angle are obtained.

A radar unit consists of a transmitter for generating the radiated signal, an antenna for radiating the signal in the direction of the reflecting object (target) and for receiving the signal the echo returned to the radar, and a receiver for detecting the returned echo signal. A display is used to detect the presence of the reflecting object, to establish characteristics of the echo signal that aid in identifying the nature of the reflecting object, and to locate its position relative to the radar.

In modern radars, sophisticated digital processing can be employed to relieve the operator of the burden of performing the detection and extracting information about the object. Radar detection is defined mathematically by the radar equation,⁷ which can be written as:

Pr = (1)

where:

Pr = echo power received at the radar, watts

Pt = power transmitted, watts

G = antenna gain, dimensionless

l = radar wavelength, meters (equals 3 x 108 m/sec 4 frequency in cycles per second)

p = Pi, 3.14

R = range to the target

(or reflecting object), meters

The type of radar used for the exploration of gas seepage has usually been a modified civil-marine navigation radar found on many small boats and large ships. It is a relatively inexpensive radar, and the modifications made for observing this phenomenon are minimal. It operates in X-band (at a frequency near 9,400 MHz, corresponding to a wavelength of about 3.2 cm), with a small fan beam antenna with 3° azimuth beamwidth and 20° elevation beamwidth revolving at a rate of 23 rpm.

Theories

Fluorescence

The theory offered in 1972 to explain the use of radar for seepage mapping was that on reflection from the gas seepage the radar echo signal was translated to a frequency considerably different from that transmitted.^2-3 Thus, there was thought to be a frequency translation on reflection, a "microwave fluorescence."

Fluorescence is a well-known phenomenon with ultraviolet radiation. It is possible to obtain fluorescence from petroleum seepage when illuminated with ultraviolet light sources.

Scientifically, fluorescence cannot be accepted to explain what is seen on radar over hydrocarbon accumulations. In molecule rotational spectra, which occur in the microwave-far infrared band, the frequency of absorption is determined by the Moment of Inertia and the intensity of absorption by the Dipole Moment.⁸

A rotating dipole generates an electric field that will interact with electromagnetic radiation. If there is no permanent dipole, it is virtually impossible for microwaves to interact with a molecule.⁹ Permanent dipoles are very, very small or nonexistent in methane and propane, and without a dipole no absorption occurs and no reradiation.

Ethane and butane the other gases likely to be found in seepage also have no or small dipoles. Furthermore, examination by NRL and others of the theory of the interaction of electromagnetic waves with gases, including laboratory measurements, could not confirm the existence of a frequency translation on reflection at the frequencies used by radar. The interesting thing, however, was that although Robert Owen started with an incorrect model of radar echoes from gas seepage, it appears he found a way to detect gas seepage with radar.

Turbulence

If there is no microwave fluorescence, or frequency translation on reflection from gas seepage, then what is causing the echo?

Radar echoes can be obtained from many solid objects such as ground, trees, large bushes, birds, insects, and dust. The only known mechanism, however, for obtaining a radar echo in clear air is from atmospheric turbulence (which is not visible to the eye). This is the mechanism believed to be the cause of the radar echo over hydrocarbon accumulations.

Radar echoes from gas seeping into the atmosphere would have the characteristics of reflections from clear-air turbulence, as there are no visible objects in the vicinity of the echoes to account for the observed radar echo.

As described by Skolnik,^6-7 the appearance on ground radar of hydrocarbon anomalies identified by Robert Owen is of individual echoes or clusters of echoes. Echoes were about the size of the radar resolution (40 ft down range by 80 ft cross range) or slightly larger. The echo was "blob-like" and changed in size and appearance from scan to scan (about 3 sec between scans), but there was no real movement or long-term drift.

The change in the appearance of the radar echo on the display may be due to a change in size from observation to observation, and/or a change in the radar echo strength, and/or the result of interference from the echoes from the individual turbulent eddies within the resolution cell to produce an apparent change in location called glint. Skolnik⁶ concluded that these echoes were likely to be due to atmospheric turbulence induced by the escaping gases interacting with the local wind.

The radar echo was weak compared to the echo from other more usual radar clutter targets, but it was much larger than would be expected from natural clear-air turbulence or from turbulent hydrocarbon gases that have a low dielectric constant (dielectric constant is a measure of polarizability and thus reflection ability). He postulated that the reflection would have to be due to a gas with a strong dielectric constant. Water vapor carried to the surface by the hydrocarbon gases might provide the high dielectric constant.

Scattering from refractive-index turbulence is an example of Bragg scattering. Mathematically the strength of the returned signal is related to the mean-square fluctuations of the refractive index. This is expressed as the Structure Constant, Cn2. The Structure Constant depends on the spatial variance of moisture and to a lesser extent temperature. For Bragg scattering the relationship between returned signal and refractive index is mathematically given by¹⁰

h = (0.38) C_n²) (l^-1/3) (2)

where:

h = radar reflectivity, meters^-1

C_n² = structure constant, meters^-2/3

l = radar wavelength, meters

Radar reflectivity, h, is a measure of the amount of electromagnetic energy a volumetrically distributed target scatters back per unit volume illuminated by a radar. It is the radar cross section (m²) divided by the volume (m³) illuminated by radar. The number 0.38 is a constant, while l is the radar wavelength. C_n² is a measure of the mean-square of the fluctuations in the refractive index of the turbulent medium. It can be written as¹¹

C_n² = (3)

where:

RI = index of refraction at point x

RI'; = index of refraction at point x';

r = distance between x and x';

The bar signifies the ensemble average. As indicated by equation 2, the radar reflectivity from turbulence is weakly dependent on radar wavelength. At X-band wavelength a powerful radar is needed to see turbulence or there has to be a short range from turbulence to radar. The X-band radar used, the Raytheon 2700, is not very powerful; but it is used at close range, less than 0.5 miles.

A hypothesis is that as gas rises through the soil to the atmosphere, water vapor rises with the hydrocarbon gas. We hypothesize that water vapor is included with the gas seepage so as to account for the magnitude of the radar echo. Turbulence, caused by wind shear, incorporates water vapor carried by the seeping hydrocarbon gas and the turbulence now becomes visible on radar.

This explanation was independently suggested by Dr. Owen Phillips of Johns Hopkins University (atmospheric physicist and a consultant to the Amoco-NRL study). As gas and water vapor move down wind they dissipate within a few tens of feet above the ground. The gas seepage will be visible on radar when it is turbulent. The radar anomaly will be seen over a spatial area close to the ground where gas and water vapor are venting.

Although it is apparent that the theory in the original patents based on frequency change of the echo signal on reflection from gas seepage is not correct, the turbulent theory is not without need of continuing research. It is offered as a reasonable hypothesis, not as a proven and verified theory. Its major advantage as a hypothesis is that atmospheric turbulence is the only known mechanism for producing a radar echo from clear air (nothing visible).

When seeping gas and water vapor encounter wind shear it would produce overturning and mixing, turbulence. One would expect, but it has not been verified, that this phenomenon can be detected any time of the night or day if there is a suitable wind. Robert Owen⁶ wrote that he monitored radar anomalies over a 24 hr period, and they were present at night. Hemenway et al.,¹² in the effort with Amoco, monitored over one night the phenomenon he saw. He was not able to detect it at night. If the phenomenon is seen at some times and not at other times, it would be important to find out if the atmospheric conditions are appropriate for generating turbulence during these times.

Another question: are there enough venting hydrocarbons to carry water vapor to the atmosphere to be seen by radar? Hydrocarbon measurements have been made in well bores from hydrocarbon accumulations to the surface.¹³ What is found is that values decrease generally and then rapidly near the atmosphere and soil interface. The upper few inches or few feet of soil is where mixing with the atmosphere occurs. Any gas that does finally reach the atmosphere is highly diluted.

The Russians¹⁴ reported measuring, with a laser, a methane increase in the atmosphere over an oil and gas field. The increase in methane matched field boundaries. In this instance it would appear then that venting gases are present over subsurface accumulations and may give a clearer picture of subsurface location. Concentrations they measured though were only elevated a couple of parts per million for methane. The other light gases, C₂, C₃, C₄, would be expected to be in even in smaller concentrations. There is another source for water and a gas carrier over a hydrocarbon accumulations.

A major oil company was studying a known field and attempted to measure gas flux at the ground surface. It found carbon dioxide but no hydrocarbons and thought the field was not seeping to the surface. Interstitial soil gas was done and confirmed hydrocarbon gases were present in the soil. Apparently the hydrocarbon gas was being consumed or oxidized before it reached the ground surface. The CO₂ measured was probably the byproduct of the destruction process.

It is the conclusion of many¹⁵ that most of the hydrocarbons that get to the soil layer are either consumed by bacteria or chemically oxidized within the soil layer. The byproducts of both are water and carbon dioxide.¹ If hydrocarbon gas is being consumed over hydrocarbon accumulations in the soil, and little free hydrocarbon gas gets to the atmosphere, could consumption in the soil be the source of water and a carrier gas, carbon dioxide?

Next: Part 2 (Conclusion), Radar exploration theory plausible but not yet proved scientifically

The Authors

Thomas C. Bailey was employed during 1968-92 by Amoco Production co., where he served in various technical and management positins in domestic and international. Before his departure he chaired Amoco's Surface Prospecting Task Force, which investigated microseepage exploration tools and methods. He has an BS in geochemistry and an MA in geology from Bowling Green State University.

Merrill I. Skolnik retired from the Naval Research Laboratory in March 1996 after serving 30 years as superintendent of the Radar Division. Previously he was with the Institute for Defense Analyses, the Research Division of Electronic Communications Inc., and the MIT Lincoln Laboratory. He is a Fellow of the Institute of Electrical and Electronic Engineers and has served as editor of IEEE Proceedings. He is author of the widely used text, "Introduction to Radar Systems," and editor of the "Radar Handbood," both published by McGraw-Hill Bood Co. In 1986 he was elected to the National Academy of Engineering. He received BE, MSE, and Dr Eng degrees, all in electrical engineering from Johns Hopkins University.

Copyright 1996 Oil & Gas Journal. All Rights Reserved.