Guide to Interpreting Radar Profiles
Note: Most of this article reefers to profiles produced with a SIR-3 analog radar
|The Ground Penetrating Radar (GPR)
records a continuous graphic profile of subsurface interfaces.
The SIR System-3 records images on a gray scale where strong
returning signals (materials with large changes in dielectric
properties), show up on the profile in black and weaker returning
signals show as shades of gray. The SIR-2000 is a digital unit
and profiles are displayed in a variety of colors.
The figure to the right is an example of a graphic profile produced by the GPR. The horizontal scale represents units of distance traveled along the transect line. This scale is dependent upon the speed at which the antenna advances along the transect and the rate of the paper advance through the graphic recorder (rate is controlled by the operator). The vertical scale is a time scale, which represents the amount of time (in nanoseconds) it takes for the radar pulse to travel through the medium and return to the antenna. This time scale may be converted into a depth scale if the velocity of the signal propagation is known (velocity x time = depth). The dashed vertical lines are event markers inserted on the profile by the radar operator to indicate known antenna locations or reference points along the transect lines. The evenly spaced horizontal lines are scale lines which provide reference planes for relative depth/time assessments.
Most graphic profiles consist of four basic components: start of scan image (A), inherent system images (B), surface images (C), and subsurface images/interfaces (D). All of these components, with the exception of the start of scan, are generally displayed in groups of multiple dark bands (positive and negative return signals) unless limited by high rates of signal attenuation or the proximity of two or more closely spaced interface signals.
The recorded data represents the total travel time for a GPR signal to pass from the antenna, through the subsurface to a reflector, and return to the antenna. A longer time between transmission and reception of a signal in a given homogeneous unit generally implies a greater distance to an interface.
The graphic image produced by the radar can be interpreted by understanding some basic concepts about the GPR principal of operation. GPR is a broad band, impulse radar system that has been specifically designed to penetrate earthen materials. Relatively high frequency (10 to 1000 MHz), short duration pulses of energy are transmitted into the ground from a coupled antenna (mono-static position). Transient electromagnetic waves are reflected, refracted and diffracted in the subsurface by changes in electrical conductivity and dielectrical properties. Travel times and amplitudes of the reflected, refracted and diffracted waves can be analyzed to give depth, geometry and material type information.
The continuous profile displayed by the radar unit produces many types of images, which must be interpreted by the radar operator. The two main patterns produced by the radar are interfaces and point objects (figure below). Interfaces are continuous returns, which typically represents a layer or strata within the subsurface. Point objects are typically displayed as a hyperbolic pattern on the profile. Point objects can represent any type of anomaly in the subsurface from an air filled void to a buried object.
Establishing depth scales:
As mentioned earlier, the vertical scale of the radar profile is a time scale which shows the time it takes for the signal to travel through the subsurface and return to the antenna. This time scale can be converted to a depth scale if the signal propagation velocity is known. Propagation velocities can be either calculated or estimated if ground truthing is not performed. Calculating the propagation velocity is usually performed by burying a reflector at a known depth and determining the velocity using the following formula:
Vm = 2D/t
Where: D = measured depth to reflecting interface.
T = elapsed time between transmitted and received pulse (nanoseconds).
Vm = effective propagation velocity (feet/nanosecond)
If propagation velocities are known, depths can be estimated using the following formula:
Depth = Vm(t)/2
Propagation velocities are often estimated using standard published values for various materials. The following are some dielectric constants (Er) of various earth materials:
Dielectric Constant (Er)
|Sand, dry (Carver soil)||4.5|
|"Average" coarse loamy soil||12|
Using the dielectric constants above, the depth to an interface may be calculated using the following formula:
D = c(t)/2[(Er)1/2]
Where: D = depth in feet.
C = velocity of light (1 foot/nanosecond).
T = pulse travel time in nanoseconds.
Er = relative dielectric constant of material
Examples of Radar Profiles:
Radar profile of a Hinesburg/Wapanucket soil, the dark continuous interface is a soil/geologic contact (lithologic discontinuity) of sandy eolian material underlain by silty lacustrine sediments.
Radar profile showing three buried underground storage tanks, producing a hyperbolic image.
Profile across a peat-filled kettle hole, the dark interface is the peat/mineral contact (click HERE for more info).
This profile shows an area of high conductivity between mark 12-17, caused by high nitrate levels in the soil. The high conductivity causes attenuation of the radar signal. The dark interface at approximately 1 meter depth is an argillic soil horizon (Bt horizon).
left: Radar profile from and archaeologic site,
the profile shows
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