Data
Purpose
To define the data of an airborne FDEM system and show the ways of visualizing them.
Definition
An abruptly-changing current in the transmitter loop is used to generate a primary time-harmonic magnetic field. This induces secondary currents in the subsurface, which in turn produce secondary magnetic fields. Both the primary and secondary magnetic fields reach the receiver. The (secondary) time-varying magnetic flux through the receiver loop induces currents which act to oppose the change in flux. The voltage in the receiver loop is what we use to define a datum.
The actual ATEM measurement is a continous time series at a certain sampling rate when the periodical source current is transmitted. During the raw data processing, the contractors often stack the opposite polarities, apply digital filters, and re-sample the time series.
An ATEM receiver measures the sum of the primary and secondary field of dB/dt. Because the primary is usually many orders of magnitude greater than the secondary, and the primary field cannot be precisely calculated for an airborne TEM system, the ATEM data mostly concern the off-time when the transmitter current is off, leaving only the secondary field in the receiver. Because the TEM responses exponentially decay after the turn-off, the off-time is usually divided into some intervals (called time gates, which exponentially expand as moving from early to late time), and the samples within each time gate are averaged to produce time channel data. The averaging over time further smooths out outliers and other noise, making the data more robust for interpretation.
Unlike an AFEM system that delivers relative reading for the secondary field, an ATEM system makes an absolute measurement. It becomes important for the users to know the convention of normalization of data. Using the rule of superpositin, ATEM responses can be scaled by the transmitter moment (product of current, number of turns, and area) and the receiver effective area (product of number of turns and area). Such information is usually available in the survey report or reflected in the unit (e.g. \(V/A\) or \(V/A/m^2\) or \(V/A/m^2/m^2\)).
Visualizing data
We create a synthetic airborne TDEM data set and illustrate how the acquired data are visualized in practice. The synthetic earth model is a 1 S/m conductive sphere of a 30 m radius buried 20 m deep in a 0.01 S/m uniform half-space. Airborne TDEM soundings are measured along 9 survey lines covering a 300 x 300 m area centered at the sphere. The line spacing and in-line sounding spacing are both 40 m. At each sounding, the TDEM system measures the time derivative of the transient magnetic field (dB/dt) in the vertical direction 20 m above the surface over 13 time channels, between \(25\;\mu s\) to \(8\;ms\). The synthetic data are generated by a 3D time-domain EM modeling code that approximates the sphere by small voxels.
Map
An airborne TDEM data map is produced by contouring data at a particular time channel as a function of the horizontal location. Click the [Play>] to go through a sequence of maps at different time channels for dBz/dt data. The maps are useful in identifying the horizontal location of the sphere. Realistic data are more complicated than the single peak anomaly in this example, but a map is still a good method for general assessment. Early time channels are most sensitive to the near-surface features, while the late time channels average over a larger volume. In this example, the bump due to the sphere is most prominent at mid-time channels.
Profile and sounding
An airborne TDEM profile is produced by plotting the data of the same time from all the soundings along a flight line. The animation below shows one profile directly over the sphere. A profile plot can be used to locate the object along the line. Analysis of the profile curves is sometimes used to infer the geometry and orientation of the object. The data can also be plotted as a function of time at individual soundings. At soundings close to the sphere, the transient curve positively deviates from the background responses, indicating the presence of a conductive object.
Why visualizing
Understanding the underlying physics - Does the system operate in a more conductive or more resistive environment? - Are the signs and units in the data compatible and consistent with the convention that the interpreter uses?
Data quality control - Can we see any suspicious data or outliers? Is there interference from cultural noise? - What is the approximate noise floor in the data?
Qualitative interpretation - Does the relative highs and lows in the data match the general geology or other a priori information we know? - Is there any indication of the sought target in the data? - What is the likelihood of making an informed decision?
Towards an inversion - What is the resolution of the data? - What physical model is appropriate for this data set? - Does the predicted data from the inversion model acceptably match the observed field data? - Is there any important feature in the observed data that is not duplicated by the inversion?