Backscatter Normalization

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This section is under development. If you have expertise in backscatter normalization, please reach out to the admins at omcadmin@ccom.unh.edu to become a contributor today!

Overview

There is growing interest in seafloor backscatter information for a variety of purposes. This section describes some of the methods for correcting systemic biases in the reported reflectivity across the swath to achieve more useful imagery.

Having the ability to display the calibrated seafloor backscatter during acquisition will assist in the efficient detection of objects during a survey and aid in the quick turnaround to produce a raw data product, e.g., a seafloor mosaic, that is as clean as possible because you do not have to worry about the changes in backscatter intensity relative to the various system modes.

Normalization vs. Calibration

There are two types of backscatter calibration:

• Relative calibration, ensuring consistency between all measurements from one system or between systems, without a reference of the absolute backscatter level. May also be called normalization.
• Absolute calibration, referenced to the absolute backscatter level.

Purposes

Seafloor characterization

Habitat Mapping

Methods

Kongsberg Discovery Backscatter Calibration Service

Kongsberg Discovery offers a Backscatter calibration service. This is a paid service.

The physical installation of the transducers on the vessel has an influence on the result, which is why it is not possible to provide these values from the factory. Relative calibration, also referred to as normalization, ensures consistency between all measurements from one system or between systems, without a reference of the absolute backscatter level. If the vessel's operating region changes significantly (e.g., different ocean), it is recommended to calibrate again.

Data collection

Kongsberg Discovery Hydrographic Customer Support can provide you with the data collection procedure. Each system has a unique set of operating modes. Every mode combination that shall be used for normal data acquisition must be calibrated. Separate lines must be run for each combination, with at least two overlapping lines in opposite directions (on top of each other). More lines can be run to improve the statistics, for example cross lines. If single (off) or dual swath (fixed or dynamic) mode is required, additional lines must be acquired for this. If using dual swath, dynamic is recommended.

Processing

Data samples are adjusted for transmitter source level, receiver sensitivity, transmitter and receiver beam pattern, receiver gain, transmit pulse length and bandwidth, spherical spreading and frequency dependent absorption, incidence angle and vessel attitude. The collected files are sent to Kongsberg Discovery Hydrographic Support for calculation of the backscatter offsets. Support will then provide a file with the resulting offsets that need to be applied to the EM processing unit to collect calibrated seafloor backscatter.