Secrets of the Ocean Depths – Part 1 of 2
Gösta Lindwall and Mikael Kindborg, July 29, 2020
Secrets from our ancient past are hidden from view at the seabeds around the world. Thanks to mapping technologies and software like Google Earth we can now get a glimpse of the underwater landscape.
This is the first of two posts about underwater maps.
Mapping the Seafloor
The study of the bottom terrain of the ocean floors is called Bathymetry.
Oceanographers have a saying that we know more about the surface of Mars and the Moon than about the geography of the seafloor. As much as 90% of the ocean floor has not yet been mapped.
Imagine a map of the world where 90% of the continents would be white (unmapped). This is the current state of what we know about the bottom of the ocean.
Sonar technology is used to map the seafloor. Sonar systems send out sound pulses through the water and register the time taken for the sound signals to reflect back to the sonar sensors.
There are two main types of sonar: multibeam sonar and side scan sonar.
Multibeam sonar systems (multibeam echosounders) provide accurate depth readings and are used by ships to chart the geography of the seafloor. Depth readings are taken as the ship travels across the surface of the ocean. A map is obtained by travelling back and forth in a grid-like pattern.
Sonar scanning is a time consuming method, and there are currently big "holes" in available measurement data, with gaps of hundreds of kilometers waiting to be mapped.
There is also a type of sonar called side scan sonar, which is used to detect objects under water. This type of sonar is commonly used for fishing, finding sunken objects, detecting wrecks, and so on.
There are side scan sonar systems available for smaller craft, like recreational boats used for fishing.
One could say that side scan sonar operates on a smaller scale than multibeam sonar. Multibeam sonar provides better depth accuracy and can map larger areas.
Satellite-Based Gravity Mapping
In addition to ship-based sonar, satellites are used to map the bottom of the ocean, based on the topography of the surface. This method is also called gravity mapping.
The ocean surface is not equally leveled. Some areas rise up higher than others, and some are lower than others. The gravitational pull from the underwater landmasses creates these height differences.
Satellites measure the distance to the ocean surface, to detect differences in gravity pull. Height data from repeated satellite flyovers is collected and processed using other data sources, such as wave heights and tides. The water depth can then be calculated from the height values.
Satellite mapping has good coverage, but the downside of using this method is that the accuracy is poor, You get a 5 km resolution, which is much less accurate than sonar scanning.
Mysteries of the Ocean Floor
Combining sonar scans with satellite readings gives us something like the equivalent of satellite images where we can see mountains and other details at the bottom of the sea. This is what we see on Google Earth.
As different mapping methods have varying levels of accuracy, the resolution of the maps produced will vary significantly. When satellite gravity scans are combined with different ship-based scans, the result may sometimes look strange.
Areas measured with ship-based sonar may look like big roads or landing strips, when they cut through low resolution areas.
In our next post, we will discuss anomalies seen on seafloor maps. What causes them, and how to distinguish visual artefacts created by the mapping process from mysterious underwater structures and potential sunken cities.
This article has also been published on Facebook in the group Forbidden Archaeology and other Mysteries.