Sub-Bottom Profiler

What?

How does it work?

A Sub-Bottom Profiler (SBP) is an acoustic technique. This system uses a transceiver—a device that both transmits and receives acoustic signals—to send sound waves to the seabed and capture the reflected signals. Due to the significantly lower frequency of the SBP’s sound waves compared to those of other acoustic methods, they can penetrate deeper into the substrate. When these waves encounter boundary layers between sediments with different physical properties, reflections occur. The reflected sound waves are then recorded and transformed into a seismic profile (Figure 1).

Verdieping X

Seismic research using the SBP is based on the principle that low-frequency sound waves:

• Can penetrate the subsurface,
• are reflected by boundary layers between different sediment layers, and
• can be represented in a seismic profile.

Whether a sound wave is (partially) reflected at a boundary between two sediment layers depends on the difference in acoustic impedance between the layers. Acoustic impedance is the product of the speed of sound wave propagation in the sediment and the density of that sediment. In mathematical terms, this can be expressed as:

acoustic impedance = v * ρ, where:

v = the speed at which a sound wave propagates through the sediment

ρ = the density of the sediment

When the difference in acoustic impedance between two layers is significant and the transition is abrupt, this results in a clear reflector in the seismic profile. This typically occurs at sharp transitions, such as a transition from sand to clay or vice versa.

Each time a sound wave reflects off a layer boundary, it loses some of its energy. After reflection, the sound wave continues to propagate into deeper layers of the subsurface. As the sound wave penetrates deeper, the energy loss increases, causing reflectors at greater depths to appear less distinct (Figure 2).

What do you need?

An SBP system consists of several components, with the most crucial being the transceiver (transmitter/receiver). It converts electrical signals into acoustic signals and receives the reflected signals.

There are different types of SBP transceivers used to map the layered structure of both deep and shallow subsurfaces. Table 1 provides an overview of common SBP systems. For archaeological purposes, pingers, chirpers, sparkers, and parametric echo sounders are typically used (table 1).

One key differentiating factor among these systems is the frequency at which they operate. Penetration depth is influenced by the amount of energy (expressed in frequency) the system imparts to the outgoing sound pulse, as well as by the properties and variability of the sediments in the subsurface (Figure 3).

In addition to frequency, the maximum penetration depth is also determined by the water depth. Due to the so-called ‘multiple‘—the reflection of the sound signal between the seabed and the water surface—a repetition of the seismic profile occurs. As a result, the effective maximum penetration depth in the seabed is limited to the water depth itself (for instance, in the IJsselmeer area, this is approximately four meters on average).

Another crucial factor is vertical resolution. This refers to the minimum distance at which two reflecting layers can still be distinguished from each other in a seismic profile. The frequency of the sound wave plays a key role in this: the lower the frequency, the longer the wavelength and the lower the vertical resolution. In other words, at lower frequencies, it becomes more difficult to differentiate between thin layers. The relationship between frequency, wavelength, penetration depth, and vertical resolution is summarized in the table below (Table 2).

Besides the transceiver, the following components make up a complete SBP system: a positioning system and software, a motion sensor, and an SBP processing program. These components are described below:

Can be used with..

The SBP can either be towed behind a survey vessel on a cable or mounted in a fixed position on the vessel. In the latter case, the accuracy of the positioning is significantly higher.

Archaeological Applications

Place in the Dutch archaeological heritage management process

Within the archaeological heritage management process (IVO opwater), the SBP is primarily used to map the structure and layering of the seabed from the water’s surface (KNA protocol 4103, pp. 5-7). Additionally, it can be used to collect information on larger buried structures within the seabed, such as known shipwrecks. In the case of known archaeological sites, remnants may be hidden beneath the sediment and not exposed at the seabed surface. Research with an SBP can help to delineate and map such sites.

What types of archaeological materials/landscapes

The SBP can be used to map buried (prehistoric) landscapes, such as filled-in channels or areas where peat and humic clay may be present. Understanding the local geology is essential to interpret features seen in seismic profiles and connect them to the geological history of the survey area. To verify the relationship between anomalies in the seismic profiles and the actual geological situation, core drilling is necessary to ground-truth the results.

Additionally, the SBP is used to determine the location and depth of buried objects. These objects, such as unexploded ordnance (UXOs) from World War II, stones, cables, or pipelines, have an acoustic impedance different from their surroundings, creating anomalies in the seismic profiles. These anomalies are referred to as “diffraction hyperbolas.” The peak of a hyperbola indicates the position of a buried point object. The depth of an object can be calculated based on the measured time interval between the seabed and the top of the hyperbola. However, the nature of an object cannot be determined solely through SBP measurements; this requires additional investigation (IVO-onderwater) using tools like a ROV.

Limitations/Uncertainties

A seismic profile cannot be directly compared to a lithostratigraphic profile due to several reasons:

• Acoustic Impedance Overlaps: Different types of sediment can share similar acoustic impedance values, which prevents reflection at the boundaries between these layers.
• Invisible Stratigraphic Boundaries: Sediments from different lithostratigraphic units can possess similar lithology and acoustic impedance, rendering the boundary between these units invisible in the seismic profile.
• Gradual Transitions: The transition between different lithostratigraphic units can be gradual, such as from sand, to sand with clay layers, to clay with sandy layers, and then to clay. In these cases, the boundary is not discernable as a reflector in the seismic profile.
• Thin and Local Units: A lithostratigraphic unit can be very thin (< 1 m) and only occur locally. Such units may merge with other units, forming a single seismic unit that is hard to distinguish.

Furthermore, certain phenomena lead to “acoustic blanking“, an attenuation of the acoustic signal. Here, most of the acoustic energy is absorbed, making deeper layers and structures invisible in the seismic profile. This commonly occurs in gas-bearing sediments like peat layers. Layers containing stones and/or gravel also act as strong reflectors, absorbing nearly all energy from the reflected signal and preventing deeper penetration. This, too, results in acoustic blanking.

Boreholes and soundings are essential to properly interpret seismic profiles in lithostratigraphic terms.

In practice, the SBP is rarely used to map previously undiscovered shipwrecks or aircraft wrecks on the seabed due to the high risk of missing buried objects. This is because an SBP only provides a profile along the navigated line. Unlike Side-Scan Sonar (SSS) and Multibeam Echo Sounders (MBES), which offer comprehensive coverage of the seabed, objects located a few meters to the left or right of the SBP’s survey line remain invisible in the seismic profile. While a 3D SBP system could mitigate this issue, it is costly and therefore only viable for small areas (< 1 ha).

References/Further reading

Cassée, R. W., van Lil, R., & Van den Brenk, S. (2023). WFZ IJmuiden Ver V and VI – An archaeological assessment of geophysical survey data. Periplus Archeomare rapport 21A032-01.

Missiaen, T., Slob, E., & Donselaar, M. E. (2008). Comparing different shallow geophysical methods in a tidal estuary, Verdronken Land van Saeftinge, Western Scheldt, the Netherlands. Netherlands Journal of Geosciences — Geologie en Mijnbouw, 87(2), 151–164. https://doi.org/10.1017/S0016774600023192

Van den Brenk, S., & Koelman, M. G. (2016). Geofysische opnamen scheepswrakken Burgzand, Waddenzee. Periplus Archeomare rapport 16A017-01. https://doi.org/10.17026/dans-xvg-3rer, DANS Data Station Archaeology, V2.

Van den Brenk, S. (2023). Geofysisch inventariserend veldonderzoek, scheepsblokkade IJ. Periplus Archeomare rapport 22A003-02.

Van den Brenk, S., & van Lil, R. (2023). Net op Zee IJmuiden Ver (Alpha/Beta/Gamma) – An archaeological assessment of geophysical & geotechnical survey results. Periplus Archeomare rapporten 21A001-02/& 21A0025-02/06.

Van den Brenk, S. (2023). Geofysisch inventariserend veldonderzoek, verdronken stad Reimerswaal. Periplus Archeomare rapport 22A001-01.