(Site investigation report) 2500 ( civil and environmental engineering )

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Introduction to Seabed Mapping and

Marine Geophysics

Aileen Bohan

INFOMAR

16th March 2021

Introduction Name: Aileen Bohan

Occupation: Geologist and Hydrographic Surveyor

BA in

Geology

MSc in

Exploration

Field

Geology

Post-Grad

Certificate in

Ocean

Bathymetry

Currently, a

Marine

Geoscientist

with

INFOMAR

Overview

• National, international & global seabed mapping – INFOMAR, MAREMAP & EMODnet

• Introduction to SONAR – Singlebeam Echosounder

– Sidescan Sonar

– Multibeam Echosounder

• Introduction to Marine Geophysics – Sub bottom profiling

– Magnetics

– Gravity

INFOMAR

• Jointly managed by Geological Survey Ireland (GSI) and the Marine Institute (MI)

• Successor to the Irish National Seabed Survey

• Integrated mapping products of the physical, chemical and biological features of the seabed

Keary

Lir RV

Galtee

The INtegrated Mapping FOr the Sustainable

Development of Ireland's MArine Resource

Mallet

RV Celtic

Voyager

Celtic Explorer RV

Geo

2026!

INFOMAR

• Nautical charts • Oil and gas exploration • Safety and storm surge/tsunami inundation models

• Ecosystem identification and management

• Emergency response • Satellite verification models

• Ocean Models • Coastal/Marine Spatial Planning

• Coastal Hazard Assessment

• Ocean Exploration • Coastal Change Analysis • Sea Level Rise Mitigation • New Energy Siting • Marine heritage

Why Collect Bathymetry Data INFOMAR

Planning offshore energy installation requires the following information

• Water depth (bathymetry)

• Seafloor features

• Shallow geology

• Seabed composition

• Seafloor dynamics

• Debris located

INFOMAR survey data can be used for initial site suitability studies

Supporting Offshore Renewable Energy (ORE)

INFOMAR

ORE- Geological Features for Site Selection

1) Seafloor Morphology, including:

• Seabed slopes and gradients

• Bedforms

• Seabed Dynamics

2) Quaternary Geology, including:

• Sediment type and classification

• Depth of transition to bedrock

3) Solid Geology, including:

• Bedrock description

• Faults

• Oil/gas accumulations

INFOMAR

• Funded by ESA under the Space for Shore consortium

• Project running from 2019 to 2021

• Providing tools for coastal erosion monitoring based on EO products

• Image catalogue spanning 25 years

Coastal Erosion from Space Capturing coastal change using satellite

data

+ 200Km of coastline

INFOMAR

1 2

3 4

CHERISH

Climate change and coastal heritage

INFOMAR

Education- MSc Module

• INFOMAR Masters module in Marine Remote Sensing

• Partnership between INFOMAR, the Dept. of

Geography at the National University of Ireland,

Maynooth and the SMART Sea School

• Includes ship time onboard the RV Celtic Voyager and

software training sponsored by QPS

Distilling 20 years of modern seabed mapping into

an educational experience

infomar.ie Data Products

Data Access

INFOMAR

MAREMAP

• Aim to improve seafloor and

shallow geological mapping

to achieve UK national

objectives

• Brings together NERC

organisations with common

geoscience objectives

Marine Environmental Mapping Programme

MAREMAP

European Marine Observation and Data Network

EMODnet

EMODnet Bathymetry

- Harmonised DTM

Seabed Habitats

- EU SeaMap

- Library of habitat maps

EMODnet Geology

- Seabed substrate

- Sea-floor geology

EMODnet

Bathymetry Data Portal

Users can retrieve depth statistics &

metadata

Users can download all data

Users can view high resolution coastal

DTM 3D viewer development Phase III

Geology Data Portal

www.emodnet.eu

Introduction to Underwater Acoustics

Underwater Acoustics During the 18th c.

1826 Lake Geneva.

• 1st recorded attempt to determine the speed

of sound in water.

Colladon and Strum: v = 1435ms-1 @ 8 degC

Only 3ms-1 different than accepted value

16km

• Until the early 1800s, water depth was measured by lowering a weighted line from the deck of a ship, which is tedious, dangerous and not very accurate.

History of Underwater Acoustic

• 1925-1927 : first large-scale scientific bathymetric survey was conducted by the German Atlantic Expedition on RV Meteor using SBES

• Ship crossed the Atlantic 13 times in 2 years at 600km line-spacing.

Underwater Acoustics in Early 19th c. History of Underwater Acoustic

• ASDIC (Anti Submarine Detection Investigation Committee) system developed Sonar

(SOund NAvigation and Ranging)

Underwater Acoustic During WWII History of Underwater Acoustic

• Military applications - detecting submarines, mines, surface vessels and

torpedoes

• Civilian applications - echosounders for hydrography and navigation,

fisheries, side-scan sonar (geotechnical), seismic (engineering to oil),

acoustic communication systems, positioning systems, current meters,

ocean tomography.

Underwater Acoustics After WW2

1977 World Ocean Floor Map created by

Heezen and Tharp GEBCO current world map

History of Underwater Acoustic

The Science of Sound Principles of Underwater Acoustics

Acoustic (Sound) Waves

• Sound is a disturbance of mechanical

energy that propagates through matter as a

wave.

• Sound is characterized by the properties of

sound waves which are

frequency (f), wavelength (l), period (T),

amplitude (γ) and velocity (v).

Principles of Underwater Acoustics

The Science of Sound

Spreading Loss

distance

am p

li tu

d e

• Spreading loss/Attenuation occurs because the total amount of

energy in a wave remains the same as it spreads out from a source.

• As the circle of a surface wave gets bigger the energy spreads to fill it.

Therefore, the energy per unit length of the wave must get smaller.

• The height of the surface wave (amplitude) decreases as the energy per

unit length of the wave crest decrease.

Principles of Underwater Acoustics

• The higher the frequency of the transmitted pulse, the higher the absorption

rate.

• Absorption is also significantly stronger in saline water than in fresh water.

Key facts

Low frequency waves travel long distances (long ranges)

High frequency waves travel short distances (short ranges)

High frequency sources (short wavelength) result in high resolution data

Low frequency sources (long wavelength) result in low resolution data

Sound Absorption Principles of Underwater Acoustics

Reflection

• The incident wave is reflected in a direction symmetrical to its direction of arrival (like

light in a mirror - specular reflection), with a loss of amplitude.

• The bathymetric derivatives from single-beam echo-sounders (SBES), multi-beam

echo-sounders (MBES) and seismic systems rely on reflected energy.

Scattering (diffuse reflection)

• The incident wave is scattered in all directions by a rough surface. The scattering of

acoustic energy back toward the sonar is called backscattering.

Reflection and Backscattering

A Brief Introduction to Underwater Acoustics

• The length (duration) of the acoustic pulse also plays a key role in the discrimination between objects on the seabed.

• Bottom detection in deep water normally requires more power, lower frequencies and an increase pulse length.

Echosounder footprint

A Brief Introduction to Underwater Acoustics

How to map the seabed? • Acoustic methods

• Primarily, multibeam echosounders

Seabed

TransmissionReflection

depth =

Time* speed of sound

in water

Principles of Underwater Acoustics

Single Beam Echo Sounder (SBES)

• Single transducer able to transmit and

receive sound.

• Vertical pulse, single frequency, usually

within 10 - 300 kHz.

• Records “two-way travel time”.

• Low frequencies (12 kHz) can map seabed

as deep as 10km.

• High frequencies (200-300 kHz) are used

for shallow water mapping.

Multi frequency SBES used to study

• water column

• fish stock

• gas flares emission

• Safety

SBES Echogram Explained

@ Anderson et al., 2007

Introduction to SingleBeam Echosounder (SBES)

Typical SBES

display

• Echo-sounders have theoretical sub-centimetre vertical resolutions in ideal conditions (depth dependent)

• To combat the effects of motion, advanced systems can correct for heave/roll/pitch.

• Tide is also a big contributor to errors in the data and needs to be corrected for.

SBES Accuracies and Errors

sea surface

DGPS

antenna

Echo-sounder transceiver

Positional

data

Depth

data

Introduction to SingleBeam Echosounder (SBES)

Side Scan Sonar (SSS)

www.tritech.co.uk

• Used to efficiently create an image of large

areas of the seafloor.

• Identification of underwater objects and

bathymetric features by casting shadows

• Search and rescue

NOTE: SSS does not measure depth.

Introduction to Side Scan Sonar (SSS)

10% of SSS range

Swath Range

The working principle of SSS is as follows

1) The instrument has two acoustic sources (antennas), one at each side. These emit two wide beams in

the vertical plane and very narrow in the horizontal plane

2) The acoustic signal spread in the water column and most of the acoustic energy is reflected off the

obstacles or the seabed.

3) The instrument antennas then receive the reflected signal. The signal is then digitised, amplified and

filtered in order to create detailed images of the reflectivity of the seabed.

Side Scan Sonar Geometry

Introduction to Side Scan Sonar (SSS)

The key to understanding and interpreting side-scan sonar records is the geometry of use:

• A - Towfish depth below the surface

• B - Towfish altitude above the bottom

• C - Slant Range to the target

• D - Acoustic shadow length

Side Scan Sonar Interpretation

Towfish altitude above the

bottom

Slant range to a target

Acoustic shadow Acoustic shadow

Introduction to Side Scan Sonar (SSS)

Water column

First return from seabed

interferences target

Introduction to MultiBeam Echo Sounder

(MBES)

• MBES are used to measure multiple depths from one transducer array

• Instead of transmitting and receiving a single vertical beam, the MBES transmits an acoustic pulse that is then received by a listening array with small beam angles (typically 0.5° or 2°) across the ship axis.

• Typically achieving 400-800 or more depth measurements each ping, imaging a wide swath up to 10 times the water depth.

• MBES are used in almost every branch of hydrographic survey: dredging / oil & gas / pipeline / telecommunication / nautical charting / construction / marine science

MBES Basic Concepts

Introduction to MultiBeam Echosounder (MBES)

Installation & Mounting Installation and configuration

@ Anzidei et all. 2015

MBES acoustic principles Introduction to MultiBeam Echosounder (MBES)

1. Multibeam transducers are typically based on Cross-fan geometry made of a Transmit

array and a receive array in a “L” or “T” configuration

2. Each array consists of multiple identical transducers elements, equally spaced in a line.

3. Each array produced a flattened main lobe which is very narrow in the arrays long

axis

4. The intersection of the two flattened main lobes results in a narrow beam.

5. Every “firing” or transmission of the array is called a “ping”.

@ Hell, 2011 Radio Holland and Kongsberg mltibeam seminar 2000

MBES Acoustic Timeline

1. The transducer emit a Ping and the time is recorded (1 to 50 Hz)

2. The acoustic signal spread in the water column and it gets reflected by the seabed and

other obstacles. Reflected sounds head back toward the transducer.

3. The receiver antenna within the MBES receives the reflected signal and the time is

recorded. The Two-Way-Travel-Time (TWTT) is calculated.

4. The intersection between the transmitted and received signal Is calculated and the

“BEAM” is formed

5. A series of additional corrections are then applied to correct for motion, offsets, sound

velocity in the water column

Introduction to MultiBeam Echosounder (MBES)

System types

Deep water systems

• Typically 12 kHz for deep ocean and 30 kHz for

continental slopes

• Regional mapping. Swath > 10 km

• Large arrays, limited to large deep-sea vessels

Shallow-water systems

• Typically 40-200 kHz

• Mapping continental shelves. Swath 1-10 km

• Best suited to hydrography

High-resolution systems

• Typically 300-500 kHz

• High-resolution imaging. Swath < 500m

• Hydrography, shipwreck location, inspections

• Small sizes suitable for deployment on ROVs and AUVs

Example of MBES array available from leading manufacturer

Introduction to MultiBeam Echosounder (MBES)

Cross survey platform management

RV Geo (GSI)RV Lir (GSI)RV Keary (GSI)RV Mallet (GSI)RV Celtic Voyager (MI)

Inshore mappingShelf mapping

30 Nm from Coast 20 Nm from Coast

Consideration: Swath shrinks and resolution increases with shallower water depths.

Survey planning

Coverage and Density Requirements

• Survey design and settings are key to meet coverage and data density targets.

• INFOMAR data meet IHO S-44 1a standard

• INFOMAR surveys follow roughly 20% overlap, achieving at least 100% coverage

Factors affecting coverage / density

Sensor specific Environmental

Swath width/beam angle capacity Depth

Pulse type (CW/FM) Bottom complexity/topography

MBES range/power Seabed composition

Single/Multi ping Weather state

Vessel speed Oceanographic / sound velocity complexity

Survey planning

Across and Along-track Resolution

• Across track resolution controlled by:

No. of beams, angular sector, beam width and beam spacing.

• Along track resolution controlled by:

Vessel speed and ping rate.

V e

ss e

l D

Ir e

ct io

Survey planning

Beam Angle and Frequency Factors Survey planning

• Beam width and acoustic frequency directly impact achievable MBES resolution

• Image shows the same seafloor imaged with 2 different systems

.E . H

u g h

e s

C la

rk e , O

M G

/ U

N B

• One of the primary advantages of modern MBES is their ability to simultaneously

acquire 3 key and precisely georeferenced datasets:

MBES Primary Outputs - Bathymetry

Bathymetry: simply the measurement of depth of water in oceans, seas, or lakes.

Bathymetry is then used and visualized in multiple ways based on the required application

(maps, contours, shaded reliefs, 3D, VR, point cloud etc.)

Introduction to MultiBeam Echosounder (MBES)

Backscatter: is the reflection of the acoustic signal back in the direction from where it

originated.. Harder bottom types (like rock) reflect more sound than softer bottom types

(like mud), and smoother bottom types (like pavement) reflect more sound than bumpier

bottom types (like coral reef)

MBES Primary Outputs - Backscatter

Combining bathymetry, backscatter, sediment sample and ecological data allows us to

create very detailed maps of the sea floor, sediment and habitats distribution.

These information are used for multiple purposes, including marine ecosystem

protection, coastal hazard preparedness, and navigational safety

Introduction to MultiBeam Echosounder (MBES)

Multi Spectral Backscatter

• Multi-frequency allows for additional discrimination of seabed properties.

• Particularly useful for bottom classification since materials can react differently to

different frequencies

Sediment bank observed on the Celtic Sea

Multispectral Backscatter- EM2040 (200kHz) EM302 (30kHz) 95kHz and EM1002 (95kHz)

Introduction to MultiBeam Echosounder (MBES)

Water column: it is a form of acoustic remote sensing that is used to explore aspects of

the marine environment that are found between the ocean surface and the sea floor.

MBES primary outputs – water column

Applications

• Imaging of mid water offshore infrastructures, shipwrecks

• Map underwater gas seeps, plumes

• Mapping seaweed resources

• Study fish behavior and dynamics

• Monitor oceanography, internal waves

• Shipwreck safety clearance – masts etc.

Introduction to MultiBeam Echosounder (MBES)

Introduction to Sub-bottom Profiling • Sub bottom profilers use acoustic signals reflected from interfaces to investigate

the seabed and shallow sub-surface.

• Geological horizons, sediment thickness, buried objects and hazards can be

identified

Introduction to Sub-bottom Profiling (SBP)

Sub-Bottom

Profiler Type

Frequency Depth of

Penetration

Resolution

Chirp 3 – 40 kHz < 100 m 0.05 m

Pinger 3.5 – 9 kHz 10-50 m 0.2 m

Boomer 500 Hz – 5kHz 30 – 100 m 0.3 – 1 m

Sparker 50 Hz – 4kHz 500 m > 2 m

Lower frequency – more penetration Lower frequency means a larger transducer Higher frequency- high resolution

Instrumentation Introduction to Sub-bottom Profiling (SBP)

Sub-bottom Profiling

• Sound from transmitter travels down to seafloor

• Some of the acoustic energy penetrates the seabed,

and can reflect off layers of material within, before

travelling back to receiver

• Time taken can be used to measure the thickness

of the layers, along with information about the

composition of layers

Identify and measure various sediment layers beneath

the sediment/water interface

Introduction to Sub-bottom Profiling (SBP)

• Locate object on or in sea floor – Pipelines

– Cables

• Map, measure and classify sediment layers

• Locate and map depth to bedrock

• Locate and map

possible hazards in area – Faults

– Shallow gas

Uses of Sub-bottom Profilers Introduction to Sub-bottom Profiling (SBP)

Artefacts in Data- Multiples

• Multiples: the high energy

acoustic wave reflecting

off the seabed, the sea

surface, and then again off

seafloor, before being

detected by transceiver.

• Results in a mirror image

of seabed at double the

depth, at twice the slope

Multiples

Multiples can obscure

data, especially in shallow

water

Introduction to Sub Bottom Profiling

Sub Bottom Profiler Case Study 1

• Celtic Sea 2017 INFOMAR

• RV Celtic Explorer

• iXblue Chirp Echoes 3500 T7

• FM 1.7 to 5.5 kHz (c3.5

kHz)

• 20 m penetration

Introduction to Sub Bottom Profiling

Sub Bottom Profiler Case Study 2

• AMETS 2008 SEAI

• Cable Route Survey

• RV Celtic Voyager

• Pinger source @ 3.5 kHz

• Coda acquisition & DP

• Depth to bedrock output

Introduction to Sub Bottom Profiling

Sub Bottom Profiler Case Study 3

• Malin Sea 2005 INSS

• Regional Mapping

• RV Celtic Explorer

• Pinger source @ 3.5 kHz

• Coda acquisition & DP

• Bedrock Isopach output

Sub Bottom Profiler Case Study 4

Sub Bottom Profiler Case Study 5

Palaeochannel, Codling Bank area, Irish Sea

• RV Celtic

Explorer

• Pinger source

@ 3.5 kHz

Introduction to Sparker

Geo-Source 400 Sparker

• Source, hydrophone, power

supply, topside

• Towed system

• Water depth: 2 – 1000 m

• Penetration: 400 ms

• Resolution: 30 cm

Introduction to Sparker

Sparker Case Study

• Irish Sea 2009 UCC GAEL

• Codling & Lamb Bay Deep

• RV Celtic Voyager

• Sparker source

• Coda acquisition & DP

• Sub bottom features

Introduction to Sparker Method

Codling Deep

Lambbay Area

Reflector

Shallow Gas

Mud

Seabed Multiple

Reflectors

Sediment Waves

Seabed Multiple

Reflectors

Introduction to Magnetic Method Magnetic pole reversals

Introduction to Magnetic Method Total Magnetic Field June 2014

Introduction to Magnetic Method

Applications of Magnetic method

1. Exploration of magnetic ores and iron.

2. Archaeological investigations.

3. Petroleum exploration in investigations on thickness

of sediments

4. Location of construction materials such as granite,

basalt and other building stones.

5. Geological investigations such as locating buried

dykes and faults.

6. Searching for buried munitions.

Introduction to Magnetic Method

Magnetic anomaly

produced by wreck

Multibeam image of

associated wreck

(INFOMAR 2017)

Introduction to Magnetic Method

INSS Case Study

Gravity & Magnetic Methods

Magnetic versus Gravity methods

1. Both techniques are passive.

2. Gravity field is always perpendicular to the Earth whereas the

magnetic field varies widely.

3. Earth’s magnetic field is less stable than it’s gravity field and can

quickly change.

4. Magnetic maps are dominated by local anomalies where as

gravity maps tend to show regional anomalies.

5. Gravity field is monopolar (always attracts) in contrast to

magnetic field which is dipolar.

6. Magnetisation of rocks differs much more greatly than density

differences in rocks.

7. Large areas can be covered more quickly using magnetic

surveys as measurements are taken very rapidly.

8. Gravimeter is a relative instrument, magnetometer is absolute.

Introduction to Gravity

Gravity meters measure the gravitational acceleration of

the Earth at a specific location.

1. Absolute Gravity Meters measure the local gravity

in absolute units (Gal). It works by directly

measuring the acceleration of a mass in freefall in

a vacuum.

2. Relative Gravity Meters measure the change in the

length of a spring carrying a fixed mass.

Sea III Marine Gravity System

MICROgLACOSTE

Introduction to Gravity

Applications of Gravity method

1. Determine shape of the Earth

2. Hydrocarbon exploration

3. Regional geological studies

4. Iso-static compensation determination

5. Engineering applications

6. Determination of glacier thickness

7. Tidal oscillations

8. Basin Geometry

9. Detection of sub-surface cavities

Introduction to Gravity

Type Rock Density

Unconsolidated Sand 1400 – 1650 kg / m3

Sedimentary Salt 2100 – 2600 kg / m3

Limestone 2000 – 2700 kg / m3

Shale 2000 – 2700 kg / m3

Igneous Granite 2500 – 2800 kg / m3

Basalt 2700 – 3000 kg / m3

Metamorphic Quartzite 2600 – 2700 kg / m3

Gneiss 2600 – 3000 kg / m3

Galena 7400 – 7600 kg / m3

Pyrite 4900 – 5200 kg / m3

Magnetite 4900 – 5300 kg / m3

Typical rock densities

Introduction to Gravity Satellite Derived Ocean Gravity Anomaly Map.

• 5 km resolution

• Produced by satellite orbit changes

• Sea height influenced by gravity

• Ridges, seamounts, troughs &

trenches

• ESA CryoSat-2

Introduction to Gravity Method

INSS Case Study

Introduction to Gravity Method

INSS Case Study

Cheers! For more information, visit

infomar.ie or gsi.ie

Any questions? aileenbohan@gsi.ie