1. Measuring beach thickness using the passive seismic technique
David Morgan and David Gunn
British Geological Survey
Southeast Regional Coastal Monitoring Programme
Annual Partners’ Meeting 2018

2. Survey Rationale
Determine the optimal instrumental methodology for the passive seismic technique in different beach environments.
Beach environments
Superficial (beach material)
Bedrock geology
Beach profile (low angle/high angle)
Key objectives of the study are to:
1. Define the interface between beach material and underlying geology/ basal layer
2. Determine the composition and variability of sub-surface beach material
3. Provide information on sub-surface geology down to 15 m below beach surface
4. Determine repeatability
5. Evaluate the best possible vertical resolution of measurements in order to determine beach thickness and base of beach

3. Passive Seismic Method What is it?
Tromino looks at the ratio between horizontal (H) and vertical (V) components of seismic ambient noise
Simple relationship between frequency, Vs and depth of layer
f0 = Vs/4H
Peaks in H/V correspond to changes in impedance
Gives a profile of velocity changes with depth
Low frequencies – tides and weather (bad weather gives better data!)
High frequencies of anthropogenic origin
Tromino response from <10m to 00’s m

4. Data Acquisition

5. Data processing
8 seconds
12 minutes

6. Medium depth response (~ 20 m)
Data interpretation
Medium depth response (~ 20 m)
Deep response (~ 200m)
Shallow response (~ 1m)
Calculated depths use H = Vs/4f equation
Depends on assumptions about seismic velocity

7. Creating a synthetic cross-section
Red Colours: H:V ratio > 3
Blue Colours: H:V ratio < 1
i.e. absence of H vibration or,
High V vibration

8. Surveys to date Chesil Beach – June. Tromino and LiDAR
Dover – July. Tromino, LiDAR, Radar and SoilSpy
Runswick Bay – August. Tromino, LiDAR and SoilSpy

9. Dover – Shakespeare Beach
East Beach
(built up by harbour mole)
West Beach
Tromino, MASW, 250 MHz GPR surveys
1. Steep to intermediate slope,
2. Fine to medium gravel beach
3. Overlying Zig Zag Chalk

10. Dover – Shakespeare Beach
Velocity of 200 m/s from MASW survey
H/V > 4 circa m  beach thickness ?
H/V > 2 circa 60 m depth: Interface in the bedrock sequence (but quite weak)

11. Soil spy/dispersion curve/velocity profile
Velocity model derived from dispersion curve
SoilSpy dispersion curve - Dover

12. Chesil - Chiswell

13. Steep; Coarse gravel and cobble beach overlying Kimmeridge Clay
Chesil - Chiswell
Steep; Coarse gravel and cobble beach overlying Kimmeridge Clay

14. Runswick Bay Tromino & SoilSpy Day 1 – Beach overlying till
Day 2 – Beach overlying bedrock
1. Shallow slope,
2. Sand and gravel (occas. cobbles) beach
3. Overlying Till and Whitby Mudstone

15. Runswick Bay ? H/V @ 2: Actual structure or artefact ?
Shallow interface with Till.
Low impedance contrast ?
Till/Mudstone or Mudstone/Ironstone interface

16. Next steps Verification of Tromino interpretation Very Shallow
PANDA penetrometer (intrusive)
Ground Penetrating RADAR – (affected by saturated layer)
Shallow to intermediate (~ 0 – 30m)
Multichannel analysis of surface waves (MASW)
Verification of Tromino Repeatability
360 (in 45o steps) tests in single location (wave influence)
Tests in same location at high and low sea levels

17. Conclusions Passive seismic method
Uses naturally occurring vibrations to investigate ground velocity structure
Tromino provides proprietary instrument and processing
Enables rapid reconnaissance surveys – useful in beach environments
Surveys undertaken for CCO
Three different beach environments tested
Steep angle, medium and coarse grained gravels
Intermediate angle, medium grained gravels
Shallow angle, mixed sand and gravel

18. Conclusions – findings
Dover – Strong evidence of interface at 6m
Suspect base of beach/top of chalk bedrock
Chesil – Variable results for beach thickness
Strong interface at ~ 40m, represents structure in bedrock – Kimmeridge Clay / Corallian?
Runswick Bay – Strong evidence of interface at shallow depth (1-2m)
Strong interface at ~ 15m probably due to Cleveland Ironstone

19. Runswick Bay data modelled at different velocities (and depth)
Shallow features can be investigated using:
CPT
MASW streamer

20. Day 2, showing approximate location of Tromino profiles.
Runswick Bay
Day 2, showing approximate location of Tromino profiles.
Sand and gravel beach over Whitby Mudstone and Mulgrave Shale (no Till observed)

21. Runswick Bay Shallow features can be investigated using: CPT
MASW streamer

22. Body vs Surface wavefields: Contribution to H/V ratio
Vibration components?
Vertical: body P-waves; vertical component of Rayleigh ellipse
Horizontal: body SH; horizontal Rayleigh ellipse h
Superficial geology or soil layer(s)
Bedrock geology
Normal
Incidence
Vertically propagating
Body waves
Alluvium (partially saturated) overlying Mudstone:
Alluvium properties: Thickness h = 30 m; S-Wave Velocity VS = 350 m.s-1; Density rS = 1.6 Mg.m-3
Mudstone properties: S-Wave Velocity VB = 950 m.s-1; Density rB = 2.25 Mg.m-3
Frequency (Hz)
Amplification
𝑘 = 2𝜋𝑓 𝑉 𝑆 .
Calculation: Based on thickness, density & shear velocity assigned to geology
Ground Motion Amplification; at selected Eigen values / frequencies
rs.Vs – assigned to superficial geology data layer via NGPD
rB.VB – to bedrock layer
GMAMax =
rB.VB
rS.VS
Bolt, 1970
fN =
(2N - 1) Vs N=1,2, 3
4.h
f1 (N = 1)
Mode 1
f2 (N = 2)
Mode 2

23. Thickness Mapping of H/V Spectra
Seht & Wohlenberg 1999
Amorosi et al. 2008
Freq (Hz)
Shallow
Deep
Shallow
Interval Times
Propagation times from different depths
Int. Time
(s)
Deep
Depth (m)

24. Chesil sites

26. Chesil – Visitor Centre

27. Chesil – Visitor Centre
1. Steep;
2. Medium to coarse gravel beach;
3. Overlying Kimmeridge Clay
Velocity of 300 m/s based on pilot study and modelling of borehole records
- Can be verified by other geophysics (MASW) on repeat survey
H/V ratio > 3 circa 60 m depth: Interface in the bedrock sequence

28. Feature Identification
Shallow Phenomenon
High Freq;
Deeper Structure
Low Freq;

29. Dover – Shakespeare Beach

30. Runswick Bay SoilSpy Dispersion Curves – Selection of Interval Velocity
Shallow interval velocity 100 – 160 m/s
Till – Mst or Mst – Ironstone interface at shallow depth

31. Field Schedule Verification of Tromino interpretation Dover
MASW: 1 m streamer:- more detailed velocity structure re: interface at 5.6 m
Invert velocity model
Runswick Bay
PANDA Penetrometer
MASW: 1 m streamer:- more detailed shallow velocity structure re: interface at 5.6 m

32. Lower Berm: which slopes down (20 degrees) to sea
Dover:- GPR Layout
Dover 23-July-2018
East beach site
Upper Berm: added aggregate fill; increasing beach height by 2.5 – 3 m?
Lower Berm: which slopes down (20 degrees) to sea
Approx. survey area.
40 m (E-W) by 24 m (N-S)

33.  40 m   24 m   Approx. 45 - 50m  Shoreline X1 (L5/5 m//L6/3m)
Up1 (L0/15 m//L1/5m)
Up2 (L2 /5m)
Up3 (L3 / 5m)
Up4 (L4/5m)
Up4 (L4/5m)
Shoulder of upper berm
 m 
LB1 (L11/3m)
LB2 (L12/3m)
 Approx m 
LB3 (L13/3m)
LB4 (L14/3m)
 Lines extended m to sea 
LB5 (L15 – ran at 44 m on tape/3m)
Just above sandy basal beach
Shoreline

34. Dover East Beach (Admiralty Pier): Upper Berm: Line 5 (5 m Depth) Shore Normal: From Upper Berm to Lower Berm (towards sea): X1 – 24 m
Metal rod at surface
North
X 1
X = 0 m
South
Upper Berm
Lower Berm
Summary Interpretation
Not to scale
Approx.
3 m
Overlapping beds
Very straight: Artefact ?
Watertable + multiples ?
Or finer material ?

35. Lower Berm: which slopes down to sea
Dover 24-July-2018
West beach site
WB1 (L16 / 3m)
Upper Berm:
WB2 (L17 / 3m)
WB3 (L18 / 3m)
Lower Berm: which slopes down to sea
Approx. survey lines.
Line 1: 300 m (E-W)
Lines 2 /3: 55 m (N-S)

36. Shingle thins over basal sand
Dover West Beach: Shore Normal Transect at 100 m (L17): From Rock armour to sea:
South
North
 1.5 m 
Shingle thins over basal sand
Sand exposed
overlapping beds