Gasperini, Luca

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China-Italy Bilateral Symposium On The Coastal Zone: Evolution And Safeguard 
4 - 8 November 2007 Bologna, ITALY 57  
GEOLOGICAL/GEOPHYSICAL STUDY OF 
SHALLOW-WATER AREAS: 
AN INTEGRATED APPROACH 
 
 
Luca Gasperini 
 
Istituto di Scienze Marine, CNR, Bologna;  luca.gasperini@ismar.cnr.it 
 
 
INTRODUCTION 
 
Due to their natural and economic importance, shallow-water 
environments, such as submerged beaches, coastal lagoons and, in general 
“wetlands”, require periodical monitoring because they are prone to ecological 
crises often related to anthropogenic impact. The study of these ecosystems 
involves a combination of investigative approaches, such as biology, physical and 
chemical oceanography, and geology. Geophysical imaging of the sediment-water 
interface and of the shallow subsurface is important for a number of applications, 
such as costal erosion, submarine earthquake geology, marine geology and 
archeology. 
We present here some case studies where high-resolution geophysical 
techniques have been successfully employed in shallow-water areas, and indicate 
some possible perspectives. 
 
 
MORPHOBATHYMETRY AND REFLECTIVITY MAPS 
 
The starting point for studies of shallow-water environments is the 
knowledge, at different levels of accuracy, of the morphology of the sediment-
water interface, i.e., the bathymetry, which is also important for describing the 
status of the system and monitoring its changes in time caused by sedimentary 
processes.  
Compilation of bathymetric map in extremely shallow-water environments 
present peculiar problems, mainly for 3 reasons 1) the average depth is often 
below the limit of conventional echosounders; 2) if these areas are in connection 
China-Italy Bilateral Symposium On The Coastal Zone: Evolution And Safeguard 
4 - 8 November 2007 Bologna, ITALY 58  
with the sea, tidal excursions might be of the same magnitude of the bathymetric 
range, and this results in potentially large errors; 3) the “soft” nature of the bottom 
favours penetration of acoustic signal, giving rise to errors in depth estimate.  
Although water-depth mapping in coastal areas can be achieved using 
satellite remote sensing techniques, conventional sonic/ultrasonic soundings are 
still more accurate, because satellite data are restricted to shallow, clear water 
areas with small changes in bottom types and free from atmospheric 
contamination. While typical multi-beam system are not effective in shallow-
waters, since their swath width is extremely limited, interferometric multi-beam 
echosounders partially overcome these problems, covering relatively large sectors 
of the seafloor (30-40 m). However, single-beam echosounders allow a more 
effective quality control on soundings, and enable reflectivity estimates of the 
sediment-water interface which can be used to infer sediment distribution 
patterns. The study of Valle Fattibello, Comacchio (Gasperini, 2005) has shown 
the potential of this type of studies. 
Valle Fattibello is part of the Valli di Comacchio, a shallow, brackish 
water, coastal lagoon system, that extends south of the Po river between 
Comacchio and the Reno River. These lagoons (valli) formed around the tenth 
Century as a consequence of subsidence and were originally fresh water basins 
supplied by river floods. Valle Fattibello is located at the northern edge of the 
lagoon system, near the town of Comacchio.  
A vertical incidence echosounder, the PSA900 manufactured by 
Datasonics, was used during the survey. This echosounder is particularly suitable 
for shallow-water environments because is characterized by a high operating 
frequency (200 kHz), a narrow (8°, conical) beam width, a short pulse length (350 
sec),  and a minimum depth range of 0.75 m. However, due to the characteristic 
of the study area, we were forced to modify the echosounder to cover the 
shallower sectors of the lagoon; the pulse length was shortened to 200 sec and 
the bottom-detection/depth-estimate section was disabled, obtaining, de facto, a 
200 kHz externally triggered ultrasonic pinger. In this way, the shallow-depth 
limit was reduced to ~0.2 m. The echosounder and the GPS antenna were 
mounted on a small catamaran towed from a flat-keel boat, equipped with the 
China-Italy Bilateral Symposium On The Coastal Zone: Evolution And Safeguard 
4 - 8 November 2007 Bologna, ITALY 59  
instruments used during the survey. The towing cable was deployed by a mobile 
arm that was used to separate the track of the catamaran relative to that of the 
boat. The tidal excursion was measured during the survey using a tide-recording 
station consisting of a mareograph, a data-logger, and a GPS receiver, which 
provided an accurate time-base.  
Bottom sediments were sampled at 47 stations using a box-corer. Samples 
were collected to a maximum subbottom depth of 17 cm and subsequently sub-
sampled at 1–2 cm intervals (Frascari et. al, 2002). Grain-size analysis involved 
an initial wet sieving of sediment samples to separate the finest (< 63 µm) from 
the coarser fractions. The finer fractions were analyzed by a X-ray sedigraph.  
The first step of the data processing was the estimate of water-depths from 
the raw echograms. The presence of soft sediments and the consequent 
penetration of the ultrasonic signal below the “soupy”, highly hydrated 
water/sediment interface, affect the accuracy of the bottom detection. In fact, 
during the acquisition of some test profiles, carried out using a conventional 
echosounder, we noted discrepancies of several tens of cm in the water-depth 
estimate of adjacent pings. If  errors of this magnitude could be neglected in 
deeper areas, they are important in such an extremely shallow environment. In 
order to avoid this effect, taking advantage of the dense spacing of the data along 
each profile (1 ping / 0.1 m, average), a trace mixing algorithm, that increased the 
lateral coherence of the bottom reflector, was applied to the data and a robus 
procedure to track the sea bottom was followed (Gasperini 2004); finally, we 
applied the tide-effect corrections using the tide-excursions time series After data 
correction, over 20,000 valid depth measures were used to compile a 
morphobathymetric map of Valle Fattibello (Figure 1). Several unexpected 
features are visible in this map, such as: a channel intersected by several small 
mounds; a sedimentary fan, in the NW sector, probably related to the loss of 
competence of the current at the lagoon entrance; a dredge cut affecting the fan; 
and other features not obviously related to sedimentary processes, such as small 
mounds and ridges flanked by sub-circular depressions. This unexpectedly uneven 
morphology might be related to human activity in Valle Fattibello, that underwent 
continuous modifications since its early formation, such as channel creation and 
China-Italy Bilateral Symposium On The Coastal Zone: Evolution And Safeguard 
4 - 8 November 2007 Bologna, ITALY 60  
maintenance. During most of these works, the material dredged from the bottom 
has been dumped partly outside and partly within the lagoon itself; currents, 
particularly in the SE sector, were not strong enough to rework these deposits.  
The acquisition of the entire echosounder sweep at each sounding point, 
rather than the simple depth estimate, gave us the opportunity to calculate the 
bottom reflectivity, and infer the distribution of sediments in the lagoon after 
“ground-truthing” the reflectivity measures with mean grain size estimates 
(Gasperini, 2005). We note that a simple working hypothesis of linear correlation 
between relative occurrence of coarse-grained sediments and normalized 
reflectivity accounts for most of the overall variability in the lagoon bottom 
sediments (Figure 1).  
 
 
Figure 1. Left: morphobathymetric map of Valle Fattibello coastal-lagoon. Right: reflectivity-strength map 
of lagoon-bottom. Reflectivity is normalized using the maximum (71.46%) and minimum 
(0.85%) occurrence of sand in the samples, and is displayed using a color palette ranging from 
red (high values) to blue (low values). The relative occurrence of sand vs. silt+clay is coded at 
each sampling station (circles) using the same red-to-blue color scale. 
 
Relating morphobathymetry and reflectivity gives us an overall picture of 
the depositional setting in Valle Fattibello. Our approach of referring the 
reflectivity data to local variability of the sediments limits generalizations, but 
greatly simplifies the interpretation of the results. In fact, the analysis of bottom 
types based on linear variations from two end-members, allows us to immediately 
translate our reflectivity images to geological features. This can be particularly 
useful in those environments where “low-energy” and “high-energy” processes 
co-exist, or are superimposed. Valle Fattibello is presently a mud-dominated, low-
energy environment. However, reflectivity and the physical properties of the 
China-Italy Bilateral Symposium On The Coastal Zone: Evolution And Safeguard 
4 - 8 November 2007 Bologna, ITALY 61  
sediment show the presence of arcuate, sand-rich features, not in equilibrium with 
the present-day sedimentary regime. An interpretation of this pattern is that the 
high-reflectivity sand-rich bands might constitute the substratum over which the 
lagoon system has been formed, in agreement with regional-scale geological 
reconstruction of the Po delta area during the Holocene.  
 
 
HIGH RESOLUTION IMAGES OF THE SUBBOTTOM 
 
Shallow-water lagoons and hinterland waters are often characterized by 
fine-grained sediments and extremely “soft” sediment-water interfaces. This 
occurrence enhances the quality of high-resolution seismic reflection profiles, 
which can be virtually multiple-free, since seismic energy is not trapped between 
water-air/water-sediment interfaces. On the other side,  is not uncommon that fine 
grained sediments of shallow-water environments are rich in organic matter and 
thus saturated by biogenic gas. High-resolution seismic refection data suffer the 
diffuse presence of gas in the sediments, because it prevents signal penetration in 
the subsurface.  
Acoustic images of the subsurface can be obtained in submerged areas 
with a variety of systems. One of the most widely used is the Chirp-sonar, a 
wideband frequency-modulated seismic reflection system able to collect acoustic 
images with vertical resolution approaching 1 cm for bandwidths exceeding 20 
kHz. This high-resolution provides detail about the sediment structure near the 
sediment-water interface and eventually allows to detect buried objects. The chirp 
sonar system transmits FM acoustic pulses to attain high pulse energy; 
consequently, the imagery has a high signal to ambient noise ratio. The images are 
normally scattering limited; that is, the subsurface depth of usable imagery is 
limited by acoustic noise scattered from inhomogeneities within the sediment 
structure. The chirp sonar has become the industry standard for generating high 
resolution images of the uppermost 10 to 100 meters of sediments in the seabed. 
In Figure 2, an example of Chirp sonar profile from a shallow (< 2 m) area is 
presented. We note that penetration of the signal reaches the ~40 meters; the high 
China-Italy Bilateral Symposium On The Coastal Zone: Evolution And Safeguard 
4 - 8 November 2007 Bologna, ITALY 62  
vertical resolution allows to analyse internal geometries of sesimostratigraphic 
units, diagnostic of tectonic and depositional processes. 
 
 
 
Figure 2. Example of high-resolution seismic reflection profile collected from a shallow (<4m) lake. We 
note reflectors dipping towards the right side of the section as a consequence of tectonic 
tilting, as well as buried shoreface deposits marking a past episode of water level low-stand. 
 
 
3-D AND PSUEDO 3-D IMAGING 
 
Although deep-towed high-resolution seismic reflection sources are widely 
diffused for surveys carried out in deep-water environments, shallow-waters 
allows to obtain high lateral resolution by using normally towed systems. This 
because shallow targets imply a narrow “Fresnel zone” and high horizontal 
resolution of seismic images; thus, it is possible to carry out close-spaced grids of 
profiles which can be used to obtain 3-D or pseudo 3-D images of the subsurface, 
which are useful to interpret geological features. A successful examples of these 
techniques is the attempt of imaging a surface buried below some tens of meters 
of mud deposited after the last sea-level rise in the Sea of Marmara. Figure 3 
shows a comparison between the seafloor morphology collected using a 
multibeam echosounder and the gradient map of the base of the Holocene in a 
China-Italy Bilateral Symposium On The Coastal Zone: Evolution And Safeguard 
4 - 8 November 2007 Bologna, ITALY 63  
shallow shelf area of the Marmara Sea, across the North Anatolian Fault trace 
(Polonia et al., 2002). The subsurface pseudo 3-D imaging, obtained by semi-
automatic picking of chirp-sonar images allows to measure the left-stepping 
strike-slip displacement of an abandoned river channel since 10,000 years before 
present, offering an accurate strain-rate estimate for this fault strand over 
geological time scales. These 3-D chirp-sonar techniques could be useful in a 
variety of fields, including archaeology, mining and environmental studies. 
 
 
Figure 3. Left: shaded relief multibeam bathymetric map (contour each 1 m, illumination from NE). 
Right: gradient map of the base of the Holocene sediments obtained through a semiautomatic 
picking of CHIRP-subbottom profiles. It images sedimentary and tectonic features below the 
holocenic drape. (A) Laplacian (gray scale) and contour lines of the Holocene base topography 
in the area where the river channel intersects the main North Anatolian Fault (NAF). (B) 
Laplacian (gray scale) and contour lines of the Holocene base topography in the area where the 
river channel intersects a southern strand of the fault. The gradient map of the Holocene base 
topography shows a meandering river channel which is displaced along the NAF fault strands. 
We calculate an offset of ~80 and ~10 m in the main and secondary strand respectively 
(modified from Polonia et al., 2004). 
 
 
CONCLUSION 
 
High-resolution geophysical techniques in shallow-water environment are 
effective in solving geological problems through an integrated approach. All these 
techniques share the use of technological advanced instruments which can be 
deployed from small boats or ROV (Remote Operating Vehicles) / AUV 
(Autonomous Underwater Vehicles) systems to collect dense-spaced grids of 
profiles. Rapidity and low-cost will be important factors for feasibility of periodic 
China-Italy Bilateral Symposium On The Coastal Zone: Evolution And Safeguard 
4 - 8 November 2007 Bologna, ITALY 64  
surveys, that will be used to take a series of “snapshots” that will describe the 
changes of the sediment-water interface in time. This would allow the 
development of predictive geological models on sediment budgets of these 
complex environments characterized by  multiple sediment sources sinks that are 
difficult to quantify. 
 
 
REFERENCES 
 
Gasperini L., Extremely shallow-water morphobathymetric surveys: the Valle 
Fattibello (Comacchio, Italy) test case. Marine Geophysical Researches (2005) 
26:97–107 DOI 10.1007/s11001-005-3710-0 
 
Polonia A, Gasperini L., Amorosi A. Bonatti E, Cagatay N, Capotondi L, Cormier 
M.-H., Gorur N., McHugh C. Seeber L. Holocene Slip rate of the North Anatolian 
Fault beneath the Sea of Marmara, EPSL, 427, 411-426, 2004 
 


GEOLOGICAL/GEOPHYSICAL STUDY OF SHALLOW-WATER AREAS: AN INTEGRATED APPROACH

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GEOLOGICAL/GEOPHYSICAL STUDY OF SHALLOW-WATER AREAS: AN INTEGRATED APPROACH

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