Some Limitations of Shallow Water Geophysical Devices Imposed by Different Oceanographic and

Geological Conditions Field Observations

César A. Félix, Thiago S. Martins, Camila H. C. Soares, Vanessa Guesser, Larissa F. Demarco, Beau

Suthard, Rodrigo C. Barletta, Jorge A. G. Souza Environmental & Infrastructure

CB&I Florianópolis, Brazil

Cesar.felix@CBI.com

Michel M. de Mahiques Dept. of Physical, Chemical and Geological Oceanography

IO-USP São Paulo, Brazil

Abstract— Applied shallow water seismic-reflection methods are an important component in designing engineering projects to address navigability, port construction and dredging activities, among others. The more these techniques are applied, the more that is known about the environment in question resulting in an increased potential to minimize costs. It is crucial to choose the correct techniques to get the desired responses, since each device has its own frequencies and operational requirements. When site-specific physical parameters and characteristics are taken into consideration when planning survey operations, high quality data can be acquired. However, this study presents some limitations of shallow water geophysical devices imposed by different oceanographic and geological conditions. Examples of a Side Scan Sonar and sub-bottom profilers were taken from four (4) survey sites along the Brazilian territory: (1) a ria-like coastal environment along the southern coast of Rio de Janeiro State; (2) the coast of Bahia State; (3) a rocky river in the central region of the country; and (4) a macro tide-dominated estuary in the Northeast. The equipment was selected and operated with specific project goals in mind including: definition of stratigraphy, depth to crystalline basement, and sediment thickness. The first site is an example of classic shallow sub-seafloor gas occurrences off the coast of Rio de Janeiro, where those gas accumulations were mapped with an integrated system consisting of a boomer (0.5 – 2.0), chirp (2.0 – 8.0) and a pinger (24 kHz), with the seismic signal being limited by strong reflections that caused phase inversions. In a similar situation, a boomer survey was conducted off the coast of Bahia State to define the depth of consolidated rock, although the signal penetration was limited by a layer of soft sediments. This layer was sampled with percussion cores and revealed very soft clay content that the cores easily penetrated. This layer also caused phase inversion in the seismic data, not very common in sediments without gas accumulations. The third site is a narrow river in which the boomer source was employed. The presence of solid rock, plus the characteristics of this impulsive source, resulted in a strong sideswipe effect, which can cause some

confusion in the interpretation process. The last site is known to have high current speeds due to the tidal amplitude. The estuary was surveyed with a Side Scan Sonar operating with dual, simultaneous frequencies of 300 and 600 kHz. The high frequency swath was dispersed and attenuated within in the water column in a way that the signal could not reach the bottom, thus resulting in no acquisition at all. The 300 kHz was used without any issue; water samples (Niskin Bottle) were taken which demonstrated a high concentration of particulate material on the water column. Along with the Side Scan, a chirp survey was conducted operating in a frequency range of 0.5 – 8.0 kHz. The chirp signal penetration was seriously degraded by a thin layer of consolidated clay, verified through surficial samples where the clay outcropped. These examples were selected to demonstrate the importance of choosing the appropriate geophysical system to achieve individual project goals at highly-variable site-specific locations However, it is important to note that there are some specific environmental conditions that limit the effectiveness of some of these methods, but that may be used for the benefit of the interpreter, depending on its particular attributes.

Keywords— Seismics; artifacts.

I. INTRODUCTION It is not uncommon to find examples in the technical

literature about the use of acoustic sources in certain projects based on the availability of the equipment, instead of the equipment’s true applicability to the project objectives [1].

It is commonly accepted that the selection of geophysical sources based on project goals is the best way to achieve success in engineering projects and quaternary research [2], [3], [1], [4].Although, while not always the case, sometimes in very shallow waters, areas with rocky outcrops or abrupt seafloor bathymetry, it is not advisable to use transducers or

IEEE/OES RIO Acoustics 2013

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heavy towed equipment within the water column due to the risk of losing or damaging the equipment.

In this study, examples from four sites were taken, all with water depths below 30 m, in which it is believed that the correct geophysical devices were used, however, specific conditions at each site affected the performance of the equipment. Even though undesirable, some of these artifacts and environmental impacts may prove beneficial by assisting the researcher with their geologic interpretations.

The main objective of this paper is not to explain the geologic formations within each project area, but to show the resulting artifacts and environmental impacts on the geophysical data, and using them as field observations, assist interpreters who might be dealing with similar issues.

II. METHODOLOGY In the following section, each example project will be

described in terms of the project goals, the geophysical equipment selected for each survey, and the reasoning behind the selection of the specific geophysical systems.

A. Site 1 – Ría do Mamanguá. The seismic survey at Ría of Mamanguá was carried out in

order to better understand the geologic setting and evolution of Brazilian Rías. Therefore, a multi-mode seismic-reflection system was employed consisting of a towed boomer with an Energos power supply (0.5 - 2.0 kHz), a towed 2.0 - 8.0 kHz chirp and a side/pole mounted 24 kHz pinger.

Ten Vibracores were taken throughout the survey area in an effort to identify the stratigraphy and deposition rates.

B. Site 2 – Bahia state coastal region A 100 J boomer was employed as the main seismic-

reflection system in order to map the foundational geology available for infrastructure engineering. Hence, the boomer was operated at full power and with larger intervals of 0.5 s between shots.

A dense coverage of Standard Penetration Tests were also acquired to corroborate with the geophysical survey.

C. Site 3 – Rocky fluvial environment This survey consisted of a boomer seismic-reflection

system used to map the depth of bedrock and sediment thickness for navigational purposes. Bathymetry data with an Odom MKIII single-beam echosounder was also collected.

The use of the 100 J C-Boom as the main seismic source for this environment is explained by two factors: (1) there’s no need for high-resolution definition of the sediment layers, or for further analysis of the strata; the only purpose of the study was to reach bedrock, with a seismic tool that also allows inferences of the sediment nature and defines it’s thickness; (2) Navigation can be very complicated in this area due to the fact that the river is very narrow (300 m), and the bedrock’s depth is highly variable and abrupt. Thus the use of a light-weight, surface-towed system, like the C-Boom system, was the most appropriate under these conditions.

Percussion cores with Standard Penetration Tests and rotary rock borings were taken to investigate the nature of the rocky basement and overlying sediment layers.

D. Site 4 – Macro tidal estuary At this site, the seismic survey was carried out in support

of a dredging and navigation project. Normally, in order to dredge a navigation channel, one only needs to know the sediment thickness between the current seafloor and the depth of interest, and some chemical characteristics of the strata. In the case of this survey, however, there was a secondary objective to locate coarse sediment for use as hydraulic fill.

An impulsive source, like the boomer, operating simultaneously with a resonant source could provide good results for the study. However, the operation of a surface towed system in 12 knot currents, which is common for this project site, can be difficult and may also result in positioning problems. In addition to this, a > 2.0 kHz resonant source could have problems penetrating coarse-grained sediments, as a result, the boomer survey might not be able to define precisely the texture of the layer to be dredged.

Therefore, an EdgeTech SB 512i chirp system operating in a frequency range of 0.5 – 8.0 kHz was employed. This seismic source is uniquely suited for this project, as the chirp signal provides a higher signal to noise ratio, and consequently, better definition of the sediment layers; and the frequency range starting below 2.0 kHz results in deeper penetration in coarser sediments when compared to other chirp systems.

This source, however, has a significant drawback due to its large size and weight, which requires larger survey boats and winches. On the other hand, in this case it gave the towfish more stability when operating in such tough environmental conditions.

Vibracores were also collected throughout the project area in order to cross-correlate the seismic results.

Sidescan sonar (SSS) data was also acquired as part of this survey in order to identify and map different seafloor sediment textures, possible rocky outcrops and surficial sandy deposits. An EdgeTech dual frequency (300/600 kHz) system with multi-pulse chirp wave technology was employed. The chirp waves provide higher signal/noise ratio and make the data less susceptible to environmental noises, and heave, pitch, roll and yaw influence.

Due to the high speed currents, a depressor wing was attached to the SSS towfish in order to increase its stability. The navigation speed was maintained between 4 and 5 knots.

Water samples were collected in the study area using a Niskin Bottle sampler. Samples were collected at eight locations and at three depth levels (surface, middle and bottom). Surficial sediment samples were also taken with a Van Veen sampler in areas with different texture and relief on the side scan mosaic.

Fig. 1. Example of boomer profile acquired in Ría do Mamanguá (B). Where the red dashed trace is shown on “A” with detail to the normal(Seafloor – SF) and phase inverted reflection (Acoustic Blanking - AB).

III. RESULTS AND DISCUSSION

A. Site 1 – Ría do Mamanguá. In Ría do Mamanguá, the results of the selected seismic-

reflection methods and systems were very limited by the accumulation of gas in sedimentary packages. This phenomenon is becoming better understood over the past decades and is very important from an economic standpoint, since sediment-laden gas also mean uneven surfaces for coastal works. In this context, some papers stand out, such as [5] and [6] and for construction [7]. In the Southeastern coast, [4] uses three distinct seismic sources to explore the effect of this phenomenon on seismic waves and its significance for the evolution of a similar environment.

The Ría do Mamanguá is a funnel-shaped environment in plain view with maximum width, at their mouths, of 1.4 and 2.6 km, respectively. It can also be classified as Tectonic Ría [8], where the adjacent emerged lands are formed by Pre-Cambrian granites and migmatites, with very strong control by three main systems of faults and fractures (SW-NE and SE-NW), which control the general arrangement of the coastline and other geomorphological features. The water depth can reach up to 20 m (Mahiques, pers. com.).

This geological setting is very similar to the Galician Rías studied by several researchers using shallow seismic to determine gas facies (e.g. [9] [10]) and source of the shallow gas accumulations (e.g. [11]).

The emerged areas are covered by an extensive vegetation of the tropical rainforest and the inner parts are characterized by the development of mangroves. The Ría has High sedimentation rates (about 60 cm.kyr-1) (Mahiques, pers. com.).

All these factors support the potential for high levels of shallow gas production in the geology of the Ría do Mamanguá.

The seismograms derived from the source chirp cannot detect phase reversal, since part of the system’s pre-processing is the rectification of the signal (wavelet transformation of negative to positive to achieve a higher signal / noise). On the other hand, it is clearly identified by trace analysis in the boomer profiles.

On Fig. 1, the typical polarity of a seafloor reflection is shown as a symmetric wavelet [12]. Where it has an initial negative peak, followed by a positive peak with a higher amplitude and then a second negative peak practically identical to the first.

In the seismograms, clumps of gas layers are extremely impactful, since the signal is dispersed and/or reflected almost completely, preventing the penetration of seismic waves and the consequent failure to identify deeper stratigraphy. The high amplitude and chaotic reflections of gas deposits require special attention because they can be misinterpreted as consolidated substrates, since they can have great similarity with reflections of rocks and coral reefs. The fact that gas accumulated in the interstices of the sediments are less dense than terrigenous sedimentary packages, or even if compared to the water column, it causes the polarity inversion and acoustic multiples/artifacts with the same phase. For more details about these phenomena caused by gas in seismic profiles acquired in shallow water see [13] and [4].

Gas accumulations are very common when surveying in enclosed and low dynamic environments, such as bays, estuaries or rias. It is known that sediments with gas content represent risks for coastal engineering structures due to the

Fig. 2. Boomer profile with an example of the reflection pattern in question (upper profile). In the lower profile, it’s shown a detail of part of the upper profile withits color pallet expressed in RMS.

potential instability of these sediments [7]. There is, however, little concern about the effects of this kind of gas returning to the atmosphere. Onshore seepage is considered an important source of methane, which can contribute with ton of CH4 per year [14]. Such a large contribution can represent around 20% of the total geologic factors [15]. This phenomenon typically happens naturally through seepage, but can also be aggravated by anthropogenic factors such as dredging activities.

B. Site 2 – Bahia state coastal region Along the coast of Bahia, a diffuse reflector appeared as

flecks on the boomer profiles. , The reflector had practically the same depth below the seafloor along the survey line (9 m). These reflections had an abrupt stop at an exact point with the same distance seaward on parallel boomer profiles.

The geometry of the reflector was so erratic and diffused that it was very difficult to define. This reflection showed two specific characteristics, which are different than what to expect from subsurface gas accumulation known as acoustic blanking (or gas blanket). Acoustic blanking is where the seismic signal is completely obliterated and the seismic wavelet (at the point of obliteration) results in a polarity inversion. The polarity inversion becomes difficult to see, or nonexistent, where the reflector becomes difficult to see or follow along the profile.

Although this reflection pattern has similarities with acoustic blanking, it has also two characteristics which differ from shallow gas reflections. In shallow gas reflections, the amplitude of the subsequent gassy reflector (less than 10 m

apart) is normally 1.5 to 2 times the sea bottom reflector [16] [6]. The relative amplitude of the Bahia reflector is much lower than the amplitude of the sea bottom. The other difference between this reflection and the ones obtained from gassy sediments comes from the very diffuse and irregular shape of the Bahia horizon. Typically, gassy sediments provide a very well-marked, horizontal and strong reflection, due to the almost complete reflection of the seismic signal on the top of the gas trapping layer. This is not evident at all on the Bahia reflector.

Percussion cores were taken that showed a downward change in the geology. The upper stratigraphic unit, a sandy mud, took a moderate amount of percussion hits to penetrate the strata, whereas immediately below that, a soft clay deposit was encountered which took very few percussion hits to penetrate before finally reaching the hard rock, reef unit that represented seismic basement.

In Fig. 2, the lighter green color of the core on the right means finer-grained sediments with some sand, the darker green core on the left is a clay material with shell fragments, which is keeping the seismic signal from propagating. The orange bottom of both cores is reef.

As shown in Fig. 2, the darker spots are points where amplitude RMS is higher and have a random occurrence. The diffuse reflections indicate that the energy is dissipating / attenuating through the sediment layer as they encounter greater concentrations of these bright spots. Therefore, it is likely that these darker spots represent random reflections of the shell and/or reef fragments found within the core samples,

Fig. 3. Boomer profile affected by sideswipe reflections, artifacts indicated by circles (vertical scale 10 millisecons).

and that the increase in spots, are due to an increase in the concentration of these rock and shell fragments, which then increase the seismic signal attenuation, resulting in a diffuse reflector as seen in Bahia [17]. This kind of reflector, shown in this area, has to be seriously considered and evaluated when investigating areas for engineering foundations, due to the potential instability of the sediment unit under this diffuse and chaotic reflector.

C. Site 3 – Rocky fluvial environment The fluvial environment in this third example is located in

northern Brazil, in the regional geologic context of a gneissic-migmatitic area, above a low-grade metamorphic Vulcan-sedimentary sequence [18]. Although it is thousands of kilometers long, the river has a low suspended sediment load due to the fact that the river drains mainly cratonic areas, has multiple hydroelectric dam facilities upstream and is located in a tropical climate region [19].

In the upstream portion of the river, some cores were acquired by geotechnical drill, percussive and rotary, drilled between 15 until 25m down. Those cores discovered basalts, gneiss (leucocratic and melanocratic) and metasedimentary rocks (as metasandstones, metasiltstones and metamudstones), the both latters as the most representative. From the firsts centimeters on the top until 1.80m, of those cores, were revealed sandy sediments.

The alluvial deposits of the river are composed of Quaternary sediments (unconsolidated sands) [20]. Though the river system is classified as dominated by sandy deposits, pelitic material (silt and clay) are present in floodplains [21].

Although there is some sediment covering bedrock at specific locations, the boomer seismic-reflection records corroborate the general absence of sedimentary layers between the Holocene deposits and Proterozoic rocks in most of the survey area.

A sideswipe is an artifact reflection event originated at a point outside of the vertical plane of the two dimensional seismic survey line [22]. This is due to the fact that sound radiates away from the seismic sound source in three dimensions, even though the seismic-reflection system is mapping in only two dimensions. As some of this radiated sound encounters geology out of plane, it is reflected back to

the seismic system and mapped as a sideswipe, out-of-plan artifact on the seismic-reflection profile.

This out-of-plane reflection is very common on deep seismic surveys, due to the fact that the pulses are generally broader and stronger than the smaller systems used in shallow water investigations. Therefore, the chances for the seismic signal to be reflected from a side target and return to the hydrophones is bigger in deep seismic survey applications.

As shown in Fig. 3, it might get difficult to define which is the real reflector, and which is the sideswipe artifact, as the sedimentary reflections and artifacts cut through each other on the profile.

In this example, the sideswipe was caused by side reflections from the highly-variable rocky basement close to, but not directly under, the sound source. Where the rocky basement is flat lying and less variable, there is little to no sideswipe artifacts present.

The high contrast response in acoustic impedance between the sediments and the crystalline basement, allied with its rough geometry could also accentuate sideswipe [23].

In this example, the subsequent apparent reflections (artifacts) caused by sideswipe were strong and continuous. Sometimes it could be mistaken as the bottom of the river, this was due to the fact that most of the project site did not have any sediment cover at all, only the interface between rocky basement and water. Hence, the strong ghost and horizontal reflections could be misinterpreted as sandy deposits, because of the similarity between the real and the sideswipe reflector.

The best indication to identify these patterns as non-geological artifacts is the fact that sometimes the sideswipe artifacts cut through or cross the rocky basement (or other real stratigraphic reflectors), which, obviously, cannot occur in nature, and therefore, can easily be identified as an artifact.

D. Site 4 – Macro tide estuary

Seismic survey results Sandy sediments are transported mainly by rolling and

saltation, and clay sediments are transported by suspension. The deposition of fine-grained sediments occurs with the decrease of the speed of the river waters, which occurs by

Fig. 4. Side scan sonar mosaic showing the transition between fine-grained sediments and consolidated clay (left). On the right, is chirp seismogram inwhich the consolidated clay layer outcrops. Where “CC” means consolidated clay. The white line shows the cited transition on the same point at the mosaicand the seismogram.

decantation and / or flocculation. Flocculation happens when ions on the surface of the clay sediments are neutralized when adsorved with others. The salinity of the sea water would act as the principal agent in the flocculation process in the area providing an increase in the deposition of clay sediments.

These flakes have a hydraulic behavior which is similar to larger particle sizes that deposit more quickly. The binding between these deposited flakes causes them to acquire greater resistance to flow, making it less susceptible to reworking and suspension on the aqueous medium.

The origin of this material is related to the sedimentary clay composition of the soil and rocks of the source area. The São Luís Basin is composed of basalt, metamorphosed sandstone and pelitic rocks, plus sandstones, shales, siltstones, conglomerates, diamictites, evaporites and carbonates [24], a rich source for minerals such as olivine, pyroxene, mica, chlorite, illite and smectite, which are minerals that are present in these source rocks, which may form this clay due to the action of weathering and erosion processes [25].

The origin and preservation of clay to sandy-clay sedimentary layers are related to intervals of varying-energy events within the depositional system. These events are typically related to the migration of the main river channel or activation of ancient rivers of the fluvial system, bringing the deposits of the floodplain to the active river system, which are basically finer sediments deposited when the channel overflows.

The clays of the Tabatinga formation are unique in that they do not disperse or absorb the energy of the seismic waves (like the Bahia state example), they reflect the seismic signal almost completely. This acoustic facies is very similar to acoustic blankets [16] or other highly concentrated gassy sediments.

Usually, this kind of fine-grained deposition would contribute to the passage of the seismic waves, with more penetration of the seismic signal. However, these clays are part of a semi-diurnal macrotidal environment. This implies that semi-diurnal tides up to 8 m amplitude act as a hammer, at least two times a day, on the bottom sediments and subsequent layers The flocculated clays submitted to this hammering process over and over throughout the years may become a

highly compacted impermeable sediment layer (even to the seismic waves).

As showed in Fig. 4 (right), this unit is synchronic with mud deposition, but its texture is very dissimilar. The 300 kHz side scan sonar on Fig. 4 (left) illustrate this difference, where the consolidated clays have clearly a rougher texture compared to adjacent fine-grained deposition, which has low backscatter and appears as dark plain patterns on the side scan.

The vibracore samples show a cemented layer composed of clayey mud, sometimes below phaser like depositions with sandy lens, which contribute to the intrinsic influence of this macrotidal dominated environment to consolidate the muddy layer.

Side Scan Sonar survey results This case shows the effects of suspended sediments

associated with high-velocity water currents on the performance of a high-resolution side scan sonar (SSS). The site where the survey took place has a macrotidal regime (about 6.5 m of variation). This generates strong currents that can reach up to 6 knots. When the direction of navigation was against the current, the speed of the boat was limited to 2 knots.

The water samples were analyzed to quantify the suspended particulate material (SPM). The results show high concentrations increasing with the depth. The mean concentration at each level was 276.2625 mg/L at surface, 982.4250 at middle and 1741.875 at bottom.

The result of a high current speed plus the high SPM meant the degradation of the SSS signal through the water column, most notably in the in higher frequencies (600 kHz). The Fig. 5 shows an example of high frequency (600 kHz) and low frequency (300 kHz) sonograms at top and bottom, respectively.

For this example, the high frequency SSS signal is attenuated and dispersed by the high concentration of SPM in the water column, preventing the high-frequency SSS signal from reaching the seafloor. In the low frequency image, the water column displays as a translucent dark brown color, but can be easily distinguished from the seafloor. However in the

Fig. 5. Sidescan sonar profiles obtained from high (600 kHz) and low(300 kHz) frequencies, top and bottom, respectively. The white arrowsindicate the same position in the both sonograms. The data was acquiredsimultaneously (vertical lines are 25 m scale).

high frequency image, the water column is a solid brown color, and is practically indistinguishable from seafloor.

The low frequency does show the discrete backscatter differences of the varying sediment textures of the seafloor, which cannot be seen with the high frequency due to the attenuation and dispersion of the high-frequency signal by the SPM in the water column. The low frequency signal seems to pass less affected by the SPM, most likely due to the lower frequency wave lengths of the signal (300 kHz). It is important to note that the lower frequency SSS results in a lower resolution image which, in this case was 60 cm along track and

2.47 cm across track. Based on the experiences of this survey, high concentrations of SPM have a negative impact on the mapping capabilities of side scan sonar systems, especially systems with higher frequency signals.

In summary, the field experiences described in this paper can be summarized as shown in table 1:

TABLE I. SUMMARY OF THE SURVEY SITES IN THIS PAPER AND SOME OBSERVED PHENOMENA ON SEISMIC SIGNALS. WHERE: “NA” – NOT

AVAILABLE.

Parameters Study sites

Site 1 Site 2 Site 3 Site 4

Main Seismic Source

300 J

Boomer

100 J

Boomer

100 J

Boomer

Low

Frequency

Chirp

Polarity inversion

Yes Yes No NA

Impedes Signal

Propagation Yes Yes NA

Yes

(generally)

Relative Amplitude

High

Low with

higher

amplitude

scatters

High Medium

Stratigraphic component

Gassy

sediments

Soft Mud

with reef

fragments

Rocky

basement

Consolidat

ed Clay

Interpretable facies

yes yes No yes

IV. CONCLUSION Even when the right geophysical tools are employed, some

environmental particularities can lower the quality of the data, although, it also can provide a reliable information for interpretation.

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