SAND WINNING - PHASE II REPORT

Transcription

1 ADRIATIC SEA - ABRUZZO GEOPHYSICAL SURVEY SAND WINNING - PHASE II REPORT Date: 11/08/2009 Ref: 09_053/MA/GP/RE Version Date Reason for Issue Written by Checked by Approved by 01 11/08/09 ARE GMO FRE Certified Safety VCA** G-tec S.A/N.V Geophysical Exploration Engineering Geology Rue Frumhy 34 B 4671 Blegny Belgium Tel: +32 (0) Fax: +32 (0) Certified ISO9001:2000 info@g-tec.eu Souverainestraat 58 Bus 4 B 9800 Deinze - Belgium Tel: +32 (0) Fax: +32 (0) Siège social : G-tec s.a. Avenue Belheid 6 B4900 Spa Belgique Numéro d'entreprise: Banque : ING Compte n

2 DOCUMENT CONTROL SHEET Document identification Title: Project: Client Reference: Project Manager Quotation ref. Adriatic Sea - Abruzzo Sidra 09_053/MA/GP/RE Aline Renson OF_09_086 Realization Item Realized by Supervised by Acquisition ARE JFR ARE Processing ARE JFR - PDE ARE Reporting ARE FRE Approval Version Date Written by Checked by Approved by 01 11/08/2009 ARE GMO FRE Distribution list Name # ex. Firm/authority Project function 09_053/MA/GP/RE Rev 01 Page 2 of 17

3 TABLE OF CONTENT 1. Introduction Method Statement High Resolution Seismic Reflection Introduction Principle Possibilities and Limitations Data Acquisition Nautical Means and Equipments Survey Boat Positioning and Navigation Seismic Reflection Equipments Geodetic Data Overview of the Survey Ortona Vasto Data Processing Seismic Reflection Presentation and Interpretation of the Results Ortona Vasto Conclusions _053/MA/GP/RE Rev 01 Page 3 of 17

4 INDEX OF FIGURES Figure 1.1: Location of survey area... 5 Figure 2.1: Seismic section example: sandy layer overlying bedrock... 6 Figure 2.2: Seismic section example: sediment horizons overlying rock basement... 6 Figure 2.3: SBP: principle of operation (left) and example of seismic section with outline of main reflectors (right)... 7 Figure 3.1: Survey boat Ermione... 9 Figure 3.2: AA300 Boomer and C-phone Hydrophones towed behind the boat...10 Figure 3.3: EdgeTech 3100 Chirp system...10 Figure 5.1: Seismic Profile at Ortona - Bedrock and layers of sediments...14 Figure 5.2: Seismic Profile at Ortona - Hyperbolas...14 Figure 5.3: Seismic profile at Vasto - Bedrock and layers of sediments...15 Figure 5.4: Seismic profile at Vasto - Difference in sediments...16 LIST OF DOCUMENTS 09_053/MA/GP/DRG001 09_053/MA/GP/DRG002 Ortona Seismic Reflection Results Vasto Seismic Reflection - Results 09_053/MA/GP/RE Rev 01 Page 4 of 17

5 1. INTRODUCTION G-tec s.a. has been contracted by SIDRA to carry out a geophysical campaign along the Adriatic coast of Italy. The survey concerns the location of sand borrow areas located offshore the cities of Ortona and Vasto, as shown on figure 1.1 below. Figure 1.1: Location of survey area Four areas have been surveyed during a first phase early 2009 for a total amount of 460 hectares. Based on the results of that first phase, two areas have been selected: Ortona and Vasto. However some extension areas had to be surveyed in order to correlate the results of some vibrocores that have been carried out outside the survey area of phase I. The dimensions of the areas are: Ortona: approx Hectares Vasto: approx. 220 Hectares The water depth mainly ranges from 7 to 20 meters. The main objective of the geophysical investigation is to assess the presence and quantity of sand on each area. One technique has been used, seismic reflection, with the objectives of mapping the sub bottom, i.e. mapping of the top of the bedrock and the sediments overlying the bedrock. This report summarizes the methodology of the survey, the results obtained after acquisition and processing and the interpretation of these results. 09_053/MA/GP/RE Rev 01 Page 5 of 17

6 2. METHOD STATEMENT 2.1. HIGH RESOLUTION SEISMIC REFLECTION Introduction High resolution single trace marine seismic reflection is an established and powerful method to investigate sub bottom conditions. The method is frequently called "Sub Bottom Profiler" (SBP). Although the principle is identical to seismic reflection as used in Oil & Gas exploration, the equipment and survey methodology are completely different, due to the shallow penetration and high resolution which are required. Many different types of equipment are available on the market with different characteristics. Like any other geophysical methods, SBP has possibilities and limitations which are strongly dependent of the hydrographical conditions (water depth, waves) and of the bottom conditions. Figure 2.1: Seismic section example: sandy layer overlying bedrock. At some places, this rock basement outcrops above sea bottom and inside the sandy horizon Figure 2.2: Seismic section example: sediment horizons overlying rock basement. Some former sand dunes covered by muddy sediments are visible in the top layers 09_053/MA/GP/RE Rev 01 Page 6 of 17

7 Principle The general principle of SBP is illustrated on the following figure. It is quite similar to single beam echo sounding but it operates at much lower frequency, enabling penetration of the signal below the bottom. Penetration : Good No Bottom Sand Clay Sand Clay Tunnel TWT in ms 100 m Depth (1500 m/s) Figure 2.3: SBP: principle of operation (left) and example of seismic section with outline of main reflectors (right). The survey boat tows a seismic source (transmitter) and a group of hydrophones (receiver). The most frequently used sources are electrical (sparkers), electromechanical (boomer) or piezoelectric (pingers). They differ in power and frequency of the generated signal. Two different types of equipment have been used for this survey: (1) Applied Acoustic Boomer. The AA300 boomer is an excellent instrument, based on a recent design, giving good penetration (40 m and more in good conditions). However, the frequency is rather low (2 to 6 khz), resulting in less resolution than pingers or similar systems. (2) EdgeTech 3100 chirp system with a SB-216S fish chirping between 4 and 16 khz which gives a good resolution on the first few meters (less than 2 meters). The source is fired at regular intervals, e.g. four times per second. Part of the signal is reflected on the bottom; part of it penetrates into the bottom and is partly reflected by any discontinuity (reflector) present in the subsurface. More or less horizontal reflectors like the bottom or the boundary between sedimentary layers appear on the raw seismic profile with their true geometry, which makes interpretation very easy. On the example given above the bottom reflection has been highlighted in yellow and the contacts between the sand and clay layers in blue. Reflectors having no lateral extension (e.g. boulders, wrecks...) are called point reflectors and have a typical signature called "diffraction hyperbola". On the example above, a tunnel below seabed produces a typical hyperbola, highlighted in green. The object causing the reflection is located at the top of the hyperbola. The vertical scale on a seismic section is a time scale (two ways travel time). On the example above, the interval between the horizontal red lines is 10 ms. The time to depth conversion is based on the formula d=t/2v where d is the depth, t the time and v the propagation velocity of seismic waves. In water and loose sediments, the velocity is in the range 1500 to 1600 m/s. A time interval of 10 ms thus corresponds to a depth of 7.5 to 8 meters. 09_053/MA/GP/RE Rev 01 Page 7 of 17

8 The resolution of the method is the ability to discriminate between closely spaced reflectors. It is dependent upon the frequency of the signal and may be estimated at λ/2 where λ is the wavelength of the signal. The penetration below the bottom depends upon the attenuation of the signal. The attenuation depends upon the type of material and upon the frequency. Lower frequencies are less attenuated than higher frequencies Possibilities and Limitations The main advantage of SBP is that it gives a continuous image of the sub bottom structure along the surveyed lines. The continuity of the layers, their thickness, the presence and depth of localized obstacles can be assessed. If conditions for SBP are good, it is by far the best available geophysical method for marine sub bottom imaging. As for any other method, there are also important limitations. The main ones are: The method shows the boundary between layers or the presence of obstacles, but gives no information as to the type of material or obstacle. Some bottom types cause a very strong attenuation of the signal, precluding the use of SBP. A typical example is mud or other fine grained material with organic content producing methane. Small gas bubbles are trapped in the bottom and absorb the seismic energy. When such bottom is present, the seismic data does not contain information about the underground below this gas containing layer. The most prominent artefact on seismic sections is produced by multiple reflections between the bottom and the surface. Bottom multiples appear at 2 times, 3 times... the water depth. On the example above, the multiples are highlighted in red. The shallower the water, the closer the spacing between the multiples, and the more difficult it is to identify true reflectors. The limiting depth for acquiring good data also depends upon the bottom type. 09_053/MA/GP/RE Rev 01 Page 8 of 17

9 3. DATA ACQUISITION 3.1. NAUTICAL MEANS AND EQUIPMENTS Survey Boat The geophysical campaign was carried out on board of the vessel Ermione from ARTA (Agenzia Regionale per la Tutela dell Ambiente). Figure 3.1: Survey boat Ermione Positioning and Navigation The positioning system was a Hemisphere vector GPS/Gyro, receiving a DGPS correction from SBAS signal. The DGPS antenna was the reference for the calculation of the positions of the geophysical equipments. The accuracy of the calculated positions is sub-metric. The navigation software EIVA/NaviPac shows on line the movement and the bearing of the vessel in reference with the survey lines. All geophysical equipments offsets were recorded and the corrected position of these equipments calculated, displayed on the screen and recorded for merging with seismic data Seismic Reflection Equipments Two systems have been used on this project. 1. Applied Acoustics Boomer AA300 The seismic source and the receivers were towed at the water surface, 30 m behind the survey boat. Method : Seismic Reflection Receivers C-Products C-phone Multi Element Hydrophone Source Applied Acoustics Boomer AA300 Acquisition Unit CodaOctopus Geosurvey DA _053/MA/GP/RE Rev 01 Page 9 of 17

10 Figure 3.2: AA300 Boomer and C-phone Hydrophones towed behind the boat 2. EdgeTech 3100 chirp system The chirp system was towed behind the survey boat, 2 m below water surface. EdgeTech 3100 chirp Fish Acquisition software Resolution (theoretical) Dominant frequencies SB-216S EdgeTech 3100 Discover 8-10 cm vertical, (dependent on frequency range) 4-16 khz Figure 3.3: EdgeTech 3100 Chirp system The use of the different systems depends on a compromise between penetration (better at low frequencies) and resolution (increasing with high frequencies). 09_053/MA/GP/RE Rev 01 Page 10 of 17

11 The Chirp system cannot penetrate gravel or coarse sand. Indirect information is that no penetration with this instrument means that no soft material is present at the surface. The resolution of the Chirp is limited to about 10 cm. This implies that a fine layer of less than 10 cm will not be detected by this system. The Chirp system has been used simultaneously to discriminate between fine layers near the seabed when needed GEODETIC DATA The coordinates system used has the following characteristics: - Projection: UTM 33N Projection type: Universal Transverse Mercator Latitude of origin: N Central Meridian: E Scale Factor: False Easting: E False Northing: 0 N - Ellipsoid: WGS 84 Semi major axis (m): Inverse Flattening: Datum Shift: None 3.3. OVERVIEW OF THE SURVEY Ortona The measurements were carried out on July 22 nd and July 23 rd. This area has a surface of approximately hectares. The bathymetry is in a range of depth between 7.0 and 14.0 m (local Z level). Eight long lines, in a direction parallel to the coast (NW-SE), and 43 cross lines, in a direction perpendicular to the coast (direction NE-SW) have been surveyed. The line spacing is 50 m between the cross lines and 200 m between the long lines. A general overview of the surveyed area and survey lines is presented on document 09_053/MA/GP/DRG _053/MA/GP/RE Rev 01 Page 11 of 17

12 Vasto The measurements were carried out on July 23 rd. The bathymetry is in a range of depth between 6 and 20 m (local Z level). Six long lines, in a direction parallel to the coast (NW-SE), and 30 cross lines, in a direction perpendicular to the coast (direction NE-SW) have been surveyed. The line spacing is 50 m between the cross lines and 200 m between the long lines. At Professor Orlando request, the cross lines have been extended to check the homogeneity of the area at deeper water level. This area has a surface of approximately 220 hectares. A general overview of the surveyed area and survey lines is presented on document 09_053/MA/GP/DRG _053/MA/GP/RE Rev 01 Page 12 of 17

13 4. DATA PROCESSING 4.1. SEISMIC REFLECTION The data processing was run on Coda Geosurvey software, Chesapeake SonarWiz.Map software and Golden Software Surfer. The following processing steps were applied to the data: - Data edition and merging with positioning data; - Filtering and image enhancement; - Bottom tracking. The interpretation consists of the following steps: - Identifying artefacts and true reflectors; - Digitalization of the main reflector(s); - Interpolation and mapping; - Production of seismic profiles. 09_053/MA/GP/RE Rev 01 Page 13 of 17

14 5. PRESENTATION AND INTERPRETATION OF THE RESULTS 5.1. ORTONA Document 09_053/MA/GP/DRG001 shows the survey trackplot, the results of the boomer measurements and their interpretation. Figures 5.1 and 5.2 below show parts of seismic profiles as recorded on site. Sediments Top of bedrock Figure 5.1: Seismic Profile at Ortona - Bedrock and layers of sediments On figure 5.1, the top of the bedrock is highlighted with the presence of hyperbolas at the interface and the presence of dipping reflectors below this interface. Above the top of the bedrock, the alternation of reflectors corresponds to layers of soft sediments. The information shown on the document 09_053/MA/GP/DRG001 consists of the representation of the thickness of soft sediments below the seabed. On this document, the results of the previous survey have been included into the results of this one. The thickness of sediments at Ortona ranges between 6 and 26 m. Hyperbolas Figure 5.2: Seismic Profile at Ortona - Hyperbolas 09_053/MA/GP/RE Rev 01 Page 14 of 17

15 As shown on figure 5.2, in some parts of the survey area, diffraction hyperbolas have been highlighted, which prevent from detecting the different reflectors. These areas with diffraction hyperbolas are shown on document 09_053/MA/GP/DRG001. The results of the seismic reflection (thickness of soft sediments) have been interpolated in these zones. The results of the Chirp measurements also highlight the presence of layers of soft sediments in the first few meters. The penetration with this system being lower, the bedrock is not highlighted. Twenty vibrocores have been sampled in the area of Ortona. In general, silty sand, fine sand and clayey silt have been identified. There are also some shells and some black striation of organic weathering. Most of the vibrocores are less than 2 m deep. The maximum depth is 4.6 m VASTO Document 09_053/MA/GP/DRG002 shows the survey trackplot, the results of the boomer measurements and their interpretation. Figures 5.3 and 5.4 below show parts of seismic profiles as recorded on site. Sediments Top of bedrock Figure 5.3: Seismic profile at Vasto - Bedrock and layers of sediments 09_053/MA/GP/RE Rev 01 Page 15 of 17

16 On figure 5.3, the top of the bedrock is highlighted with the presence of hyperbolas at the interface. Above the top of the bedrock, the alternation of reflectors corresponds to layers of soft sediments. The information shown on the document 09_053/MA/GP/DRG002 consists of the representation of the thickness of soft sediments. On this document, the results of the previous survey have been included into the results of this one. The thickness of sediments at Vasto ranges between 7 and 15 m. Zone A Figure 5.4: Seismic profile at Vasto - Difference in sediments As shown on figure 5.4, a difference in sediments has been highlighted based on seismic reflection results. The sediments in zone A (right part of the above example area located offshore) are coarser than the sediments located near the shore. On document 09_053/MA/GP/DRG002, this area is shown on the seismic profile. The document also shows the thickness of the sediments in zone A which varies from 2.6 to 11.5 m. The results of the Chirp measurements also indicate the presence of layers of soft sediments in the first few meters. The penetration with this system being lower, the bedrock has not been detected. Six vibrocores have been sampled in the area of Vasto. In general, fine sand has been identified. Some remains of shells are present. All of the vibrocores are less than 1.5 m deep. 09_053/MA/GP/RE Rev 01 Page 16 of 17

17 6. CONCLUSIONS A geophysical survey, consisting of seismic reflection measurements, has been carried out along the Adriatic coast of Italy, in order to find possible sand borrow areas located offshore the cities of Ortona and Vasto. The aim of the geophysical investigation was the mapping of the sub bottom. The results of the seismic reflection survey are presented on documents 09_053/MA/GP/DRG001 and DRG002. In the two areas, Ortona and Vasto, the bedrock as been highlighted and the thickness of sediments calculated. In Ortona, the thickness of soft sediments varies between 6 and 26 m. In some parts of the survey area, diffraction hyperbolas prevent from digitalizing the bedrock. The results have been interpolated in these areas. In Vasto, the thickness of sediments ranges from 7 to 15 m. A lens of coarser sediment, shown as zone A on the document, has been highlighted. This lens is 2.6 to 11.5 thick. 09_053/MA/GP/RE Rev 01 Page 17 of 17