| Offshore and Onshore Survey
Hydrography and Bathymetry
A hydrography survey is a survey to obtain water parameters, and this activity includes measurement, mapping, and visualization.
The results of the hydrographic survey can be used for various needs, including:
- Navigation and navigation safety.
- Determination of territorial boundaries or areas at sea.
- Laying of submarine cables and pipelines, as well as.
- Study of coastal dynamics and marine resource management.
Bathymetry is the part of hydrography that is a measurement work to map the morphology of the seabed based on the depth value against a predetermined reference (Datum). Because the number of results must have a reference, the results of depth measurements (using an echosounder) must be tied to the reference to become the final bathymetry map. In terms of technology/generalization technique, there is currently a MultiBeam Echo Sounder system (MBES) which has a wide-scale measurement capability, so measurement work can be faster than using the Single Beam Echo Sounder system (SBES).
In a measurement system using a single beam echosounder, the correction factor that is applied in data processing to produce True Depth values is simpler than the MBES system. Common corrections in the two measurement system techniques are tide correction, sound wave velocity correction, and heave correction. Meanwhile, in the MBES system, some special corrections need to be applied such as correction of headings, beam angles, etc.
In bathymetric survey work (using a special echosounder) integration of the following devices is required:
- Positioning system (navigation software and GPS / GNSS)
- Echosounder (single/dual-frequency)
- Sound Velocity Profiler (SVP)
- Tide Recorder
- Leveling System
Side Scan Sonar
Side Scan Sonar is the most effective tool for underwater exploration because it can quickly and accurately image the vast seabed surface in producing detailed and detailed images and is not affected by water turbidity factors. The search for underwater obstacles is possible by conducting surveys with this tool, including surveys for early identification of marine construction planning. the existence of obstacle pipes, cables, corals, or other debris objects such as shipwrecks, falling material during loading and unloading of the material and the distribution of existing sediments on the surface can be clearly identified and have spatial data information.
The side scan sonar transmits a narrow acoustic beam to the side of the survey track line which propagates out across the seabed. As the acoustic beam travels outward from the side scan sonar, the seabed and other obstructions reflect some of the incidents sound energy back in the direction of the side scan sonar (known as backscatter). The travel time of the acoustic pulses from the side scan sonar is recorded together with the amplitude of the returned signal as a time series and sent to a topside console for interpretation and display.
As with any acoustic sonar, side scan sonars only show echoes of objects that reflect sound back to the side scan sonar transducer, such that hard shiny surfaces are sometimes only seen when they are at right angles to the side scan sonar and rough seabed textures can blot out smaller targets completely. Some types of material, such as metals, boulders, gravel, or recently extruded volcanic rock, are very efficient at reflecting acoustic pulses (high backscatter). Finer sediments like clay and silt, on the other hand, do not reflect sound well (low backscatter). Strong reflectors create strong echoes, while weak reflectors create weaker echoes. Knowing these characteristics, you can use the strength of acoustic returns from the side scan sonar to examine the composition of the seafloor.
Interpretation of side scan sonar data develops with experience. Side scan sonar reflections of isolated small objects do not indicate shape or attitude. Manmade structures, such as platforms or rock walls tend to have regular patterns that are easier to identify. Using a side scan sonar is rather like looking at a world made of shiny black plastic, in the dark, with only a narrow torch beam for illumination. Remember that when close to large objects, or in a depression in the seabed, that the viewing range of the side scan sonar may be severely limited. Very strong reflectors may give multiple echoes along a bearing line and are identified by being equispaced in range. The plan view provided by the side scan sonar also does not show how high an object is unless an acoustic shadow is a cast, in which case the length of the acoustic shadow is related to the height of the object, its range, and the height of the side scan sonar.
Experience with the side scan sonar will enable the side scan operator to be able to quickly and effectively set controls such as receiver gain and dynamic range to give as even a background as possible, without swamping the side scan display, and maximize the performance capabilities of the side scan sonar. Separate controls are available for Port and Starboard side scan transducers. Although normally the settings would be the same, under some conditions (e.g. sloping seabed) different settings may be needed from port to starboard.
Slicing The Sediment Layer
Sub-bottom profiling systems identify and measure various sediment layers that exist below the sediment/water interface. These acoustic systems use a technique that is similar to simple echosounders. A sound source emits a signal vertically downwards into the water and a receiver monitors the return signal that has been reflected off the seafloor. Some of the acoustic signal will penetrate the seabed and be reflected when it encounters a boundary between two layers that have different acoustical properties (acoustic impedance). The system uses this reflected energy to provide information on sediment layers beneath the sediment-water interface.
Acoustic impedance is related to the density of the material and the rate at which sound travels through the material. When there is a change in acoustic impedance, such as the water-sediment interface, part of the transmitted sound is reflected. However, some of the sound energy penetrates through the boundary and into the sediments. This energy is reflected when it encounters boundaries between deeper sediment layers having different acoustic impedance. The system uses the energy reflected by these layers to create a profile of the sub-bottom sediments. Several sonar parameters (output power, signal frequency, and pulse length) affect the instrument performance.
High-resolution sub-bottom systems have been used to detect and measure the thickness of dredged material deposits, detect hard substrate that has been covered by sedimentation, identify buried objects (such as cables and pipelines), and define the basement (or bedrock) layer for potential confined aquatic disposal sites for dredged material
Survey-ocean data are very important in engineering planning to work both onshore and offshore. Information on the physical properties of seawater in the form of current, wave, wind, humidity, temperature, conductivity, salinity, including tides, is the basic data in modeling the dynamics of the characteristics of water conditions both locally and regionally.
A weather station is a data logger system that can record data on air temperature, humidity, rainfall, wind speed, and direction at a very station. The characteristics of current and wave conditions as well as the tides of water can predict its periodicity cycle based on weather data modeling.
Water sampler is a tool for taking seawater samples that work manually, the use of seawater sampling in oceanic studies is for laboratory analysis of seawater samples taken (generally taken in several different conditions, times, and positions. ). From laboratory analysis, it will be possible to know the level of turbidity which is correlated with velocity and direction data as well as other data on the physical properties of seawater which can predict the sedimentation rate in certain waters.
Sediment sampling is generally related to the need for granulometric analysis, kurtosis, or kurtosisis, namely laboratory analysis for the purposes of soil analysis in terms of grain size identification, sorting, and more specific characteristics of sediment grains. A sampling at sea level can be done by using the grab method or the gravity corer. The advantage of using a gravity corer is to obtain a vertically shallow sample surface.
With a simple technique, surface sediment sampling will be obtained using this tool. This tool can take fine sediment samples (fine sand) to hard (coral) even because it has pointed teeth. For very fine sediments, the sampling speed can be obtained successfully (considering the construction of this tool that is not impermeable when closed so that it is possible that any samples escaped during the lifting process.
Gravity corer is designed for surface sediment sampling using gravity techniques. The use of the tool itself is carried out by being dropped at the location of the desired target sample point. part of the gravity core system consists of a nose cone in which a cather unit is installed as a sediment trap, a sample pipe (1-5m), a wing equipped with weight for gravity loading. The results of the sediment samples obtained from the gravity corer system are in the tube and for analysis purposes, a vertical division of the core sample pipe is carried out.
| Topographic Survey
An unmanned aerial photogrammetric survey using an Unmanned Aerial Vehicle (UAV) is a method for taking photographs for use in photogrammetry
(the science of making measurements from photos).
Instruments manufactured for UAVs can be mounted on unmanned flying platforms of various sizes and types. This machine can be used and is suitable for complete geodetic survey activities at the study site by making quality measurement points and a nearly homogeneous level of accuracy. These detailed point clouds (of different types of data) can be used in line with orthophoto, etc.
The UAV is suitable for obtaining three-dimensional (3D) digital models and orthophoto mosaics for a specific survey area.
This UAV survey is very suitable for use in small survey areas to get good survey data.
LiDAR (Light Detection and Ranging) is a method of determining a variable distance by targeting an object using a laser and measuring the time the light is reflected back to the receiver. LiDAR can be used to create digital 3-D representations of areas on the earth’s surface and the seabed, due to the difference in laser turnaround times, and by varying the laser wavelengths. Lidar has terrestrial, aerial, and cellular applications.
Lidar can be used to create high-resolution maps and is applied in surveying, geodesy, geomatics, geography, geology, geomorphology, seismology, aerial laser grid mapping (ALSM), and laser altimetry.
LiDAR technology is also commonly used in the control and navigation of some autonomous cars.
GPS, Total Station Survey
Total Station-based location positioning and/or measurement systems provide a high level of accuracy. Total Station-based systems have a limited range compared to GNSS-based systems and are better suited for projects where accuracy is a key factor.
Static GPS Baseline is a technique used to determine accurate coordinates for survey points. Basic measurement by recording GPS observations from time to time, then the data is processed to get accurate results. This technique works by using two GPS receivers.
| Geophysical Survey
Geoelectric (DC Resistivity and Induced Polarization)
The DC resistivity and IP methods are used to map subsurface electrical properties, resistivity for DC resistivity and chargeability for IP. Fundamentally this method has the concept that the ground response is like a bunch of electrical resistors. The conventional DC resistivity method injects current at the transmit electrodes and measures the potential difference between the potential electrodes. Meanwhile, the IP method measures the voltage decay in the time or frequency domain after which the current injected is off, potential in the rock is not zero and will decay until it reaches zero. This indicates the ability of the rock to store instantaneous currents.
This method is useful for groundwater exploration, mineral exploration, potential landslide mapping, and environmental study.
The gravity method is a passive, non-invasive technique used to measure the Earth’s gravitational field at a specific place on the surface. Through the processing and modeling process, this method can provide an overview of the subsurface density model.
This method can be used on a variety of scales from regional to micro. This method is intended for basin geometry, oil and gas exploration, geothermal exploration, mineral exploration, land subsidence (4D microgravity, combined with GPS surveying).
Geomagnetic (Land and Marine)
The geomagnetic method is similar to the gravity method, except that it measures the earth’s magnetic field. The measured magnetic field can be either a total magnetic field or a vertical gradient magnetic. Anomalies in the Earth’s magnetic field come from the rock / magnetic material that is induced by the magnetic field of the earth to produce a secondary magnetic field that is local.
Qyudos geosurvey Indonesia provides land and marine magnetic survey services. This method is useful for mineral exploration, geothermal exploration, and mapping buried submarine pipelines.
GPR (Ground Penetrating Radar)
GPR (Ground Penetrating Radar) works by transmitting a radio frequency signal pulse into the material and recording the amplitude and time required by the wave after it is reflected. Reflections will occur if there is a material that has a different dielectric constant (relative permittivity), if the radar wave propagates from a material that has a low dielectric constant to a material that has a higher dielectric constant, it will produce a strong reflector, and vice versa.
This method can be applied to sinkhole & void locating, concrete scanning, utility locating, roadway inspection, bridge deck inspection, and for mineral exploration that related to paleochannel deposits or lateritic mineral deposits.
Seismic refraction utilizing wave propagates below the surface along refraction ray path between two layers with contrasting seismic velocity. The source of the waves in this method can be a sledgehammer or a weight drop. The refractive waves will return to the surface and are recorded by a group of sensors called a geophone. At a certain distance (on each geophone), the signal that has been refracted is observed as the first-arrival signal (first break). Through data processing, the signal can be derived to a depth so that a 2-D cross-section of seismic velocity can be obtained.
This method can be used to perform rockhead/bedrock profiling for the purpose of piling, or excavation.