NATURE OF PROJECT: SUB-BOTTOM PROFILER SURVEY
CLIENT: JABATAN LAUT MALAYSIA
LOCATION: PULAU KLANG, SELANGOR
INTRODUCTION
Geomapping Technology Sdn Bhd has been awarded a contract by Jabatan Laut Malaysia to perform the Sub-Bottom Profiler survey to identify the position of HDPE pipeline with dimension of 40cm diameter at Sungai Terusan Bagan, Pulau Klang, nearshore Port Klang, Selangor, Malaysia.
DISCUSSION AND RESULT
Sub-Bottom Profiler transmit a relatively high frequency acoustic pulse, a portion of the outgoing pulse is transmitted below the seabed and will reflect off layers in the sediment sequence, thus producing a pseudo cross-section of the subsurface. The penetration of the acoustic signal into the sediment column is inversely proportional to the frequency of the acoustic signal.
The Innomar SES 2000 Compact system was used for sub-bottom profiling during the survey. The raw sub-bottom signals were recorded digitally by the Innomar SES2000 recording system in ses format. The digital copy was processed and interpreted on-screen using ISE2 processing software package.
Seismic stratigraphic techniques were used to interpret the sub-bottom profiler data. The seismo-stratigraphy was divided into acoustic units that generally equated to seismic sequences. The depths to reflectors from the seafloor were estimated by converting the two-way time (TWT) to the acoustic events to depth (D) using an assumed seismic velocity (Vp) of 1600ms-1 with the formula overleaf:
D = (Vp x TWT) / 2
Seismic facies techniques were employed to characterize the units. These included reflection character (amplitude, continuity and frequency) and reflection configuration (well-stratified, chaotic, parallel, reflection-free or transparent).
One central line was used to create UHR stratigraphic cross-sections of the top-hole section. The centre line reflectors and any significant geological features were placed on a tracing. This was digitized into AutoCAD and a time to depth factor was applied to convert the time seismic profile to a depth cross-section.
The ISE2 processing system was also used to perform interpretation and produce data examples. Prior to interpretation, correlation between the analogue single channel and digital multi-channel data upper section seismo-stratigraphy was checked.
The parameters for the sub-bottom profiler system are as follows:
- Process: Staking 1
- Range: 20m, (Adjust Ping Rate 80%)
- Gain: LF = 5 Db
- Transmit: LF- Frequency 3-5 KHz
- Filters: raw
- AGC: raw
The maximum seismic penetration achieved was approximately 20m below seabed. Descriptions of the shallow geological and sub-seabed sediment interpretations are based on reflection characteristics, sedimentary structures and/or sub-seabed features noted on the profiler data
The shallow zone geology consists of sub horizontal events and can be divided into three (3) major units, designated as Units A, B and C. The anticipated shallow sedimentary sequence beneath the pipeline supply water from Mainland to Pulau Ketam at Sungai Terusan Bagan.
Figure 1: Sub-Bottom Profiler data show the litho-stratigraphic crossing the existing pipeline supply water.
Figure 2: Sub-Bottom Profiler record show the pipeline water supply within the survey area
CONCLUSION
The shallow zone geology consists of sub horizontal events and can be divided into five (5) major units, designated as Units A (very soft to firm CLAY), B (possible form to stiff CLAY) and C (possible stiff to hard CLAY)
The shallow geology within the survey corridor consists of a series of depositional environments. The shallow sedimentary sequence is interpreted as alternating horizons of predominantly CLAY. The surficial sediments are expected to provide good settling conditions along the survey area.
One (1) pipeline supply water was observed within the survey area based on the Sub-Bottom Profiler data. Pipeline was marked by high to moderate and parabolic reflector with slight to severe acoustic masking of underlying reflector due to the fresh water in the pipeline.
The as-found position for the pipeline supply water was depicted well with maximum discrepancy of 5m. Caution is advised during dredging operation as this pipeline may have damage/ broken.
