PROCESSING AND INTERPRETATION OF

 SHALLOW SEISMIC REFLECTION SURVEY OFF UMERGAM AND NANDGAON, MAHARASHTRA COAST, ARABIAN SEA.

INTRODUCTION

Geophysical surveys were carried out in the territorial waters off Umergam and Nandgaon, Maharashtra coast, Arabian Sea in the integrated cruise SD-183 covering an area of about 1160 sq. km under F.S.P. of Marine Wing, Geological Survey of India. The survey was bounded by the latitudes 19⁰42’N and 20⁰10’N and longitudes 72⁰5’E and 72⁰40’E. Shallow Reflection Seismic (Sparker Source) Survey and Bathymetric surveys were carried out in the area to map the sea bed. The various reflecting horizons were deciphered within a depth in Two Way Time (TWT) varying from 13 to 55 msec. Based on the reflectivity the marker reflector bed R1 was tentatively identified as the top of the acoustic basement rock, i.e., Deccan Traps as inferred from the geology of the hinterland. Apart from the prominent reflector R1 a number of reflecting horizons were identified. These acoustic reflectors may be attributed as different flows of Trap rocks at different epochs. The sea floor and the prominent sub-bottom reflecting horizons are gently dipping towards sea. Sea floor n the nearest shore side is of compact nature indicating the trap rocks of basalt origin exposed at sea floor. No sedimentation occurred in the near shore horizon confirmed during the geological sampling.

OBJECTIVE

The prime objective of the geophysical survey comprising of shallow seismic (sparker source) along with bathymetry was to map the seabed and beneath within territorial waters off Umergam and Nandgaon, Maharashtra coast in Arabian Sea and is to generate data base of the area which can be gainfully utilized for coastal and near shore developmental activity.

SHALLOW REFLECTION SEISMIC SURVEY

          The shallow reflection seismic surveys were carried out by deploying EG & G make sparker system comprising of power supply , trigger capacitor bank, electrode sparker array for transmitting energy and two sections of receiving suits, i.e., the hydrophone streamers each of 50m length containing 50 hydrophones. The recording system consisting of a preamplifier and filter and EPC graphic recorder were used for analog recording of the seismic signals. The area of operation was confined to water depths of about 7m to 35m. Considering the various geological aspects, the input energy level was fixed at 400 Joules. The firing rate was set at two pulses per second. The sweep rate of EPC analog recorder was kept at ¼ sec/sweep.

            A total of 298 line km shallow reflection seismic and 345 km of bathymetric surveys were conducted within the water depth of 7m to 35 m. the shallow seismic survey was carried out continuously in real time mode along all the lines and sot points were marked at every 5 min interval.

PROCESSING AND INTERPRETATION

The marine seismic data obtained from reflection surveys are mostly in the analog form. To process and interpret this data, one has to, at first, digitize it so that the data can be handled properly and more accurate interpretation can be done.

The processing and interpretation of the analog seismic data is carried out in the following steps:-

1)            Digitization of the analog data from the seismic section.

2)          Plotting of Two Way Time curves for the sea floor and each marker    bed.

3)            Preparing the isochron and isopach maps for the reflectors that are of interest.

DIGITIZATION: -

This involves identification of each marker bed including the seafloor, on the analogue seismic section. After identifying the trend that each reflector follows, we have to mark its top at each shot point and read it in terms of cm. Thus we are actually digitizing the analog data so that we can process it according to our needs and also fed the data to the software for TWT plotting and contouring. In this process we have to make out where and at what depth a particular reflector has overridden its overlying reflector and where it has disappeared or originated. The presence of faults and fractures are also to be identified for the proper determination of the formation pattern of each reflector beds. This is repeated for each line and correlated with the cross line also so as to maintain continuity of each reflector bed throughout the area.   

Now, since the sweep rate of the analog recorder is ¼ sec/sweep and the total length covered is 50 cm, we have to multiply it with 5 to get the Two Way Travel time of each marker bed in msec. This process is repeated for each shot point in a particular profile line and then repeated for each line parallel as well as perpendicular to the coast so that we can get a continuous picture of the subsurface over the whole area.

PLOTTING OF TWT CURVES:- 

After the reflector beds have been identified and heir tops have been defined in terms of two way time from the sea surface, we plot TWT curves to find out the pattern how the sea floor and each of the identified reflectors are present at the subsurface and to determine their depositional environment and other related facts.

To do this, profile maps are created to exactly position each shot point in the latitude and longitude scale. This is particularly done because the shot points are not equispaced and since the shot points are generally somewhat haphazardly located, the profile lines are also not straight lines. So an excel sheet is prepared in which the latitude and longitude of each shot point is entered and then the data is fed to the software SURFER 8 which prepares the profile map.

To plot the TWT curves manually for each profile line, a transparent graph sheet is superimposed on the profile map and the best fit profile line is drawn, taking into consideration the deviation of each shot point from the drawn line. After this the shot points are projected on the profile line to bring all the points on the same line. Now the TWT values are plotted for each reflector in that profile covering all the shot points. While joining the points in the graph, we have to take into consideration the pattern each reflector bed follows in the original section, such as the existence of any faults or folds in between the shot points. Any discontinuity in the formation of the marker beds is generally avoided by extrapolating the formation according to the trend or simply keeping it as it is if the analog section shows that the formation is actually disappearing at that place. This gives us a clear picture of the formation pattern of each marker bed and also the way they have superposed on their overlying bed or diminished in course of time. The presence of faults or fractures can also be easily identified by the abrupt change of the dip of some of the reflector beds at a particular point.

The TWT curves can also be plotted on the computer using the software SURFER 8

or Grapher. For that, the latitude and longitude of each shot point has to be entered and the seabed and all the reflectors are to be assigned a name depth wise. The TWTs of all the reflector beds corresponding to each shot point has to be entered and continued for the whole area covering all the profiles in the map including the cross line. But in doing this, the points will be joined in straight line which will not show the actual pattern as seen in the original analog section. So manual plotting is more suitable for correct interpretation.

PREPARING ISOCHRON AND ISOPACH MAPS

                        For making isochron maps of each reflector bed, an excel sheet is created where the latitude and longitude of each shot point is entered along with the two way travel time in msec. This excel file is then converted to a grid file by giving necessary commands in the software SURFER 8. After that, the contour map is created in SURFER, which gives the formation pattern of which each reflector in the subsurface. The contour map is superposed with the profile map and only the area that comes within the profile lines is chosen and the rest is blanked out. Separate color is assigned to each contour value to distinguish each contour with the other. The closures in the contour map indicate local highs or lows according to the contour value. These may sometimes indicate folding in the subsurface due to which the surface has elevated above the surroundings. Correlating the isochron map of any reflector bed with the profile map also gives us an idea in which area the particular reflector is present n the total surveyed zone.

            For preparation of isopach maps, generally those two reflectors are chosen which show uniform presence over the whole area. Now another excel file is created which will contain all the shot points where these two reflectors are present, their latitudes, longitudes, two way times and the difference in TWTs between these reflector beds. The shot points where any of these two reflectors are absent are generally not taken to avoid discrepancy between data. These TWT differences are plotted on the maps and contours are created with the help of SURFER, which shows the depositional feature between any two reflector beds. This depositional feature gives an indication of the depositional history of each formation in that area and its surroundings.

Activities carried out:-

For detailed study of the Arabia Sea off Maharastra coast the analog seismic data was digitized and excel sheets were created for each reflector containing the shot points, the corresponding longitudes, latitudes and two way travel time in msec. The two way travel time curves of all the identified reflectors were plotted manually on the transparent graph sheets separately.