Very High Resolution Multichannel Recording For Shallow Seismic
Authors: Marsset Bruno , Blarez Emmanuel , Girault Robert 
Published: October 2008

Introduction

Numerical recording of Very High Resolution seismic data is only a few years old and we can say that to- day most of the acquisition of this type of data is not yet recorded in a digital way, leading to a poor Signal/Noise ratio and forbidding any further reprocessing of the data. The use of several hydrophones, in parallel connection, and summed together in an analogical way might be seen as the first step towards multichannel techniques, in term of Signal/Noise ratio !

Single channel seismic numerical acquisition has been an important step which allows, using powerful signal processing and graphical analysis algorithms, to improve the output quality significantly

Two main hurdles limit the exploitation of single channel seismic data:The lack of information concerning wave propagation velocity laws in the sediments, leads to a two way travel time representation which cannot be closely related to depth information extracted from geotechnical boreholes.The lack of redundancy by the use of a single trace for each shot: weather and operating conditions become of outstanding importance for data quality acquisition.

The use of multichannel techniques for Very High Resolution purposes may solve the two previous problems by providing :A performing tool for velocity analysis using the principle of dynamic correction.An improvement of the Signal/Noise ratio using the principle of summation.An access to true depth information migration algorithms.Last, a redundancy of information over each Shot-Receiver position. This allows to sustain site-surveys in worse sea conditions than in the case of single channel technique.

Acquisition system

The configuration, -source-receiver-recording system-, of the acquisition device should be carefully reviewed in order to fit the acquired data with the expected goals of the survey :

The streamer

The definition of four parameters are mandatory to build up a streamer :Hydrophone bandwidth, in term of spatial resolution are closely related to the maximal immersion depth of the receivers (ghost effect).The preservation of apparent velocities on a trace (group of hydrophones) determines the trace length.The different algorithms for velocity analysis rely on the time arrival differences between near and far traces and therefore define the streamer length.From the previous parameters, we may now compute the adequated number of traces.

Concerning the surveys which results are presented here, the geotechnical goals were included in a time slot between 150 and 200 milliseconds (two way travel time) for water depths ranging from 20 to 60 meters. The streamer was thus defined as follows: a maximal bandwidth of 3 kHz, 24 groups of hydrophones with 2 meters spacing between traces. The immersion depth should then be theoretically of 25 centimeters, i.e. the streamer was operated on surface.

The source

To take into account the requirements concerning the frequency bandwidth of the signal, the following sources were used in different surveys:A sparker (used together with a power supply lower than 300 joules).A boomer (used together with a power supply lower than 300 joules).