Attenuation of Seismic Multiples in Very Shallow Water: An Application in Archaeological Prospection Using Data Driven Approaches

Michaela Schwardt,Wolfgang Rabbel,Dennis Wilken

doi:10.3390/rs13101871

Abstract

Water-layer multiples pose a major problem in shallow water seismic investigations as they interfere with primaries reflected from layer boundaries or archaeology buried only a few meters below the water bottom. In the present study we evaluate two model-driven approaches (“Prediction and Subtraction” and “RTM-Deco”) to attenuate water-layer multiple reflections in very shallow water using synthetic and field data. The tests comprise both multi- and constant-offset data. We compare the multiple removal efficiency of the evaluated methods with two traditional methods (Predictive Deconvolution and SRME). Both model-driven approaches yield satisfactory results concerning the enhancement of primary energy and the attenuation of multiple energy. For the synthetic test cases, the multiple energy is reduced by at least 80% for the Prediction and Subtraction approach, and by more than 60% for the RTM-Deco approach. The application to two field data sets shows a significant amplification of primaries formerly hidden by the first water-layer multiple, with a reduction of multiple energy of up to 50%. The waveforms obtained from FD modeling match the true waveforms of the field data well and small deviations in time and amplitude can be removed by a time shift of the traces as well as an amplitude adaption to the field data. The field data examples should be emphasized, where the tested Prediction and Subtraction approach works significantly better than the traditional methods: the multiples are effectively predicted and attenuated while primary signals are highlighted. In conclusion, this shows that this method is particularly suitable in shallow water applications. Both evaluated multiple attenuation approaches could be successfully transferred to two other 3D systems used in shallow water near surface investigations. Especially the Prediction and Subtraction approach is able to enhance the primaries for both tested 3D systems with the multiple energy being reduced by more than 50%.

Highlights

During the recently completed Priority Program of the German Science Foundation “Harbors from the Roman Period to the Middle Ages” [1], a large number of possible ancient harbor sites with different subsurface properties were geophysically investigated.The investigation and identification of such sites often includes the shallow water area close to the shore, where common depth resolving geophysical prospection methods are limited in depth penetration and resolution due to high signal attenuation caused by high salinity
Compared to the results of method A (Figures 9d and 10d) and B (Figure 9e), multiple energy, and primary energy that was masked by the multiple reflection, was attenuated by the application of the traditional methods
We evaluated two data driven methods using FD modeling of the acoustic wave equation for the attenuation of shallow water-layer multiples for multi- and constantoffset seismic data as recorded for archaeological prospection purposes

Summary

Introduction

During the recently completed Priority Program of the German Science Foundation “Harbors from the Roman Period to the Middle Ages” [1], a large number of possible ancient harbor sites with different subsurface properties were geophysically investigated. The investigation and identification of such sites often includes the shallow water area close to the shore, where common depth resolving geophysical prospection methods (i.e., electrical resistivity tomography, ground penetrating radar) are limited in depth penetration and resolution due to high signal attenuation caused by high salinity. There are a few shallow water applications of these methods. Examples of a successful application of ERT are shown in [2,4,5,6]. [5] showed that ERT Ref. [5] showed that ERT

Methods

Results

Discussion

Conclusion