A solution to the airwave-removal problem in shallow-water marine EM

Abstract

A new method is presented for removing the airwave in shallow-water electromagnetic (EM) data. By using pairs of receivers or pairs of source points along a towline and forming weighted differences of an EM field component, the airwave is significantly attenuated. The weights are related to the geometric spreading of the airwave. Specifically, the horizontal electric and magnetic field components are weighted with the horizontal offset to the power of three before their differences are calculated. One can also first calculate difference data without weights in the common-source domain to remove correlated noise and then use the resulting data to perform a second-order difference with weights. In this case, the weights should equal the horizontal offset to the power of four. Moreover, the weighted difference data can be integrated to obtain an estimate of the physical field void of the airwave. Additionally, the airwave contribution to the physical field can be estimated. The theory behind the method is presented and demonstrated on a real controlled-source EM (CSEM) data set.