Methodologies
Seismic waves (P-waves) energy is generated using an acoustic source at predetermined locations (S). The travel times of acoustic energy that has been critically refracted at horizons of interest (L1) is recorded at predetermined receiver locations (R1, R2, etc.). The recorded travel time information is used to generate a velocity–structure profile of the subsurface along the length of the refraction profile.
Modified from “Introduction to Geotechnical Geophysics – N. Anderson and N. Croxton, Circular – Number E-C130, October 2008”.
There are two main Marine Seismic Refraction (MSR) systems, Underway and Static.
Underway:
MSR has been developed to quantify and qualify seismic P-waves velocities distribution in challenging terrain/environment, the sub-seabed.
A seismic streamer (receivers) with various spacing and number of hydrophones is secured and towed behind a vessel in line with a seismic source, which consist of various levels of power (pressure chamber size) air-pressure-gun. The streamer and seismic source, which move in sync, are connected to a seismograph and position/navigational system on board vessel.
The system can be surface, mid, or bottom towed.
Static:
This system is normally used for river crossing, very shallow water bodies that preclude normal navigation or short investigation requiring high resolution.
The system is a combination/adaptation of SRT and MSR. Basically, the system and geometries are as per SRT, but with hydrophones instead of geophones.
Example result of an Underway MSR