Welcome to Drisana Geotechnical Consultancy!
For the seismic refraction survey work sledge-hammer is struck vertically on an iron plate to generate P-wave.
Low frequency (10Hz) vertical geophones shall be used to record seismic signals. The general layout of the seismic survey lines will be supplied in accordance with the co-ordinate system provided by the client. Shooting progressed along the lines. Depending on the length available for seismic lines 5-meter geophone spacing shall be used in a spread depending on the length available along seismic lines 24 channels were used. The seismic spread consisted of number of shots with a minimum of two offset shots, two end shots and one center shot. Care was taken to ensure that the pointed ends (spikes) of the geophones are fully embedded in the topsoil.
With seismic techniques, in general, a stress applied at the surface of an elastic media creates the conditions for the associated strains to propagate as elastic waves (P & S) in the subsurface material as pattern of particle deformation traveling with velocities that are dependent on the elastic properties and densities of the media through which they travel.
The basic principle behind seismic refraction surveys is to initiate elastic waves at a point at or near the ground surface and to determine at a number of other positions the arrival times of the seismic energy that has traveled along discontinuities or interfaces between surface layers and totally refracted back to the surface. The seismic energy source creates the seismic elastic waves. The travel times of these elastic waves are detected by series of geophones, placed in line into the ground and connected to the seismograph, via the geophone cable. The seismograph will register these times and display them as traces of time for individual geophones.
The data will be stored in the hard disk of the seismograph at the time of data acquisition and transferred to the computer for further processing. The processing involves picking the first arrivals, input of data with geophone elevations and first arrivals, and final interpretation for velocity model. In case of noisy data there are intermediate steps of data processing using filtering, amplitude corrections etc.
WinSism can accept any field geometry, receiver spacing, shot spacing, shot distance to receiver. All distances are computed from first End-of-line shot. This shot must be always distance 0 and must exist. First Out-of-line shot must have a negative distance. Line length is computed from first End-of-line shot to the last End-of-Shot. Usually, the first End-of-line shot distance to the first receiver is half receiver spacing. Remember that two offset shots are necessary to use Delay or GRM method.
WinSism Travel time can be composed of more than one spread. In this case, first pick all original records using picking module, without to take care of exact geometry then see below. It is strongly recommend recording at least 5 shots; offset shots allow to use ABC method and to compute depth below all geophones.
