Introduction

During my stay at SEP, I participated in the processing of several ``passive'' datasets, with the goal of locating seismic sources, either primary (endogenous sources of microseismic background noise) or secondary (scatterers). (Vanyan & Cole, 1990). In addition, similar source location techniques are being applied to novel source datasets, including pump noise (Cole & Vanyan, 1990) and drill-bit (Cole, 1991) sources.

To locate seismic sources an algorithm based on computing the so-called semblance coefficient for points in a 3-D grid (Nikolaev & Troitskiy,1987) is used:  

$$Sem(x,y,z)={\sum_t{(\sum_i{S_i(t+t_i)})^2}\over {M\sum_t{\sum_i{S_i^2(t+t_i)}}}}.$$ (1)

Here x, y, and z are the coordinates of a point in the medium, t_i is the travel time from that point to geophone i, and M is the number of channels.

The idea is that if there is a source at some point in the medium, then semblance computed at this point will have a large value. The possibility of locating sources depends on different conditions: correspondence of the chosen model to reality, knowledge of velocity structure, configuration of the seismic array, presence of obstacles, etc. The impact of some of these aspects on the quality of the results can be considered using synthetic data. Such analysis can reveal how well, for a given array geometry, we can hope to locate sources. Such computations can also aid in interpreting semblance results obtained for real data.

In this paper I consider the effects on the source location method of (1) velocity variations and (2) spatial aliasing. The synthetic datasets were obtained for two different seismic array geometries. The first is a sparse 24 channel array used for registering microseismic background noise in Iceland (Cole & Vanyan, 1990). This array is shown in Figure .

The second one is the 240 channel seismic array used in the SEP drill-bit source experiment (Cole, 1991). This array is shown in Figure . Actually the array used in these studies differs slightly from the one used in the field. In the field experiment the layout of geophones is not quite so regular. But the dimensions and spacing are as shown here.

 

ar1
Figure 1

Geometry of the Iceland array.

 

ar2
Figure 2

Geometry of the drill-bit array.