Page 242 - Fundamentals of Gas Shale Reservoirs

222 PASSIVE SEISMIC METHODS FOR UNCONVENTIONAL RESOURCE DEVELOPMENT

10.5 MONITORING PASSIVE SEISMIC The most important difference between recording data
EMISSIONS WITH SURFACE AND SHALLOW using surface geophone arrays and buried geophone arrays is
BURIED ARRAYS the presence of coherent surface wave noise. This noise is
almost always present on surface grids. Coherent surface
This section describes the use of geophone arrays on or near waves are generated by activity on the surface such as well
the surface of the earth to record passive seismic emissions. head activity, highways, construction sites, frac pumps, and
Surface and near‐surface monitoring are rapidly coming into drilling and pumping activity in the area. This noise can be
wide use for passive seismic monitoring. Many geophysical of very high amplitude in the low‐frequency band. The sur­
service companies that traditionally offered only downhole face noise can propagate along the surface and scatter off of
monitoring are adding surface monitoring to their service inhomogeneities in the near surface. The scattering points
offerings. Surface monitoring has many advantages over become new noise sources. As a result, every geophone on the
downhole monitoring. These advantages include uniform cov­ surface records surface wave noise arrivals from all azimuths.
erage, wide array aperture, no need for one or more dedicated The scattered coherent surface wave noise is very difficult to
monitoring wells, and much reduced sensitivity to velocity predict and remove using most available filters. Geophones
changes in the target formation due to the hydraulic fracture buried sufficiently deeply in shallow wells are hidden from
treatment. Wide array aperture and uniform coverage allow this surface wave noise. This difference in coherent surface
generation of important new products that are not possible wave noise present on surface geophones versus buried geo­
with downhole data. These products are cumulative seismic phones allows for the use of many fewer geophones for a
activity volumes and direct fracture images (Sections 10.5.4 buried array than for a surface array. The removal of coherent
and 10.5.5). Another advantage is that SET, which is used to noise from the surface geophones is the largest challenge to
process surface data, captures total seismic activity and not obtaining good detection of MEQs and making good‐quality
just discrete MEQs. On the downside, surface and near‐sur­ maps of the fracture networks using surface arrays.
face arrays suffer from reduced sensitivity resulting in higher While the quality of the surface geophone data suffers
detection threshold, limited frequency response, and lower from the surface wave noise, the buried phone suffers from
signal‐to‐noise (S/N) ratio compared to downhole arrays. An lower signal level and ghosting. The geophone on the sur­
additional problem is that landowner permits must be obtained face has twice the signal amplitude for upcoming waves
over large areas to deploy a surface array, whereas downhole compared to the buried geophones because the free surface
monitoring does not require such permitting. is essentially a perfect reflector and causes the amplitude of
the up‐going signal at the free surface to be multiplied by 2.
The signal that hits the free surface is propagated back down
10.5.1 Recording
with the same amplitude as the up‐going signal. Ghosting
Geophones may be installed on the surface or buried in shallow occurs when the buried geophone records the up‐going
wells. Surface geophones are either planted directly in the signal and then the down‐going signal after the reflection off
ground surface or (preferably) are planted in the ground after of the free surface. The delay between the up‐going signal
scraping away 20–30 cm of soil and covering with the displaced and the down‐going signal (the “ghost” signal) at the buried
soil. The soil slightly below the ground surface is firmer and geophone can be on the order of 10 ms for a very high
provides better geophone coupling. Alternating layers of soil velocity near surface and shallow geophone and be on the
and coils of geophone cable while burying the phones, damps order of 100 ms for a very low velocity in the near surface
wind, rain, and other vibrations traveling down the cable to the and a deep geophone.
geophones. However, burying the geophones adds substantially Consider these examples:
to deployment costs and hence is often not performed.
The geophones may either be single component (vertical) • In the Eagle Ford of South Texas the near surface
or three components (vertical and orthogonal horizontal velocity may be on the order of 1800 m/s (6000 ft/s)
components). The wells for shallow buried arrays are typi­ and the buried phone may be 90 m (300 ft) deep. The
cally 20–100 m (65–330 ft) deep. The well depth depends on delay between the up‐going and the down‐going
near‐surface conditions and the depth to which surface signals will be 100 ms.
waves can penetrate. One or more three‐component sondes • The Permian Basin of Texas has a very high velocity near
are cemented permanently into the well. Sondes that have surface layer that can be as thick as 365 m (1200 ft). The
only the vertical component are sometimes used. Hammer surface wave noise can penetrate hundreds of meters in
shots or small impulse‐generating devices are used to orient this kind of rock, so the buried phone may need to be 300
multicomponent geophones and sondes deployed in shallow m (1000 ft) deep in order to avoid surface noise. For the
wells by striking the surface at eight azimuths around the case of the geophone buried 300 m and the near‐surface
device. Both surface and buried array receivers record con­ velocity of 6100 m/s (20,000 ft/s), the delay between
tinuously for many days. the up‐going and down‐going signal will be 100 ms.